HVAC/R Electronics # 6

Improving air quality is another opportunity for engineers to apply the capabilities of digital systems. Energy conservation strategies require buildings to be airtight to prevent the escape of expensive heated or cooled air. This usually comes at the expense of ventilation. “Sick building syndrome” has become a national concern as researchers continue to uncover the effects of indoor air quality (IAQ) on human health and productivity in the workplace. Concerns over health hazards in the workplace and the spread of airborne contaminants are issues that have reached the forefront of public attention. The control of building ventilation is a problem that is being solved through the application of digital controls. So let’s take a look at a ventilation program.

Ventilating Control Program



Sequence of Operation:

1. Supply fan starts and enables return fan start and system controls.
2. SA smoke detector stops supply fan when smoke detected.
3. RA smoke detector stops supply fan when smoke detected.
4. Controller stops fan when low temperature detected.
5. SA high static pressure control stops fan when unsafe pressure exists.
6. Automatic fan system control subject to commandable on-off-auto software
point.
7. Control program turns supply, return, and exhaust fan on and off dependent
upon optimized time schedule, unoccupied space temperatures, and occupant
override requests.
8. Occupant override switch provides after hours operation when pressed.
9. Duration of operation for override request.
10. Space temperature (perimeter zone) inputs to optimum start-stop, unoccupied
purge, and low limit programs.
11. Set-point at which unoccupied low-limit program executes.
12. OA temperature input to optimum start-stop program.
13. Return fan operation enables exhaust fan control program.
14. Exhaust fan status (operator information).
15. Warm-up mode status (operator information).
16. Supply fan load (VAV type systems-operator information).
17. Return fan load (VAV type systems-operator information).

Air handling system shall be under program control, subject to supply air (SA) and return air (RA) smoke detectors, SA high pressure cut-out, and heating coil leaving air low-temperature limit control; and shall be subject to system software on-off-auto function.

Supply fan shall be started and stopped by an optimum start-stop seven-day time schedule program, an unoccupied low space temperature limit program, or by an occupant via push button request. The push button shall be integral with the space temperature sensor. Any push button request shall provide sixty minutes (operator adjustable) of full system operation. Return fan shall operate anytime the supply fan proves flow (via a current sensing relay). The exhaust fan shall operate during scheduled occupancy periods and during occupant requested after-hour periods anytime the return fan proves flow.

In figure 1 we see a supply air control loop with the sequence of operation.



Sequence of Operation:

The DDC controller uses a temperature sensor mounted in the supply air duct to modulate control valves or mixing dampers to maintain a supply air temperature set-point. In most systems that employ a heating and cooling coil, the hot water valve and the chilled water valve should be modulated in sequence.

When the supply air temperature falls below set-point, the hot water valve begins to modulate open and consequently, the cooling valve begins to modulate closed. If the supply air temperature continues to fall below the set-point, the heating valve will open fully and the cooling valve will close completely.

When the supply air temperature rises above set-point, the hot water valve begins to modulate closed and consequently, the cooling valve begins to modulate open. If the supply air temperature continues to rise above the set-point, the heating valve will fully close and the cooling valve will open completely.

A temperature sensor located in the mixed air stream (between the unit filters and the coils) is used to provide mixed air low limit control. When the temperature sensed by this element falls below the setpoint, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper closes to a minimum position, and the valves on all coils will fully open. This sequence should always be used on systems with wetted coils.

When the unit fan is turned off, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper fully closes, and all control valves return to their “normal” positions.

Design Considerations:

Control Valves:

· Avoid using spring return actuators on control valves for wetted coil
applications.

· When selecting two-way valves for control
of wetted coils:

· For hot water coils, have the valve configured in the “normally” open position.

· For chilled water coils, have the valve configured in the “normally” closed
position.

When selecting three-way valves for control of wetted coils:

· For hot water coils, have the valve piped such that when the valve is in the
“normal” position, the water flows through the coil.

· For chilled water coils, have the valve piped such that when the valve is
in the “normal” position, the water bypasses the coil.It is recommended that
mixing valves be used in all three-way applications unless otherwise
specified.Also be cautious to not pipe globe valves that are designed
for mixing applications for diverting service. The fluid flow will
cause a“hammering”effect and severe noise and damage will follow.

In Electronic #5 I discussed how to program a control loop, following are some of the most common loops that can be controlled that you will find on sophisticated controls.

1. Discharge Air Temperature
2. Mixed Air Temperature
3. Hot Cold Deck Temperature
4. Cold Deck Temperature
5. Humidity or Dew Point Control
6. Indoor Air Quality Control
7. Ventilation Control
8. Supply Fan Static Pressure Control
9. Supply Fan Start/Stop Control
10. Return Fan Start/Stop Control

One of the last steps is to connect the analog inputs and outputs to an 8x computrol controller as shown in figure 1 below.



From this diagram you can see that the DDC is powered with 24 volts supply at the terminal strip in the lower left side. The discrete (on-off) inputs are called binary inputs. They are connected to any terminal that you designate as a binary, because you have the option to designate any of the terminals you like, which is a feature of these controls. Therefore point 1 thru 8 can be an analog input or output or it can be a binary input or output.

