Converting an Air Conditioner to a Heat Pump
Can I convert my air conditioner to a heat pump?
Yes you can do this if you:

I get this question a lot in my emails and wanted to address it. The time and materials it would take to change an air conditioner into a heat pump is not worth it. You would be better off having a new heat pump installed replacing the old air conditioning unit. For me, or another HVAC technician, this would be a pet project for teaching students how to braze for refrigeration and wire heat pump controls for the proper sequence of operation. It would be a good lesson in metering devices for air conditioners and heat pumps. It would also be a good lesson in finding all the appropriate parts for the size and model number of the air conditioner equipment.

Converting an Air Conditioner to a Heat Pump - The Control Wiring

Converting an Air Conditioner to a Heat PumpConverting an Air Conditioner to a Heat Pump - I would start with the condenser. I would completely gut all the piping which connects the compressor to the coils. Then refit the new piping to account for the metering device, accumulator, and reversing valve. After I got all that sorted out I would clean all the piping and joints. Then instruct the students to the fragility of using too much heat when soldering or brazing in new metering devices and a new reversing valve.

Some the parts in the reversing valve are Teflon and the metering device has Teflon in it also. Too much heat on Teflon and you cause problems for the devices, whether it be the reversing valve or the metering device, to malfunction before it is even put into service for the first time. Using wet rags and maybe heat sink paste I would carefully solder or braze the new parts and piping into the old air conditioner.

Brazing and Leak Tests

After the brazing is complete and the piping and parts cooled off I would perform a leak check. This is done to make sure we were finished with the soldering of the old air conditioner which now the new heat pump as far as piping is concerned.

No leaks and we would move on to wiring the condenser for defrost control. After attaching the bulb, for the new metering device to the appropriate pipe, for heat pump refrigerant metering. Next, I would install the new heat pump bi-flow filter drier.

Wiring the Controls

This is a little complex to explain in words and without diagrams. However, we would gut all the control wiring in the old air conditioner to make way for the new heat pump control wiring. The reversing valve and sensors would be hooked up to the new parts we add to the system. I would use a new solid state defrost circuit board for defrost control and hopefully there are a few spare wires running to the air handler and then to the thermostat so we can hook up defrost backup heat and the reversing valve control in the thermostat.

The condenser fan motor would now be controlled by the defrost board and not the compressor contactor and the line voltage wiring would have to be modified to make this happen. It’s a simple jumper wire from the line side of the compressor contactor to the new defrost control board and then moving the wire for fan power over to the board.

After double checking all the wiring and making all is correct and the wiring is tightly connected we would move on to the air handler. If there are no spare wires running to the air handler from the condenser we would have to pull new thermostat wire from the condenser to the air handler.

Converting an Air Conditioner to a Heat Pump - Defrost and Back-up Heat Considerations

The next part is a little tricky depending on what type of heating system you have. If you had a straight air conditioner with electric heat then we are good to go but if you have a gas furnace we’ll have to modify a few things to make this work efficiently. On a call for defrost which originates at the defrost control board in the heat pump condensing unit we need to make the heat turn because when the heat pump condenser switches to defrost it changes itself into an air conditioner.

This means if you don’t have a secondary source of heat cold air will blow out the supply vents and we want to prevent cold air from coming out of the supply vents so we need a good backup source of heat which with most heat pumps is electric. We will energize the heating source from the heat pump condenser and not the thermostat because it is the defrost control board in the heat pump condenser which initiates defrost.

How It Works

As soon as the defrost cycle is finished the secondary or backup heat shuts down. That is unless the thermostat is falling behind. If the thermostat temperature has fallen behind more than 3° the secondary heat remains energized. This is done to help the heat pump unit catch up. When the thermostat comes within 3° of the manual set point on the thermostat the secondary or backup heat de-energizes. Hopefully, the thermostat satisfies shortly thereafter and everything shuts down.
After taking care of all the controls wiring to make this work properly I would install the metering device. It needs to have a bypass for the evaporator coil and condenser. Then perform a leak check to ensure there were no leaks after brazing in the new metering device. To handle the secondary heating we need to install a heat pump thermostat. This is done because the old thermostat that controlled the air conditioner and will not work in this case.

Converting an Air Conditioner to a Heat Pump - Final Testing

Before moving on to the next step I would get my vacuum pump and nitrogen tanks. I need these to begin the process of performing a triple evacuation for the refrigeration system. Next, I would install the thermostat. Hopefully, there were extra wires there coming from the air handler. I need the extra wires so I could make the reversing valve connection. Additionally, any other connections necessary for heat pump staging. Staging depends on the number of stages of heat I had in the air handler.

After that then it’s back to the triple evacuation and nitrogen charge into the refrigeration system. After this was completed to my satisfaction I would charge the system with refrigerant. Then test everything including the refrigerant for proper charge. I would run the new heat pump system through a series of test. This is to ensure it will do what I want it to do when I want it to do it. Using a refrigerant pressure temperature chart or PT chart I would make sure the system has the correct charge using the sub-cooling method.

Converting an Air Conditioner to a Heat Pump - Conclusion

This is what it would take to change your air conditioner over to a heat pump. I would spend at least a weekend doing this not counting the time it took to find all the right parts to make this modification. If you do not have electric heat but need electric heat for backup heat an electrical permit will be needed. This is needed for the new branch circuit for the electric heating. Hopefully, there is room enough in the circuit breaker panel to accommodate the new branch circuit. That is without overloading the panel based on the NEC code. A refrigeration license is also necessary to purchase new refrigerant if so needed.

Converting an Air Conditioner to a Heat Pump - Summary

I don’t mean to make this sound complicated. However, this is what it would take to convert your air conditioner over to a heat pump. This is why I am telling you that you would be better off having a new heat pump unit installed. That is versus having the air conditioner converted into a heat pump. To change an air conditioning unit into a heat pump will take more than what is worth. You can buy a new heat pump with less hassle and cost. Plus with an air conditioner to heat pump conversion, you will only get a one year warranty on the new heat pump parts. Whereas with a new heat pump some manufacturers offer up to a ten-year warranty.

  1. Install a reversing valve, accumulator, metering devices, heat pump thermostat, and defrost controls in the condensing unit. I would buy a thermostat with an outside temperature sensor and heat pump ambient temperature control. This is so I could program the thermostat to shut the outside heat pump condenser unit off when the outside air temperature fell below 38° Fahrenheit. Heat pump condensers become inefficient below this temperature.
  2. Install electric heat strips in the air handler for backup heat or if you have a gas furnace change the wiring for heat pump heat staging control.
  3. Install a bypass on the metering device which now serves as your air conditioning metering device.
  4. Lest I forget to change the filter drier from a mono-flow filter drier to a bi-flow filter drier.

Additional external resources concerning heat pumps can be found here.

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