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Heat Pump Components – Heat Pumps are similar to air conditioners with a few extra components. Components added to allow the heat pump to offer both heating and cooling. The two major differences between a heat pump and an air conditioner are the controls and the refrigeration circuit. While there are many different types of heat pumps and air conditioners the methods and means these two types of heating and cooling systems use are basically the same.

Heat Pump Components | HVAC Heating & Cooling

Heat Pump Components
An air conditioner will only cool. If you want heat you need some other type of system added to the air conditioner to provide heating. While a heat pump looks like an air conditioner the heatpump can provide both heating and cooling. It uses the same principles and methods an air conditioner uses to provide cooling. A heatpump simply reverses the refrigeration process and moves the heat in a different direction when in heating mode. If you have read about refrigeration on High Performance HVAC then you know the process of refrigeration is moving heat. Refrigeration uses mechanical processes to absorb heat. It moves this heat somewhere where it is not wanted. Heatpumps, when in heating mode, moves heat to a place where it is wanted.

Heat Pump Components – Heat Pump Controls

Depending on the type of heat pump you have will depend on the type of controls you have. It will also depend on many other factors such as proper setup and installation. Some installers install the system and never install an outdoor thermostat. An outdoor thermostat to cut the heat pump condenser off when the temperature outdoors makes the unit useless. This is only applicable to air source heat pumps and not geothermal or ground source heat pumps. Air source heat pumps use the typical outdoor condenser many conventional air conditioners use. In fact, many people often mistake a heat pump for an air conditioner. Because air source heat pumps and air source air conditioners look the same. The difference between the heat pump and the air conditioner is the controls and refrigeration circuits. These heat pump components are different than air conditioners, as described above.

Heat Pump Thermostat

reversing valve in a heat pumpIf you have a heat pump then you have a heat pump thermostat. The thermostat controls the heating mode through the reversing valve or the 4-way valve. The heat pump thermostat reverses the refrigeration process when the heat pump thermostat is set to heating mode. When you need a new thermostat it is important to know whether you have an air conditioner or a heat pump. This is because there is a difference between a heat pump thermostat and an air conditioner thermostat. While some thermostat manufacturers make all-purpose thermostats all thermostats are not the same. It is very important to make sure the thermostat is the right thermostat for the application, especially for heat pumps.

High Performance HVAC recommends the Honeywell VisionPro thermostat (Model # RTH8500D) for heat pumps and air conditioners. It is an all-purpose thermostat that will work with both heat pumps and air conditioners. Plus it is easy to use and program if you want to program it. The Honeywell VisionPro can be programmed or used manually according to the users manual settings. There also lots of other heat pump thermostats available that offers lots of features for heat pump control. Many modern thermostats are touchscreen and offer WiFi remote access. It is important when selecting a thermostat for a heat pump that you get a heat pump thermostat and not an air conditioner thermostat.

Heat Pump Defrost Controls - Heat Pump Components

Other controls for the system are the defrost control board. Heat pumps typically operate below the dew point outdoors. When they run below the dew point temperature they collect frost or ice on the condenser coils. For this reason, you need a defrost control to initiate a defrost cycle from time to time. There are different methods used to initiate the defrost mode. The most common method is to time the defrost cycle like a freezer in your kitchen defrosts. Depending on how the installation technician sets the defrost timer will depend on when your system will change to defrost mode.

Sequence for Defrost - Heat Pump Components

The sequence for defrost mode is timed for defrost mode. When the defrost mode initiates, the defrost control shifts the reversing valve to the air conditioning mode. This makes the condenser coils hot so the frost or ice will melt off the condenser. After the defrost time has expired the reversing valve shifts back to its heating position. The system is now a heater and not an air conditioner. When the unit is in defrost mode you want the unit to continue to provide heating. However, how does it do this if it shifted to air conditioning mode when in the defrost mode?

Related link: Learn How Heat Pumps Work. Delve deeper and learn more (opens in a new window)

The defrost control board sends a signal to the indoor air handler unit. Sometimes directly to the air handler unit and sometimes through the heat pump thermostat. This all depends on how the unit is wired and controlled by the manufacturers design and recommendation. The signal the defrost control sends is to activate the heat pump backup heat. The heat pump backup heat engages and counters the cooling effect from the air conditioning cycle.