One of the main advantages of a DDC system is that it can be connected to a network and be controlled remotely. In some cases the control is setup in a special room, where the HVAC technicians for a large building or campus can monitor the entire system from a single console. Another feature of the network is that its data can be transmitted over dedicated telephone lines or connected through the Internet so that the data are available worldwide. Other features such as energy management, security, fire control and other essential functions can be controlled over these networks.

Conclusion:

This brings me to the end of this series of discussions on the wonderful world of DDC Controls and some of the attributes and marvelous things that can be done with these controls. My intention was not to teach you all about these controls but more of a fundamental introduction. If I aroused your interest into looking further into how these controls work then I will feel that I have done some good for the betterment of this industry. If you are interested in learning more about DDC Controls I urge you to peruse the following web sites:
DDC-Online
Computrols.com

copyright(c)2009
Roger J. Desrosiers



About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.Lean More About Roger!

Air-Source Heat Pump (ASHP) Auxiliary-Supplemental Heat Sizing

Most ASHPs will have electric heaters installed within the “air handler” (where the supply air duct work originates). Outdoor temperatures lower than the structure “balance point” requires an additional heat source since the ASHP refrigeration cycle alone will not produce enough heat for indoor comfort needs during winter months. This additional heat is often called either auxiliary or supplemental heat.

Typically, around 30 - 40 degrees the ASHP will generally be able to produce the same amount of heat with the compressor and refrigerant cycle alone that the structure is losing (this is considered the structure "balance point”).Every structure has a distinctive “balance point” which can be calculated.

If the ASHP is located where the heating requirements are very much greater than the cooling requirements, there will be a need for this additional heat to help meet the load of the structure during heating. This is required as it is harder to extract heat from colder air.

Since the indoor coil of an "all electric heat pump" can be located upstream of the electric heating elements used as auxiliary or supplemental heat and emergency heat, the ASHP and the backup heating can operate simultaneously in the heating mode. These heaters, of course, will not, and should not, energize during the cooling mode. If additional heat is required due to unexpected heat loss above the “balance point,” the heaters energize briefly, controlled by the thermostat.

Note in the diagram shown that the RA (return air) enters the indoor coil, not the heated air from the heating elements. An ASHP is always sized for the cooling load of the structure so that it has a long run cycle and can properly dehumidify the space.
When installing an ASHP, choosing the right auxiliary/supplemental heating system is almost as important as proper heat pump sizing.

Unfortunately, in the past and even now, it has become common practice by some contractors and/or technicians to install an ASHP with larger electric heat strips than necessary. But remember, any time the temperature is below the structure “balance point” the auxiliary/supplemental heat may come on, called for by the “second heating stage” of the indoor thermostat.

Most ASHPs operating in the heating mode around these temperatures will run most of the time. This is actually normal operation, At or below the “balance point” the heat is leaving the structure faster than the ASHP compressor can transfer it into the conditioned space. This is a function of 1) the temperature difference between indoors and outdoors, 2) the insulating value of the structure envelope, and 3) the amount of air leakage into or out of the structure. Without supplemental heating, the indoor temperature will fall below the thermostat setting.

You should always calculate the proper size of the heaters to meet the entire structure heat loss in Btu/h from the indoor design temperature (say 70 degrees) down to the typical outdoor design temperature for the locale (for example 0 degrees). This will provide enough electric heat to meet the indoor thermostat setting alone without the compressor. However, remember that the electric heaters operating alone are much less efficient than the heat pump compressor during heat pump heating. The heat pump should be configured to provide heating throughout the heating needs (say from 65 degrees outside down to the outdoor design temperature) to achieve the efficiency of the electric heat pump compressor, with or without the heaters. Basically, always allow the compressor to provide heat during the heating cycle and never wire the system to turn the compressor off at some specific outdoor temperature above the outdoor design except if combined with a gas furnace (an add-on or dual fuel configuration).

The best way to provide effective auxiliary/supplemental heating is to have multiple levels of heat if necessary. These banks of heat then energize as necessary. Most manufacturers provide heaters in sizes that meet most structure heating needs (5, 10, 15, 20 kW banks, etc.).

Emergency heat is always necessary with ASHPs if the need arises. This heating is actually provided by the auxiliary/supplemental heaters as well. This allows the occupant to energize the heaters and the indoor blower to meet the heating requirements of the particular structure as a backup if malfunctions occur with the ASHP refrigerant cycle. Homeowners can energize the emergency heat mode typically with a switch on the indoor thermostat. In fact, most ASHP thermostats are now electronic and equipped with these selector switches or other means of energizing emergency heat. Always study the particular thermostat specifications for the model you use to assure the emergency heat can operate properly if necessary.

Emergency heat relays should always be used in conjunction with outdoor thermostats to allow electric heat operation in the event the heat pump compressor and refrigerant cycle become inoperative. An outdoor thermostat disallows the second stage (if provided) of electric heat above a selected outdoor temperature. If the outdoor temperature falls below the setting on the outdoor thermostat, this additional heater stage will come on. When the outdoor air temperature rises, and the outdoor thermostat set point is reached, the system will revert back to first stage electric heating.

Codes are very strict concerning electric heaters in ASHPs. The code governs where the electric heaters are to be installed, how they must be wired, and, in some cases, how much additional heat must be added to the ASHP to compensate for capacity loss during the heating mode. If the ASHP fails, the code specifies how much heat must be added to serve as both supplemental heat and emergency heat. It is imperative that you as the technician check the governing codes and follow them exactly.

