High Performance HVAC Air Conditioning & Heating: Air Conditioner Conversion to Heat Pump

Converting an Air Conditioner to a Heat Pump? Is this Possible?

Can I convert my air conditioner to a heat pump?

Yes you can do this if you:

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 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 back-up 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.

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 and wire heat pump controls. 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.

I would start with the condenser and completely gut all the piping which connects the compressor to the coils and 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 and 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. After the brazing is complete and the piping and parts cooled off I would perform a leak check 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 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.

This is a little complex to explain in words and without diagrams but 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. 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 back-up 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.

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 back up 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. As soon as the defrost cycle is finished this secondary or back-up heat shuts down unless the thermostat is falling behind. If the thermostat temperature has fallen behind more than 3° the secondary heat remains energized to help the heat pump unit catch up. When the thermostat comes with in 3° of the manual set point on the thermostat the secondary or back-up heat de-energizes and 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 with a bypass for the evaporator coil and perform a leak check to ensure there were no leaks after brazing in the new metering device.

Before moving on to the next step I would get my vacuum pump and nitrogen tanks and begin the process of performing a triple evacuation for the refrigeration system. Next, I would install the thermostat and hopefully there were extra wires there coming from the air handler so I could make the reversing valve connection and any other connections necessary for heat pump staging depending on the number of stages of heat I had. After that then it's back to the triple evacuation and nitrogen charges into the refrigeration system. After this was completed to my satisfaction I would charge the system with refrigerant and then test everything including the refrigerant for proper charge. I would the new heat pump system through a series of test to ensure it will do what I want it to do when I want it to do it.

This is what it would take to change your air conditioner over to a heat pump and 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 back-up heat an electrical permit will be needed for the new branch circuit for the electric heating and hopefully there is room enough in the circuit breaker panel to accommodate the new branch circuit without overloading the panel.

I don't mean to make this sound complicated but 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 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 to just buy a new heat pump. 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.

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landlord
written by kay ryon, May 01, 2010

Hi, I have many units and am trying to save the best I can. At the moment I have found a heat pump but my unit previously used a straight cool. Can you use a heat pump with the original air handler and how would you connect if? Would the heat still come from the heat strips from the air handler? thanks, k

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Point I didn't see
written by Brian S., June 03, 2010

Richard, you probably didn't mention it because it's so basic - but another important point is that heat pumps can't be installed directly on the ground. We just dealt with one here in Indianapolis where a two year old system died because of this installation flaw. Great products die quickly when installed incorrectly.



Thanks for your insights!

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Heat and cool
written by Braxton, August 23, 2010

I want to install heating and aircon in my house. I have tried getting qoutes from about 6 companies for geothermal but only one got back to me and the price tag was as much as my house was worth. I want to go green and it has put me off so i thought maybe i just do it my self. Is it possible to run split systems as you normally would and feed them into a ground loop. Or how do i do this so my power bill does not go through the roof. Any suggestion, i am in Australia and we are way behind in all of this. So i want the most economical system electric (do not have gas) split systems are cheap now so is that the way to go and can i modify them to work with a ground loop or a heat pump. Open to ideas

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Aussy Split System
written by Richard, August 24, 2010

The reason geothermal is expensive is because you need a water source for geothermal systems to work and for many people this means you have to drill a (closed loop preferable) well that will accommodate the tonnage needed to heat and cool the house or dwelling where the geothermal will be installed. There are comparable split system air to air models that come close to the efficiency of geothermal and these systems definitely cost more to heat and cool the home or business because these split system heat pumps (air to air) are more complex than the average air to air heat pump system is. Invertor duty mini-splits are comparable in SEER and HSPF ratings for heat pumps. There is a few manuafacturers that make invertor duty heat pumps and air conditioner split systems that have very high efficiency ratings comparable to geothermal units. Invertor duty means the unit will modulate from on demand from low load condition to peak load conditions and even above average load conditions when necessary. In layman's terms this means you will only use the energy needed to satisfy the load or conditions.

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