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Residential Thermostat Call or Switching for Cooling

The first thing that must happen is that the control thermostat must provide a voltage signal both to the furnace and air conditioning unit at the same time. 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 thermostat calling for cooling the thermostat switches on the power from the “R” or in some cases the “RC” terminal which is the 24 volt power source to the “Y” terminal on the thermostat which is in essence the call for cooling. These terminals are located on the back or on the wall mounted face plate of the thermostat.

All the wires connected to the thermostat will usually always go directly to the furnace and another set of wires will then go out to the outdoor air conditioning unit and if wired correctly 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 furnace heating system.

The second set of wires going to the outdoor unit also connect to the "Y" and "C" terminals in the furnace and then they either connect to another terminal or simply to wires inside the outdoor unit. The same 24 volt power applied to the Y terminal at the furnace also tells the air conditioner to run

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 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.

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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