Copyright © Phil Rains

About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.

HVAC/R Electronics # 5-DDC Controls

DDC control consists of microprocessor-based controllers with the control logic performed by software. Analog-to-Digital (A/D) converters transform analog values into digital signals that a microprocessor can use. Analog sensors can be resistance, voltage or current generators.

The microprocessor unit (MPU) in the controller provides the computation. Therefore, the term digital in DDC refers to digital processing of data and not that HVAC sensor inputs or control outputs from the controller are necessarily in digital format. Nearly all sensor inputs are analog and most output devices are also analog. In order to accept signals from these I/O devices, A/D and D/A converters are included in the microprocessor-based controller. The figure below shows several inputs and outputs. The microprocessor usually performs several control functions.


DDC provides more effective control of HVAC systems by providing the potential for more accurately sensed data. Electronic sensors for measuring the common HVAC parameters of temperature, humidity and pressure are inherently more accurate than their pneumatic predecessors. Since the logic of a control loop is now included in the software, this logic can be readily changed. In this sense, DDC is far more flexible in changing reset schedules, set points and the overall control logic. Users are apt to apply more complex strategies, implement energy saving features and optimize their system performance since there is less cost associated with these changes than there would be when the logic is distributed to individual components. This of course assumes the user possesses the knowledge to make the changes.

Elements of a Direct Digital Control System

8X DDC Controller: Figure 1


Figure 1 above is a typical state of the art DDC controller in use today.

Any point can be configured through software to be Analog In, Analog Out, Binary In, or Binary Out – no jumpers with Bright on-board LEDs assist in troubleshooting. You can directly connect a normal web browser for simple management. It's high speed communications allow the ultimate in flexibility and snappy response, 3 decimal rotary switches (0-9) allow simple addressing – no hex, no binary and all of the electronics are on one easily replaceable brain board for quick repairs.

POINTS

All field devices and any logic or calculations associated with those devices are points. The word "points" is used to describe data storage locations within a DDC system. Data can come from sensors or from software calculations and logic. Data can also be sent to controlled devices or software calculations and logic. Each data storage location has a unique means of identification or addressing. A point can be an actuator, a temperature sensor, a control sequence or any other quantity or status that can be monitored or controlled. We recommend naming your points based on their function to make it easier for the operator. For example, if you have a temperature sensor that reads the outside air temperature, name the point “Outside Air Temp.” There are two categories of points: Hardware and Software

HARDWARE POINTS

Hardware points are points that can be physically wired or connected through a wireless sensor to the terminal strip of a controller. They include field devices such as relays, actuators and sensors. Their function is to transmit data back to the controller or physically carry through a building automated control command.

There are four main types of hardware points. They are analog inputs, analog outputs, binary inputs, and binary outputs. Binary points have only two states such as ON/OFF, OPEN/CLOSE, or START/STOP. Analog points on the other hand, represent a range of measurement such as a temperature of 0°F to 110°F, a pressure of 1psi to 5psi, or a flow rate of 100 CFM to 200 CFM.

Whether a point is binary or analog, it must be either an input or an output. Points that monitor the status of a field device are inputs. Field devices send their condition or quantity to an input on the controller.

Points that control the status of a field device are outputs. The user can either control outputs manually, or allow for automatic control based on schedule, logic, PID, or other software outputs programmed in the building automated control.

SOFTWARE POINTS

Software points include calculations, points of reference, and logic statements. They are intelligent points that are not physically connected to the controller. Instead, they gather data and send commands to hardware points. An example of gathering data is the average supply temperature of all AHUs in the building. An example of sending commands is, "if Outside Air Temp is less than 50°F, then start VAV heat strips.

CONTROLLER CONFIGURATION

- The microprocessor
- A program memory
- A working memory
- A clock or timing devices
- A means of getting data in the basic elements of a microprocessor-based (or microprocessor) controller. (Fig. 3)

In addition, a communications port is not only a desirable feature but a requirement for program tuning or interfacing with a central computer or building management system.

Timing for microprocessor operation is provided by a battery-backed clock. The clock operates in the microsecond range controlling execution of program instructions.

Program memory holds the basic instruction set for controller operation as well as for the application programs. Memory size and type vary depending on the application and whether the controller is considered a dedicated purpose or general purpose device.

Dedicated purpose configurable controllers normally have standard programs and are furnished with read only memory (ROM) or programmable read only memory (PROM.)

General purpose controllers often accommodate a variety of individual custom programs and are supplied with field-alterable memories such as electrically erasable,programmable, read only memory (EEPROM) or flash memory. Memories used to hold the program for a controller must be nonvolatile, that is, they retain the program data during power outages.


Fig. 3. Microprocessor Controller Configuration for Automatic Control Applications.

All input signals, whether analog or digital, undergo conditioning (Fig. 3) to eliminate the adverse affects of contact bounce, induced voltage, or electrical transients. Time delay circuits, electronic filters, and optical coupling are commonly used for this purpose. Analog inputs must also be linear zed, scaled, and converted to digital values prior to entering the microprocessor unit. Resistance sensor inputs can also be compensated for lead wire resistance...
Performance and reliability of temperature control applications can be enhanced by using a single 12-bit A/D converter for all controller multiplexed inputs, and simple two-wire high resistance RTDs as inputs.

