Lowering the Cost of Heating – Converting a Window A/C to a Heat Pump

November 27, 2008

Lowering the Cost of Heating – Converting a Window A/C to a Heat Pump
~James Dunn


Do NOT attempt to act upon or disseminate any portion of the following unless you read the full content of the Disclaimer linked herein: Click Here

Using a Window Air Conditioner as a Heat Pump to Lower Energy Costs

Safety Precautions (partial list): Ensure your installation and environmental conditions will not damage the air conditioner and living space structure, shock and fire hazards due to workmanship, electrical properties of components must be compatible

Currently, people use window air conditioners to cool their living space during the summer months. During the winter months they use traditional heating systems like electric heating elements, gas burners, oil fired boilers…

Often, the most efficient heating of choice is the electric heat pump. What is a heat pump? A heat pump is an air conditioner that is turned around and pushing heat into the living space, and has the thermostat inside the living space. A heat pump is only efficient above 40 degrees Fahrenheit; shutting off the heat pump and providing an alternative source of supplemental heating is required for temperatures below 40F.

An air conditioner captures the heat from the air and the heat from the refrigeration pump and shoves that heat out through the condenser. Turn the air conditioner around and now the air conditioner is taking the heat from outside plus the heat from the refrigeration pump and shoving it into the living space. You get much more heat doing this per watt of power used than by any other commonly available commercial method.

So the concept is to turn the window air conditioner around so as to transfer the heat from outside and push it into the living space to supplement traditional heating systems, and mount a thermostat inside the living space to automatically control the temperature during the winter months. This method would not work well in Northern States, but should work very effectively in Southern States.

A weather-proof awning would need to go over the air conditioner to protect the controls; since they will now be out in the weather. On some window air conditioners there is an internal vent, this should be closed to prevent losses of heat from inside the living space. The controls will be turned to the lowest setting or jumpered out so as to run continuously. The plug inside the house can not be manually plugged in and unplugged as heat is desired; because the compressor will try to start as soon as you try to plug the unit in. This will damage the plug outlet over time and may cause the plug to get hot enough to cause a fire. Instead, make a small thermostatically controlled relay to install between the air conditioner plug and the power source.

You can use your present heating system to turn your “heat pump” on and off by installing a power relay off of your heater fan control relay coil (usually 24 Vac, check your schematic). When your heater fan comes on, it remotely pulls in your power relay (mounted in your circuit breaker panel) to energize the outlet circuit that feeds your air conditioner. This allows the heat to circulate throughout the entire house. This gets rid of all of the parts needed except the power relay (~$15) , some low voltage wire, and some terminations.

However, if you need an independent system completely separate from the your normal heating and cooling system, maybe for your garage or storage shed, the following describes how to make an external thermostat for your “heat pump”.

There are several sources for getting a thermostat to control your heat pump/air conditioner as described herein.

* salvage the thermostat from your air conditioner
* salvage the thermostat from an old air conditioner
* get an old junked out refrigerator from an apartment complex maintenance man
* salvage the thermostat from an old space heater
* purchase a new thermostat from an appliance parts suppiler

* While salvaging the thermostat, salvage the power relays as well. They might not be useful, but one never knows.

You might want to salvage a thermostat from an old air conditioner or refrigerator, so you don’t destroy your air conditioner; while you find out if the following is going to work.

If your window air conditioner uses 220Vac instead of the 110Vac commonly available at household outlets, you may need to find a thermostat from an old 220Vac air conditioner; make sure you salvage the power cord and the 220Vac plug.

Assuming the refrigerator is someplace around 70F and unplugged, listen for the contacts to click as you rotate the thermostat to warmer settings. If it doesn’t, it may be incapable of controlling at room temperatures. No use having to dispose of a refrigerator if you can’t use the thermostat. Find another junked out refrigerator to use.

When you find a workable thermostat, salvage the thermostat along with the terminations and the attached wires, and the power cord & plug. Most thermostats of this type are designed to start and stop the refrigeration pump without the need for an additional relay, however, you are going to be using the thermostat for heating, not cooling.

Wire the thermostats made for cooling to a normally-closed power relay.
Wire the thermostats made for heating to a normally-open power relay.

