Please Allow me to spend a few moments explaining heat pump operation. The heat pump is basically a compressor in which the refrigerant can change directions. In a traditional air conditioner, you have a compressor outside an outdoor condensing coil, and an indoor evaporator coil. The condenser coil takes the hot compressed gas and condenses it into a liquid refrigerant and sends it to the indoor coil and the indoor coil boils off that refrigerant and makes cold air. Now with a heat pump, we simply call it indoor coil an outdoor coil because the outdoor unit has a reversing valve inside of it that changes the refrigerant flow so it makes either coil be either a condenser or an evaporator.
Now you have an orange wire on O/B and that’s the wire that controls that reversing valve. Some systems the reversing valve when not energized will be in cooling mode, and some systems when the valve is not energized, default to heating mode. Now on a heat pump, there is a defrost control board on the outdoor unit with a temperature sensor on the coil to detect when ice is building up. So when the heat pump is in heating mode, the outdoor coil is where we reject the cold air. Therefore it tends to ice up. The defrost board will detect the ice and have the heat pump reversing valve switch so to take the heat from inside the house and put it in the outdoor coil to melt the ice. When that happens, the heat pump will typically tell the indoor unit to either turn the blower off, or it will activate auxiliary heat to heat the air.
out sometimes contractors will wire the emergency heat to the auxiliary heat strips. The reason for this is that if the heat pump fails during the winter, the thermostat could turn on emergency heat and that will make the heating coils come on and the blower stay on and not energize the outdoor unit. I would have to see how your indoor unit is wired to see if they’ve put both the auxiliary and the emergency heat on the same connector. Normally, when you have a auxiliary and emergency heat, you would have a gas or oil burner on the indoor unit, with electric heat strips has a emergency back up. It does not have to be done that way, but when it is done, that way is called a dual fuel system. The heat pump is is one fuel, electricity, basically, and then the gas furnace or oil. Furnace is the second fuel tight.
I hope this explains a little bit about how a heat pump operates for you. In order to use multiple stage heating by the way, you have to use a nest learning thermostat. The basic nest thermostat cannot support multiple heating modes.
Ken, The AC Cooling Wizard
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
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