A/D converters for DDC applications normally range from 8 to 12 bits depending on the application. An 8-bit A/D converter provides a resolution of one count in 256. A 12-bit A/D converter provides a resolution of one count in 4096. If the A/D converter is set up to provide a binary coded decimal (BCD) output, a 12-bit converter can provide values from 0 to 999, 0 to 99.9, or 0 to 9.99 depending on the decimal placement. This range of outputs adequately covers normal control and display ranges for most HVAC control applications. D/A converters generally range from 6 to 10 bits.

The output multiplexer (Fig. 3) provides the reverse operation from the input multiplexer. It takes a serial string of output values from the D/A converter and routes them to the terminals connected to a transducer or a valve or damper actuator.
The communication port (Fig. 3) allows interconnection of controllers to each other, to a master controller, to a central computer, or to local or portable terminals.

Types of controllers:
DDC can be designed for system level or zone level control like that shown below.


Zone level

Zone-level controllers can be applied to a variety of types of HVAC unitary equipment. Several control sequences can be resident in a single zone-level controller to meet various application requirements. The appropriate control sequence is selected and set up through either a PC for the system or through a portable operator's terminal. The following two examples discuss typical control sequences for one type of zone-level controller used specifically for VAV air terminal units.

VAV sequence of operation

In a pressure independent VAV cooling only air terminal unit application the zone-level controller controls the primary airflow independent of varying supply air pressures. The airflow set point of the controller is reset by the thermostat to vary airflow between field programmable minimum and maximum settings to satisfy space temperatures. On a call for less cooling, the damper modulates toward minimum. On a call for more cooling, the damper modulates toward maximum. The airflow control maintains the airflow at whatever level the thermostat demands, and holds the volume constant at that level until a new level is called for. The minimum airflow setting assures continuous ventilation during light loads. The maximum setting limits fan loading, excessive use of cool air, and/or noise during heavy loads.

System-Level Controller

System level controllers have more capacity and are more flexible then zone level controllers. System level controllers are used in central chiller and boiler plants, equipment rooms, and built up air handlers. Control sequences usually contain customized programs written to handle the specific application. The application of the controller must allow both the number and mix of inputs and outputs to be variable. The number of inputs and outputs required for the system level controller is usually not predictable.

Programming a DDC Loop

Most DDC systems use tables similar to the one shown below to set up a loop control for each part of the system. From this diagram you can see the loop is separated into several sections. The first section is the controller, which takes in a set point from an operator /programmer. It also takes in a feedback signal from a sensor and sends it to a control algorithm, which compares it to the set point. Any error found is the difference between the set point and the signal from the sensor also called the process variable. The output signal from this area goes through the digital/analog convertor. The corrective signal is sent as an analog signal from the controller to the final control element. (Chilled water/hot water valve etc.) For example, this loop could be controlling the temperature of a hot deck by modulating a hot water valve. The sensor could be an RTD thermostat, which would send back a feedback signal to the controller so the actual temperature in the hot deck can be compared to the set-point. If the set-point is 80* and the RTD says its 78* the controller would determine that the system needs to be warmer.

Simple Control Loop.

In Electronic # 6 we will talk about sequence of operations of various applications and web sites you can look up the gain more knowledge on DDC Controls. In the mean time: Be happy in your work and learn a lot!

copyright(c)2009
Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.

R-410A Air Conditioner Refrigerant Charging

An R-410A air conditioner’s ability to operate as designed is dependent upon the amount of refrigerant it contains. U.S. Environmental Protection Agency (EPA) studies suggest that approximately 75 % of installed air conditioners possibly have incorrect refrigerant levels, which can reduce system capacity and efficiency by 20 percent or more.

The level of refrigerant charge is unique to each R-410A air conditioner and is determined by every component, including the outdoor coil and compressor, the indoor coil, and the refrigeration lines that carry the refrigerant between the coils. Correct refrigerant charge and proper refrigerant line sizing protect the compressor from damage, ensure efficiency, and improve performance. You should always verify the refrigerant charge for proper installation of an R - 410A air conditioner.

R-410A air conditioners should be leak-checked during the installation and during each service call. Most R-410A air conditioners are charged with refrigerant at the factory, and are seldom incorrectly charged. R-410A air conditioners that have the correct refrigerant charge and airflow typically perform very close to manufacturers listed cooling efficiencies. Over-charging or under-charging refrigerant however, reduces R-410A air conditioner performance and efficiency.

For satisfactory performance and efficiency, an R-410A air conditioner should be within a few ounces of the correct charge, specified by the manufacturer. You must measure airflow prior to checking refrigerant charge because the refrigerant measurements aren't accurate unless air flow is correct. Several simple methods have been utilized in the field for years by technicians to estimate the Cubic Feet per Minute (CFM) per Ton crossing the evaporator. They include velocity multiplied by area for registers and grilles, and pressure drop across a system or coil. These particular types are useful when you check for airflow across an evaporator during cooling and you should become familiar with these in lieu of simply guessing. Airflow should be between 350 and 450 CFM/Ton per Air-Conditioning, Heating and Refrigeration Institute (AHRI) standards.

