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Central Air Pool Heater

Babies don’t have a fully developed temperature regulation system and lose a lot of body heat to the water, most people recommend a pool temp of 87^ to 92^. I started using my 400,000 BTU pool heater to keep the pool in that range, but found that heating 22,610 gallons of water was not a low cost thing to do!

Since I live in Houston, TX and like to keep my house at about 73^, my central air systems stay on quite a bit throughout the day. That sounded like a lot of heat that was being pulled out of my house and blown into the air every day. I wondered if I could use that heat to warm up my pool!

First lets look at how a basic air conditioner works:


It is actually quite simple system composed of a hot high pressure side and a cold low pressure side. The compressor is the first division point between the high and low pressure sides of the system and is used to compress the refrigerant (in my case R22). The hot gas enters a condenser, a big copper coil with a fan that blows air through it. The air cools down the gas causing it to condense into a liquid by the time it leaves the condenser. The liquid runs through a small pipe into the evaporator unit where it enters the refrigerant control device. The control devices limits the amount of refrigerant that enters the evaporator and is the second barrier between the high pressure and low pressure. As the liquid refrigerant moves to the low pressure side it evaporates taking up heat from the air moving over the evaporator coils. Finally, the refrigerant is sucked up as low temp gas into the compressor to start the cycle all over again. – For more information check out “Heating and Cooling Essentials” ISBN 1-56637-433-2.

Some key components:

One of the first components I needed that I did not have was a heat exchanger. In my setup it would act like a condenser, but rather then using air to cool it would use water from my pool to cool. I selected a CXN-S 250 from Doucette Industries.

CXN-S 250

CXN-S 250

When opening a HVAC system you always want to replace the filter / dryer, this thing gets rid of any junk you may have in your system and more even more important removes small traces of water that may be left in the system.

Parker 053S Filter Dryer

Parker 053S Filter Dryer

In early experiments working on this concept I quickly found the need for some valves. The can get a little expensive because you have to buy a valve body and then a control coil that typically operates at 24 Volts AC. One hard lesson I learned about HVAC valves is that just because they are off does NOT mean that fluid or gas can not flow through them, HVAC valves are directional the IN side needs to be the higher pressure side. If you design a system where the OUT side of the value can have a higher pressure you need to insert a check valve to keep the back pressure from pushing the wrong way through the value.

Parker R20E84 Solenoid Valve

Parker R20E84 Solenoid Valve

I always add a site glass to any HVAC system I work on. Technically they are not needed, but they make it very easy when your charging a system by letting you know when all the bubbles are out of the liquid line going to the evaporator.

Parker Site Glass

Parker Site Glass


With House Cool Mode, hot gas leaves the compressor runs through the reversing value into the condenser where it condenses into a liquid. Valve #1 is ON so liquid is able to leave the outside unit and run through the filter dryer and site glass into the evaporator upstairs in the house where the liquid flashes into a gas as it passes through the expansion valve and absorbs heat from the air passing through the evaporator. The cold gas travels downstairs and outside to the compressor and the cycle starts all over again.


In Pool Heat Mode the reversing value is ON, allowing the hot gas leaving the compressor to be sent to the pool heat exchanger / condenser. Valve #2 is on, so liquid leaving the condenser is able to pass through the valve around the evaporator loop as in above. In order to save electricity the fan to the normal condenser is powered off.


Pool Cool Mode is a new addition that lets me cool the pool at night when the system is not busy cooling my house. Valve #3 allows liquid to enter a constant pressure expansion valve that flashes refrigerant that absorbs heat from the pool via the heat exchanger.


Some have have asked why I use a reversing valve at all. Originally I just used a few valves, but found that the unused condenser filled with refrigerant requiring a huge amount of refrigerant to be charged into the system. With the valve, the unused parts of the system are pumped down via the suction line.

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