On a recent Boise a/c repair I was asked. What is a heat pump and how does it work? Is a heat pump right for me? A heat pump is an air conditioner that will do both heating and cooling from the same unit. To understand how a heat pump works you have to understand how an air conditioner uses Freon to move heat around from one place to another. The compressor in the a/c unit takes Freon in a gas state and pumps it through a coil outside that has a fan on it. The air from the fan changes the hot gas from a gas to a liquid. That liquid Freon is then pumped toward another coil and thru a small hole called a metering device. At the metering device the high pressure liquid passes thru and a portion of that liquid picks up heat by boiling back into a gas. This process cools the remaining liquid which is then passed thru the coil removing heat from your home by boiling off the remaining liquid as it absorbs the heat from the air passing through it. At this point the (mostly cool gas) heads back to the compressor to start the job over again. On a typical day where the outside temp is 90 degrees and your home is 75 degrees, the air coming from your cold coil (inside) would be about 55 degrees and the air coming from the hot coil (outside) would be about 120 degrees. These temperatures will vary as the outside and interior temperatures change. Freon does not go bad under normal use and never wears out. Think of it as water boiling to steam as you heat it and then turning back to water as it cools again.
So a heat pump then, has what is called a reversing valve that simply reverses the flow of the Freon so that the outside coil becomes the cold coil and the inside coil becomes the hot coil thus removing heat from outside and returning it to you inside. So it stands to figure then that the warmer it is outside to more heat a heat pump will return to your home and the more efficient it will work. On most heat pumps you will also have (back-up) heating elements. These heaters are electric coils that heat up as electricity passes through them (like a space heater or electric stove element). These (heat strips) are used in several ways. When set up correctly they add extra heat to your home when the heat pump cannot do the job alone, they can be used instead of the heat pump when the temperatures outside are too cold for the heat pump to work well, and they provide heat to warm the air during a defrost cycle.
Good question “what is a defrost cycle”? When the temperatures are very cold outside, that cold coil we spoke of is below 32 degrees and will start to form ice on it from the surrounding humidity in the air. Too much ice means low heat transfer, so we need a way to remove the ice so we can get back to heating correctly again. This is called the defrost cycle. In defrost the reversing valve changes back to cooling mode (causing the outside coil to get hot) it also shuts down the fan motor so the coil gets hot faster to melt the ice in the fastest time possible. During defrost the unit sends a signal to the heat strips to turn them on so that the air entering the house is not so cold. A typical defrost will happen about every 30 to 60 min. of running time and last for between 1 and 15 min. Your unit knows when to go into and out of defrost by using a sensor, most sensors measure the coil temperature and will start defrost when the coil is below 30 degrees and end it when it reaches 60 degrees.
For this Boise a/c repair a heat pump was the correct choice. The only other option to her was propane which for our area is very expensive. Other important factors are how cold does it get where you are? Heat pumps are better in areas where the temps stay above 25 in the winter such as Las Vegas or Phoenix. In colder areas such as Boise, often times natural gas heating is a much smarter move.