When the charge is correct, specific refrigerant temperatures and pressures listed by the manufacturer will match temperatures and pressures measured in the field. Always verify these measurements. If the manufacturer's temperatures and pressures don't match the measured ones, refrigerant should be added or recovered, according to standards specified by the EPA.

R-410A air conditioners charged with refrigerant at the factory are shipped with the refrigerant charge typically noted on the unit nameplate. This charge is for a typical application of between 15 to 25 feet of equivalent line length, depending on the particular manufacturer. Occasionally, you may have to field charge or adjust charge when servicing existing systems. The best method to insure that the R-410A air conditioner is properly charged is by weighing in the amount of refrigerant specified on the outdoor sections nameplate, or per installation and operation manuals. Many contractors and technicians utilize the superheat method (for orifices/pistons), and the sub cooling method (for TXVs) when charging in the field.

R-410A air conditioners installed with more than 25 feet of refrigerant line should be charged following the charging method described in the installation and operation "long-line application" instructions provided by the particular manufacturer, or the superheat method, and/or sub cooling method, as necessary. No additional refrigerant oil charge is usually required for these applications.

Many field variations exist which may affect the operating temperature and pressure readings of an R-410A air conditioner. Some R-410A air conditioners utilize fixed orifice refrigerant control devices prior to the evaporator. The following procedure is for this type of refrigerant control device:

1. Check the condition of coils, blower wheels, and the blower motor speed. Measure airflow. The airflow calculation is very important because it helps you determine evaporator load, and therefore will have a significant effect on system pressures. Correct airflow if necessary prior to performing this check.

2. With both valves fully open, connect a set of manifold gauges to the valves' service ports, being careful to purge the lines.

3. Allow the system to operate at least 10 minutes or until the pressures stabilize.

4. Temporarily install a thermometer on the suction (large) line near the condensing unit's service valve. Make sure that there is good contact.

5. Determine the systems superheat as follows:

a. Read the system's suction pressure on the compound gauge.
b. Using the compound gauge (or a P/T chart) determine the system's saturated
suction temperature.
c. Read the suction line temperature with a temperature measurement device.
d. Superheat = the suction line temperature - the saturated liquid temperature.

6. Adjust the charge as necessary to meet the manufacturer’s requirements by adding refrigerant to lower the superheat, or recovering refrigerant to raise the superheat. Superheat charts are provided by manufacturers.

Most R-410A air conditioners manufactured today are equipped with a TXV prior to the evaporator. You can’t check a unit’s charge by using the previous Superheat Charging Method if this is the case. You must use the Sub-Cooling Charging Method.

TXVs control refrigerant flow by maintaining a constant superheat (for instance, 8ºF to 10ºF). With constant superheat values, the condition of the system charge cannot be determined using superheat. You must look to the condenser and the liquid side to verify proper charge. The following procedure is for this type of refrigerant control device:

1. Check the condition of coils, blower wheels, and the blower motor speed. Measure airflow by using the temperature rise method. Check pressure drop across coils using the manufacturer’s coil specification sheets. Or, use the velocity pressure to calculate airflow. The airflow calculation is very important because it helps you determine evaporator load, and therefore will have a significant effect on system pressures.

2. Check the system operating pressures. Connect the hoses from your manifold gauge set to the pressure taps on the liquid and suction service valves. Measure and record the liquid (discharge) and suction pressures.

3. Measure and record the outdoor ambient temperature.

4. Measure the wet bulb and dry bulb of the air entering the indoor unit in the return duct. This step is very important because it also helps you determine the evaporator load, and therefore will have a significant effect on system pressures.

5. Measure the liquid-line temperature so that sub cooling can be calculated. Use a good thermometer with a probe that can be strapped tightly to the line. Install the probe on the liquid line about 6-in. from the liquid service valve, then measure and record the liquid-line temperature.

6. Measure the high side pressure at the liquid-line service valve pressure tap. Using the discharge gauge (or a P/T chart) convert high side pressure to saturation temperature. Then simply subtract the liquid-line temperature from the saturation temperature of the refrigerant in the condenser to determine the sub cooling value.

Always refer to manufacturer’s data sheets to find the proper operating pressures for the conditions of the air that you’ve measured. Do the same for required sub cooling levels. Some manufacturers have tested their systems in laboratories and developed specific sub cooling requirements. If this has been accomplished, the sub cooling target will typically be visible on the label on the condenser. You should attempt to stay within 3 degrees of the target sub cooling.

If sub cooling is too low, there may be an insufficient amount of refrigerant. Add refrigerant as necessary.

If sub cooling is too high, there may be too much refrigerant in the outdoor coil. Recover refrigerant as necessary.

Zeotropic refrigerants like R-410A must be charged as a liquid from a canister due the possibility of fractionation of the blend of refrigerants it contains. You must consider its temperature glide, which refers to the range of temperatures at which components in a blended refrigerant boil or condense at a given pressures. R-410A’s temperature glide is < .3 º F, making it a near-azeotropic refrigerant mixture.

Liquid charging is much faster than vapor due to the density of liquid refrigerant. R-410A must be “liquid charged” into the high side of the system if it is empty, so the components in the blend do not separate. Charging by weight is the preferred method of admitting the liquid charge.

