Thermostat issues..I think

Understanding a Heat Pump

A heat pump absorbs heat energy from the outdoor air and rejects it into your home at a higher temperature level. Basically, it's an air conditioner in reverse.



As the outdoor temp. decreases, so does the amount of heat energy that the heat pump can pick up and put into the house. This is why the air coming out of your registers gets cooler as the outside temp. drops. Conversely, as the temp goes down outside, the heating load increases for the house. The temp at which these two roads meet is called the "balance point". The capacity of the heat pump is going down while the demand is increasing. On average, this happens somewhere around 30 - 35 deg. outside (every house is different). If the outdoor temp. drops below this point, the heat pump cannot sufficiently heat the house by itself. That is why some form of auxiliary heat is needed. In most cases it's electric resistance elements that are installed in the air handler.



Now, here is how this is usually controlled. Your thermostat has two stages. The first brings on the heat pump. The second brings on the heat elements. If your stat is set at 72, the first stage brings the heat pump on when room temp. drops to 71. If the outside temp is below the balance point, the heat pump is going to begin to loose the battle and the room temp is going to continue to fall. At 70, the second stage of the stat will bring on the heat elements and the heat output will increase until the second stage of the stat is satisfied and the elements go off. We're back to 71, but the stat is still set at 72 and the heat pump continues to run because the room never made it to 72 to satisfy the first stage and turn the heat pump off. Because it's still very cold outside (below the balance point), the heat pump will begin to loose the battle again and the process repeats itself until the outside temp rises above the balance point and then the heat pump can do the job alone.



Most newer digital thermostats have a feature that will run the auxiliary heat until both stages are satisfied and thus the heat pump will shut off briefly when it's below balance point temp outside. A mechanical thermostat does not have this feature.



Another thing to be aware of is that heat pumps have a defrost function to clear frost off the outdoor coil. The unit can go into defrost as much as twice an hour. When it does the outside fan will stop and water vapor (steam) will rise from the unit. When defrosting completes, the fan will resume, a large puff of steam will come from the unit, followed by a "whooshing" sound. This is all normal, but can be very alarming if you didn't know that it's supposed to work that way. :-)