When the room temperature drops and the air conditioner thermostat contacts open, the normally-closed power relay will start the heat pump to warm the living space.

When the room temperature drops and the space heater thermostat contacts close, the normally-open power relay will start the heat pump to warm the living space.

Air conditioner/refrigerator thermostats open the contacts as it gets colder.
Heater thermostats close the contacts as it gets colder.

To build the 110 Vac thermostat (if you have a 220 Vac, email me and I’ll post instructions):

Get the following from Lowes or some similar electrical supply (about $30 in parts, less if you can salvage parts)

* 1 – electrical outlet (< $2)
* 1 – power relay (<$15)
o normally-closed if using an a/c thermostat
o normally-open if using a heating thermostat
o with contacts of sufficient voltage and current to run compressor
o with a coil rating of 110 Vac
* 1 – triple outlet electrical box, metal preferred ( < $6)
o like what you would use for having 3 outlets side by side in the bathroom or kitchen
o ensure the thermostat you salvaged and power relay will fit in the box with the above outlet before you buy the electrical box
* 1 – strain relief for the cord (< $1)
* 1 – metal cover plate for 1 outlet and 2 switches (< $4)

1. Cut off 12″ of the corded plug wire to use for wiring the thermostat and relay. Ensure the insulation is in good condition.
2. Feed the cord with the plug through the strain relief and attach the neutral lead to the silver terminal of the outlet.
3. Wire from the silver terminal of the outlet to the neutral side of the 110 Vac relay coil.
4. Wire the line side of the plug wire to the line side of the thermostat.
5. Wire the control side of the thermostat to the line side of the relay coil.
6. Create an opening in the electrical box cover plate to allow mounting the thermostat in the cover plate.
1. Ensure you leave enough room for the outlet and the coiled tubing for the temperature bulb when installed in the box.
2. Mount the thermostat temperature bulb outside of the electrical box
1. ensure you do not kink the tubing
8. Screw in the outlet into the electrical box.
9. Secure the cover to the electrical box.
1. You may need to pull the cord out as you put on the cover to give enough room inside the box.
10. Secure the strain relief.

To test your work:

1. Plug in a lamp to test the circuits.
1. check to see if the lamp works before starting
2. With the lamp turned on, dial the thermostat up and down until the lamp turns on and off.
3. Repeat by hooking the unit between the wall outlet and your window air conditioner.
1. After stopping an air conditioner, let it rest for 5 minutes before you attempt to restart the unit.

Hang your thermostatically controlled outlet on the wall adjacent to your air conditioner about 4 to 5 feet off the floor and 12″ from the window if cord length allows.

This type of external thermostat is also useful in the summer time when you turn the air conditioner around to cool the house. Because normally the fan must run all the time in window air conditioners so the internal thermostat can sense room temperature. With a modification of the new wall mounted thermostat you made, the fan will only have to run when your external thermostat energizes the air conditioner.

There is a limitation. The outside air can not be less than 20F for the system to work effectively. In this case you would have to rely totally upon your present heating system. One such air conditioning unit will probably not heat your entire living space, though it may.

You can buy a heat pump for this purpose, but it is significantly more than a standard air conditioner. You can buy a new small window air conditioner without a thermostat for less than $200. Or find a used one for much less than that.

If you do not want to be flipping the window air conditioner around every season, and you have two windows in the same living space, set one up for air conditioning and the other for heating. You only need the external thermostat for the heat pump.

General tips to save on heating and air conditioning bills

* Keep in mind, the fewer living spaces you have to heat/cool, the lower the power bills
o don’t heat/cool rooms that you are not using
+ close the doors and windows
+ shut the forced air vents, or put a rug / book over them
o but make sure you have at least 2 vents open
o make sure there is a path for the air to flow from the vent to the return
* Keep the thermostat turned down for heaters /turned up for air conditioners so as to minimize the energy usage.
* Change your air filters once a month with the cheapest air filter you can find
o the cheaper the filter the less air restriction there will likely be
o maximize the air flow, but do NOT run an air conditioner without an air filter or it will damage the air conditioner
o a really dirty air filter can cost you over $500 in one month
* If you are only living in one room of your house, even if you don’t heat that room with the above heat pump, use a small space heater and leave your central heating and air conditioner shut off.


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