A heat pump uses the outdoor unit for both heating and cooling. Often the heat pump also has some sort of backup or auxiliary heat for when it is cold outside. The backup or auxiliary heat could be electric heat strips or a fossil fuel furnace. Thermostats that control heat pumps with backup or auxiliary heat typically have an Em Heat (Emergency heat) position. When the system switch is moved to the Em Ht. position, the auxiliary heat is activated on a call for heat. The compressor is disabled. If replacing a thermostat on a heat pump system, verify your thermostat is compatible.
Honeywell digital thermostats are designed to display the room temperature in a way similar to how people sense temperature. This means the thermostat takes into account not just the ambient air temperature but also the radiant temperature of objects in the room (i.e., wall and furniture). Room thermometers often only read ambient air temperature so they will not match the thermostat reading. Also, Honeywell thermostats do round in the display to the nearest whole number (half number in Celsius) and to the temperature setting. For example, if you have the thermostat set to 72 degrees the system will turn on and off but the inside reading on the thermostat never changes from 72. The actual temperature did fall to 71 or up to 73 and that is what turned on the heating or cooling but the thermostat display will stay at 72 to avoid jumping up and down constantly.
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