The Reversing Valve or 4-Way Valve and Metering Device – Heat Pump Components

To separate a heat pump from an air conditioner the heat pump needs a way to reverse the process of refrigeration. This process moves the heat in the opposite direction an air conditioner moves the heat. That is, when in air conditioning mode the heat pump moves the heat outside from inside. When in heating mode it moves the heat inside from the outside. The refrigerant absorbs heat from the air. It then moves the heat in the direction where it is not wanted or where it is wanted. The reversing valve accomplishes this process by reversing the refrigeration flow. The condenser has a metering device inside it. This is there so the refrigerant temperature can be dropped below the temperature of the air outdoors.

When the refrigerant is colder than the temperature of the air outdoors it absorbs this heat. If the temperature outdoors is 45° F. then the refrigerant needs to be 35° F. to absorb enough heat to be effective. The metering device in the condenser causes a pressure drop of the refrigerant which drops the temperature of the refrigerant. This results in the refrigerant being colder than the outdoor air temperature and allows it to absorb heat from air that is 45° F. or colder to approximately 38° F. When the outdoor air temperature is colder than 38° F. the heat pump must struggle to absorb enough heat to be effective for providing heating for indoor comfort factors. This is the tipping point or the point at which the heat pump starts falling behind in its ability to provide heating for comfort factors inside.

The Outdoor Thermostat

Heat Pumps should have a method to turn the condenser off when the unit reaches the tipping point and stops being able to produce effective heat for indoor comfort factors. Many heat pumps are installed without this method to turn it off when it reaches this tipping point of ineffectiveness. There are two methods that can easily turn the heat pump off when it reaches the tipping point described. One is to have an outdoor thermostat that will turn the heat pump condenser off when it reaches the tipping point. The other method is to use an indoor thermostat that reads the outside temperature with a remote temperature sensor.

Related link: Learn more about heat pumps - Heat Pump Sequence of Operation (opens in a new window)

Methods for Control and the Heat Pump Components

One is to have an outdoor thermostat that will turn the heat pump condenser off when it reaches the tipping point. The other method is to use an indoor thermostat that reads the outside temperature and allow the indoor thermostat to control the heat pump condenser based on the outside temperature. The Honeywell VisionPro heat pump thermostat has this ability. Two terminals in the Honeywell VisionPro are used for remote temperature sensing.

It does require running an additional shielded wire to the remote solid state temperature sensor (thermistor) but it is far more reliable and easier to control through the thermostat than it is from an outdoor thermostat hooked up to the heat pump condenser. Plus, if it is set up properly you will be able to visually see the temperature outdoors from you thermostat. The Honeywell VisionPro has in its display a reading for outdoor air temperature if the outdoor air temperature sensor is hooked up.

The real bonus to have the heat pump condenser shut down when it reaches the tipping point is that you will save energy because you do not have the run the heat pump condenser when it is not doing anything. This is why heat pumps are installed in geographical climate zones where the air temperature rarely falls below 38° F.

Related Link: Learn more about HVAC and Heat Pumps – Air Source & Geothermal Heat Pump Heating Systems (opens in a new window)

Heat Pump Components – The Heat Pump Accumulator

Because the heat pump operates in cold whether there is a chance the compressor can eat some liquid when it is operating in cold weather. For this reason, heat pump condensers have accumulators installed in the refrigeration circuit. The accumulator collects liquid refrigerant and allows it to boil into a vapor before it enters the compressor. This prevents liquid slugging of the compressor.

Liquid slugging can damage a heat pump or air conditioner compressor beyond repair. Put simply, liquid does not compress as well as vapor compresses and that is exactly what a compressor does is compress vapor. Heat Pump and air conditioner compressors are designed to compress vapor and not liquid. Heat Pump Manufacturers outfit their heat pumps with accumulators to prevent liquid slugging when the heat pump is operating in cold weather.

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Heat Pump Components

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