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Residential Thermostat Calling or Switching for Heating

The first thing that must happen is that the heat pump control thermostat must provide a voltage signal to the indoor air handler or electric unit. The thermostat can be most understood as simply a switch that turns a low voltage 24 volt power on and off to the respective terminals.

In the case of the heat pump thermostat calling for heating or cooling the thermostat switches on the power from the “R” terminal which is the 24 volt power source to the “Y” terminal on the thermostat which is in essence the call for the outdoor unit compressor to run. These terminals are located on the back or on the wall mounted face plate of the thermostat.

The heat pump thermostat determines the mode of operation, cooling or heating, by switching the power from the "R" terminal to the reversing valve located in the outdoor unit, this terminal on the heat pump thermostat is most often the "O" or "B" or "O/B" terminal.

Most heat pump thermostats also have either an emergency heat or second stage heating terminal as well, this controls when the back up heat turns on and the thermostat switches the power from the "R" terminal to the "E" terminal for the back up emergency heat, this is usually only active when the thermostat emergency heat switch or mode is selected to be on manually. The second stage or back up heat source, which is basically the same as the emergency heat except that it is automatic and does not require the operator (that's you) to manually turn it on. this is normally accomplished by the thermostat switching power from the "R" terminal to the "AUX" or "W2" terminal.

All the wires connected to the thermostat will usually always go directly to the air handler or electric furnace and if wired correctly most of the specific terminals that the wires are connected to will match at both the thermostat and at the furnace. For instance the wire connected to the “Y” terminal should be the same wire at both the thermostat and the air handler or electric furnace heating system.  

One important thing to note is that all electrical circuits must have both a positive and negative side to complete the circuit, much like a battery has both a plus ” + “  and a minus ” – “ side. Without both the positive and negative sides this electrical circuit will not be complete and will not function.

Most digital and programmable thermostats, thermostats with an LCD display of some type, will have both the positive and negative. Some other types are either battery powered or are not digital but are simply some sort of a mechanical switch and do not have the negative circuit at the thermostat. Regardless the negative side of the circuit at the thermostat is normally used only to power the thermostat itself.

For the purpose of understanding where the positive and negative are located on the thermostat and on the furnace in most if not all cases the positive side is always on the “R” “RH” or “RC” terminal and the negative side is almost always on the “C” terminal.  The “RH” is usually designated as the heating power source and the “RC” is usually designated as the cooling power source and in most cases these are connected together. Most dedicated heat pump thermostats do not have the "RH" or "RC" terminals.

CAUTION - Always be very careful with electricity as it can be lethal, if you are not comfortable performing any of these steps please do not perform them, of course you are performing these steps at your own risk.

Important Note: The heating system control board is sometimes very sensitive to static electricity, avoid touching this board to prevent failure.

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