If you are “topping off” the charge, it is necessary to charge R-410A refrigerant into the low side of an operating system. You will need to invert most R-410A refrigerant cylinders, as most do not have dip tubes anymore. This will allow liquid refrigerant to flow freely from the cylinder. Connect the service hose to a commercially available throttling device and then to the suction service valve to charge the system. Using the throttling device is recommended, but some technicians have simply “hand throttled” the refrigerant into the system with the low side valve, with very little problem. Either way, you must avoid the possibility of fractionation.

Pressures are 50% to 70% higher within an R - 410A air conditioner than what you were used to finding with R-22 refrigerant air conditioners. Always practice safe procedures when working with R-410A refrigerant.

For more information about our R-410 online course, Click Here.

Copyright © Phil Rains

About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.

HVAC/R Electronics #4

In Electronics # 3 we looked at some working circuits of op amps, diac/triacs and transistors. Now as we talk about electronic controls, also known as conventional or single loop controls that control only one loop in an entire control system, let’s start to discuss the components as a working system that integrates many electronic components to a whole series of accurate control systems. Let’s begin by talking about sensors and output devices.

The sensors and output devices (e.g., actuators, relays) used for electronic control systems are usually the same ones used on microprocessor-based systems, which we will discuss in Electronic # 5. The distinction between electronic control systems and microprocessor-based systems is in the handling of the input signals. In an electronic control system, the analog sensor signal is amplified, and then compared to a set-point or override signal through voltage or current comparison and control circuits.

In a microprocessor-based system, the sensor input is converted to a digital form, where discrete instructions (algorithms) perform the process of comparison and control. Direct Digital Control Systems (DDC) can control many control sequences simultaneously.

Analog Electronic Controllers perform control functions based on inputs that can handle only one control loop at a time.

Convential Electronic control systems usually have the following characteristics:

Controller: Low voltage, solid state.
Inputs: 0 to 1V dc, 0 to 10V dc, 4 to 20 mA, resistance element,
thermister,thermocouple.
Outputs: 2 to 10V dc or 4 to 20 mA device.
Control Mode: Two-position, proportional, proportional plus integral (PI), step.

Figure 1 shows a simple electronic control system with a controller that regulates supply water temperature by mixing return water with water from the boiler. The main temperature sensor is located in the hot water supply from the valve. To increase efficiency and energy savings, the controller resets the supply water temperature setpoint as a function of the outdoor air temperature. The controller analyzes the sensor data and sends a signal to the valve actuator to regulate the mixture of hot water to the unit heaters.


Figure 1

These controllers measure signals from sensors, perform control routines in software programs, and take corrective action in the form of output signals to actuators. Since the programs are in digital form, the controllers perform what is known as direct digital control (DDC). Microprocessor-based controllers can be used as stand-alone controllers or they can be used as controllers incorporated into a building management system utilizing a personal computer (PC) as a host to provide additional functions.

Some electronic sensors use an inherent attribute of their material (e.g., wire resistance) to provide a signal and can be directly connected to the electronic controller. Other sensors require conversion of the sensor signal to a type or level that can be used by the electronic controller. For example, a sensor that detects pressure requires a transducer or transmitter to convert the pressure signal to a voltage that can be used by the electronic controller.

Electronic control, temperature sensors are classified as follows: Thermister, such as Resistance Temperature Devices (RTDs), change resistance with varying temperature. RTDs have a positive temperature coefficient (resistance increases with temperature).
Thermistors are solid-state resistance-temperature sensors with a negative temperature coefficient.

Controller:
The electronic controller receives a sensor signal, amplifies and/or conditions it, compares it with the set-point, and derives a correction if necessary. The output signal typically positions an actuator. Electronic controller circuits allow a wide variety of control functions and sequences from very simple to multiple input circuits with several sequential outputs. Controller circuits use solid-state components such as transistors, diodes, and integrated circuits and include the power supply and all the adjustments required for proper control.

Universal Controllers:
The input circuits of universal controllers can accept one or more of the standard transmitter/transducer signals. The most common input ranges are 0 to 10V dc and 4 to 20 mA. Other input variations in this category include a 2 to 10V dc and a 0 to 20 mA signal. Because these inputs can represent a variety of sensed variables such as a current of 0 to 15 amperes or pressure of 0 to 3000 psi, the settings and scales are often expressed in percent of full scale only.

Output Control:
Electronic controllers provide outputs to a relay or actuator for the final control element. The output is not dependent on the input types or control method. The simplest form of output is two-position, where the final control element can be in one of two states. For example, an exhaust fan in a mechanical room can be turned either on or off. The most common output form, however, provides a modulating output signal which can adjust the final control device (actuator) between 0 and 100 percent such as in the control of a chilled water valve.

Output Devices:
Actuator, relay, and transducer are output devices which use the controller output signal (voltage, current, or relay contact) to perform a physical function on the final control element, such as starting a fan or modulating a valve. Actuators can be divided into devices that provide two-position action and those that provide modulating action.

Two-Position:
Two-position devices such as relays, motor starters, and solenoid valves have only two discrete states. These devices interface between the controller and the final control element. For example, when a solenoid valve is energized, it allows steam to enter a coil which heats a room The solenoid valve provides the final action on the controlled media, steam. Damper actuators can also be designed to be two-position devices.

