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How Your Heat Pump Functions
By:New Jersey Home Inspector Michael Del Greco
How Your Heat Pump Functions
Submitted by: Michael Del Greco, New Jersey Home Inspector Lic GI 0121, President of Accurate Inspections, Inc, a New Jersey home inspection company
By learning how the systems in your home operate, you can often troubleshoot problems when they develop. At the very least, you will be able to talk intelligently with a contractor who is brought in to correct a problem. Showing that you "know your stuff" can gain you respect and possibly better service.
A heat pump serves two functions: to heat and cool your home. A central air system simply cools the house. Air conditioning units operate the same way the heat pump's cooling side works. Here's how your heat pump operates:
With your heat pump unit in the air conditioning mode, the compressor compresses Freon gas. (It does this in both the heating and cooling mode.) When you compress gas, as with anything, you develop heat. You get a high pressure gas that is very hot -- typically between 190-240 degrees. The hot gas gets pushed into the outside coil. The outside coil's fan, which is on, is drawing the outside air across the coil. The 90 degree outside air is significantly cooler than the gas inside the coil. This is why you feel warm air coming off an air conditioning unit.
The cooling of the gas causes a change in state. The hot gas converts into a warm liquid, one of the unique properties of Freon. The warm liquid continues through the final portion of the coil and moves inside to the evaporator, which creates space for the liquid to expand. The liquid expands into a low-pressure gas, which is now cold. The cold gas goes through the "A coil," which is inside the plenum of the heating system ductwork. The cold gas makes the coil cold.
The cooling into the house occurs from drawing the warm house air across the coil. The velocity and volume of air across the coil dictates the temperature of the air on the other side of the coil. The technician will set the fan speed up so you get a 15-17 degree differential between the supply and return air.
The cold gas moves through the coil and is pushed back into the suction line to the outside unit, which draws the cold gas back to the compressor where the whole process starts over.
In the heating mode, the compressor does the same thing. However, instead of pushing the hot gas to the outside coil, it pushes it to the inside coil. Once into the inside coil, a similar process happens. Hot gas moves around the coil. The air from the house takes the heat off the coil, which is desirable, but while it does this it drops the temperature of the gas. When it does this enough, the high pressure gas changes into a liquid. This continues through the coil, then through the expansion valve which allows the liquid to change to a low-pressure cold gas. This cold gas is taken to the outside coil -- the opposite of the AC mode -- but simply by moving gas/liquid the other way. Once it goes through the outside coil, it ends up back at the compressor and then starts the process over again. In either mode, the compressor simply compresses the Freon gas.
There is one more important point when the system is in the heating mode and the cold gas comes to the outside coil. If the temperature is low enough, you may start to freeze the outside coil. When this happens, you will develop ice. When you start to form ice, the unit will automatically change to a defrost cycle. An outside thermostat controls this function. The defrost cycle will reverse or change the function back to the AC mode and defrost that outside coil.
Note: While defrosting the outside coil, you will have back-up heat engaged on the inside. The back-up heat could be electric or a furnace operation. This will be dictated by the electric and fossil-fuel costs.
When the temperature drops too low and too frequently, this situation becomes economically impractical and the compressor will shut down and will heat with the back-up process.
Information provided by Michael Del Greco, New Jersey Home Inspector Lic. GI 0121, American Society of Home Inspectors Member 102273, Pesident of Accurate Inspections, Inc. A West Paterson New Jersey Home Inspection firm.
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