Heat Pump Thermostat Operation

 
Operation of a Heat Pump HVAC system may be a little unfamiliar to those of you that are custom to other forms of heat were gas and oil utilities are available.
 
A Heat Pump is an air conditioner that can operate in reverse in the wintertime.
 
Instead of moving heat from the inside of the house and discharging it to the exterior, a Heat Pump can also remove heat from the outside air and discharge it into the house. This process is performed by rather complex and sophisticated control devices, one of which is the thermostat.
 
You will notice on a heat pump thermostat that there are extra settings that you can select from. The one we will discuss here is the mode setting "EM heat or Aux heat".
 
 
A heat pump has  two individual heating systems combined into one. They can work together or individually. When outdoor weather conditions become very cold, there is less heat in the air for the heat pump to absorb and the rate of heat loss through your house increases because of the temperature differential. This requires more heat to maintain the setpoint temperature.
 
The thermostat monitors this condition and if the setpoint temperature cannot be maintained within 2° Fahrenheit, an electric heater (similar to your toaster) will be turned on to raise the temperature backup to the setpoint temperature and will then shut off. The indoor fan never stops blowing . When this happens people often complain of uncomfortable conditions between on/off cycles of the electric heater.
 
Another operation of the heat pump is to remove ice on the outside HVAC unit which forms due to the cold air temperatures and the  coil temperature which is significantly colder than the outdoor air. Every 45 minutes a computer board checks a sensor to see if ice is present. If it is, the Heat Pump goes into a "defrost mode", at which time the electric heater is turned on and the Heat Pump is switched into Air Conditioning Mode. This absorbs heat from inside the house to melt the ice on the outside coil. As you might guess, this takes a lot of heat that you had to produce to melt the ice outdoors (it's like leaving your window open for five minutes).
 
The situation primarily becomes an issue when the outdoor air temperatures get extremely cold. Under these conditions where there is insufficient amounts of heat in the air to effectively operate the Heat Pump and you actually begin to lose money because of the air-conditioning cycle which occurs every 45 minutes.
 
The mode selection switch on the thermostat can be turned to the emergency/auxiliary heat position when these conditions exist. This changes the way the heat pump operates. 
 
It turns off the refrigeration compressor system so it does not continually run 100% of the time causing cold drafty conditions in the house. 
 
When the electric heaters bring the room temperature up to the setpoint temperature, everything will turn off (no cold blowing air from the heat pump).
 
It does not freeze the outside coil so there is no need for a defrost cycle to occur every 45 minutes. 
 
Is a known fact by many that operating the electric heat by itself is expensive. But the reality is that this auxiliary heat is already turning on and off most of the time, whether you realize it or not. A Heat Pump will save you money throughout most of the heating season but when things get extreme outdoors you can save money by switching off the Heat Pump until the weather changes.
 
When should you turn off the Heat Pump? 
You'll notice that when outdoor temperatures fall, there comes a point when the Heat Pump never turns off. This is the "balance point" for your particular building. It is when the amount of heat that can be produced by the Heat Pump is equal to or less than the amount of heat that passes through the building envelope. This is when the indoor air temperature will gradually fall 2° and the electric heat will be turned on.
 
 
 Another indicator is when it is raining and ice is forming on your car or other objects outdoors. Under these weather conditions the outdoor HVAC coil rapidly freezes solid and restricts air flow through the coil. Heat cannot be absorbed from the air if the air does not flow through the coil, so the heat pump runs for 40 minutes without producing any heat. It then turns on the air conditioner and you pull heat out of the house to melt the ice. The ice doesn't stay melted for more than five minutes and the process repeats itself.