Modulating:
Modulating actuators use a varying control signal to adjust the final control element. For example, a modulating valve controls the amount of chilled water entering a coil so that cool supply air is just sufficient to match the load at a desired set-point (Fig. 17). The most common modulating actuators accept a varying voltage input of 0 to 10 or 2 to 10V dc or a current input of 4 to 20 mA. Another form of actuator requires a pulsating (intermittent) or duty cycling signal to perform modulating functions. One form of pulsating signal is a Pulse Width Modulation (PWM) signal.




In modulating control, when an actuator is energized, it moves the damper or valve a distance proportional to the sensed change in the controlled variable. For example, a Series 90 thermostat with a 10-degree throttling range moves the actuator 1/10 of the total travel for each degree change in temperature.

Series 90 controllers differ from controllers of other series in that the electrical mechanism is a variable potentiometer rather than an electric switch. The potentiometer has a wiper that moves across a 135-ohm coil of resistance wire. Typically the wiper is positioned by the temperature, pressure, or humidity sensing element of the controller.

The actuator has a low-voltage, reversible-drive motor which turns a drive shaft by means of a gear train. Limit switches limit drive shaft rotation to 90 or 160 degrees depending on the actuator model. The motor is started, stopped, and reversed by the electronic relay.

The feedback potentiometer is electrically identical to the one in the controller and consists of a resistance path and a movable wiper. The wiper is moved by the actuator drive shaft and can travel from one end of the resistance path to the other as the actuator drive shaft travels through its full stroke. For any given position of the actuator drive shaft, there is a corresponding position for the potentiometer wiper.

All Series 90 actuators have low-voltage motors. A line-voltage model has a built-in trnsformer to change the incoming line voltage to low voltage for the control circuit and the motor. Low-voltage models’ require an external transformer to supply the actuator (Fig. 19). Notice the Triacs in this circuit.



In reiterating the difference between Electronic Control and Direct Digital Control the electronic controls were quite good for the time but DDC is now the state of the art. These controllers measure signals from sensors, perform control routines in software programs, and take corrective action in the form of output signals to actuators. Since the programs are in digital form, the controllers perform what is known as direct digital control (DDC). Microprocessor-based controllers can be used as stand-alone controllers or they can be used as controllers incorporated into a building management system utilizing a personal computer (PC) as a host to provide additional functions. This is called distributed direct digital control. In part #5 we will talk about the various functions of DDC controls.

copyright(c)2009
Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.

Residential Load Calculations – Why they are Important to Contractors

A residential load calculation is a way for a contractor (or technician) to determine the envelope loads for a particular residential dwelling.

Every residential structure has these envelope loads which are determined by local weather patterns, features of the structure, and all the building materials and techniques that are or were used in its construction. Other parameters include appliances and the number of occupants in the structure, duct work loads, ventilation loads, and motor heat loads. Effectively, the envelope loads are the total heating and cooling loads for the components that surround the conditioned space (walls, ceilings, roofs, floors, doors, windows, etc.)

The residential heating and cooling system must be selected and designed to provide comfort conditions in all occupied spaces regardless of whether is it winter or summer. The installed system must be able to control temperature, humidity, air movement and ventilation simultaneously.

Load calculations procedures produce improved equipment sizing loads for single-family detached homes, small multi-unit structures, condominiums, town houses and manufactured homes. These procedures are also compatible with different types of comfort systems and applications such as:

.Central single-zone systems
.Central multi-zone systems
.Distributed multi-zone systems
.Dwellings with limited exposure or no exposure diversity

The load calculation is the most important step in determining the size and type of cooling and heating equipment required to maintain comfortable indoor air conditions.

The Air Conditioning Contractors of America (ACCA) is a group of air conditioning contractors who work together to improve the HVAC industry, promote good practices, and keep homes and buildings safe, clean and comfortable. As part of their effort, ACCA has developed the HVAC Quality Installation Specifications (QI) from contribution from contractors and other interested parties. These contributors include original equipment manufacturers (OEMs), public, private, and federal electric utilities, and industry associations. Many contractors now follow the concepts and requirements of the QI, either by desire or requirement.

The QI is designed to assist contractors as they go through the process of determining which approach to follow when they design, install and service a system. Also, the QI requires that contractors perform load calculations for new structures and when dealing with existing structures. The QI states the following concerning load calculations:

“The contractor shall provide evidence that for new residential and commercial buildings, or when adding new ducts to an existing structure, room-by-room heat gain/loss load calculations are completed…”

When discussing load calculations, we need to review equipment sizing considerations. Significantly undersized HVAC equipment will typically be unable to maintain the desired set-point temperature when a design load is imposed on the heating and cooling equipment. However, slightly undersized equipment sometimes will often provide acceptable comfort at a lower cost, but never undersized by more than around 10 % of the design loads.


When someone has oversized the HVAC equipment, short-cycles can occur during the cooling mode. This provides only marginal part-load temperature control for the structure, as well as allowing stagnate air pockets to materialize (unless the indoor blower is operating continuously). There will typically be a degradation of humidity control as well, as the system does not run long enough to condense the moisture out of the air. It is only running basically to meet the sensible load controlled by the indoor thermostat. Oversizing requires larger equipment and larger duct systems, increasing the installed cost and results in increased operating costs as well. Utilities also oppose oversing as it results in increased demand on their systems. And finally, oversizing adds unnecessary stress to the HVAC equipment.

The best way to avoid oversizing or undersizing HVAC equipment is to perform a load calculation. Don’t rely on “rule of thumb” methods or on past experience. At best, these can only provide quick design however they are often not precise and can lead to problems.

Also, remember that a well- insulated house is much tighter than an un-insulated house. Always recommend sufficient levels of insulation in lieu of increased system size. But, if the consumer cannot facilitate more insulation, you will have to calculate the system size with the levels present.

As such, a quality installation begins with a load calculation as part of the professional design process, even for a home.

Year-round, comfort is always the goal when performing a load calculation. In the cooling mode the HVAC system not only cools the indoor air (sensible cooling), it also removes moisture (latent cooling). In the winter, your heating system must keep you comfortable without causing high utility bills.

Concepts and fundamentals of HVAC/R equipment sizing is based on heat gain and losses in a dwelling. You will need to remove the amount of heat gain in the summer and add in the amount of heat loss in the winter with the equipment. Heat gain and loss must be equally balanced by heat removal and addition to get the desired comfort.

Many residential load calculation methods are available in the HVAC/R industry. Two of the most recognizable and frequently utilized are ACCA’s Manual J, Eighth Edition (MJ8), and ACCA’s Manual J, Eighth Abridged Edition (MJ8AE). Regardless of the method, a residential load calculation is always a good idea prior to installing or retrofitting any HVAC/R system.

To learn more about HVACReducation.net's online Hvac Load Calculations course,
CLICK HERE

Phillip A. Rains
Copyright © Phil Rains

About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.

Electronics and HVAC/R Systems #3

In electronics # 2 we discussed how thermisters work, and some of their uses. Now we will use one as a temperature sensor with an op amp as in the circuit below, and see how it will affect the voltage to the negative input to the amplifier pulling in the DPDT relay.

You will remember how a thermostat’s resistance changes with temperature. If an increase in temperature results in an increase in resistance, the thermister has a Positive Temperature Coefficient and is called a PTC thermister. These are not common.

If an increase in temperature results in a decrease in resistance, the thermister
has a Negative Temperature Coefficient and is called a NTC thermister. These are the normal types usually used in our work.

Thermisters have their value given in ohms based on a temperature of 25°C, the
NTC1 that is used in this circuit is 20k ohms @ 25°C (77*F).

The Operational Amplifier

The operational amplifier (op-amp) is used as a voltage comparator.
When the voltage on pin 3 is more positive than the voltage on pin 2 the output will be high (9V). When the voltage on pin 3 is less positive than the voltage on pin 2 the output will be low (0V). This op-amp can be used to switch something on or off such as a Refrigeration Compressor or a Supply Fan, a Boiler, etc.


CIRCUIT DESCRIPTION:

The voltage on pin 3 will increase with a rise in temperature. When this voltage is greater than the voltage on pin 2, the output voltage on pin 1 will quickly rise to 9V. With pin 1 at 9V base, current to the transistor will flow and the transistor will be switched on, the LED will be on and the relay energized. When pin 3 voltage falls below that of pin 2, then the output of pin 1 will fall to 0V, the LED will be off and the relay de-energized. Changing the voltage at pin 2 by changing VR1 or R1 will result in the circuit responding to different temperatures. In other words,
VR1 could be called the thermostat set point.

DIAC/TRIAC

The diac is used primarily as a triggering device, so when a positive or negative reaches its break over voltage, the diac rapidly switches from a high resistance state to a low resistance state. Since the diac is a bi-directional device, it is ideal for controlling triacs, which are also bi-directional. Following is the symbol for the diac and break over voltage graph.


Now let’s look at a simple circuit where a diac is controlling a triac and driving a universal motor.

In this circuit, capacitor C1 charges up to the firing voltage of the diac in either direction (positive or negative). Once fired, the diac will apply voltage to the gate of the triac. The triac will conduct and apply power to the load. The speed of the load (universal motor) may be changed by varying the resistance of the potentiometer R1, which in turn varies the time of the firing voltage on the gate. Notice that since the universal motor is basically a series D.C. motor current flowing in either direction, and will cause rotation in only one direction. You will see this type of circuit in many electronic controls.

Here is a circuit for controlling an ordinary DC motor using two pairs of transistors (1 NPN and 1 PNP for each pair). Can you tell which transistors is NPN?


This DC motor runs in one direction if the required voltage is applied across its winding, and runs in the opposite direction if the polarity of the applied voltage is reversed. This function can easily be achieved by the circuit above.

In this circuit, a "logic 1" (on) voltage at Control 1 and a "logic 0" (off) voltage at Control 2 will turn on Q1 and Q4 and turn off Q2 and Q3, causing the motor to turn in one direction. Reversing the voltage levels at Control 1 and Control 2 will reverse the pairs of transistors that are "on" and "off", causing the motor to turn in the opposite direction. Putting the same logic input at Control 1 and Control 2 (both "1" and both "0") will cause the motor to stop turning.

The values of the base resistors of the transistors (or even the transistors themselves) required by the circuit may be different from those shown in Figure 1, depending on the motor being driven. Experimentation may therefore be required on the part of the hobbyist to make this circuit work.

In Electronics # 4, I will go into fundamental of electronic controls such as features of electronic control systems, sensors and output devices.

copyright(c)2009
Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.