Arkansas HVACR NewsMagazine March 2025

HVACR NewsMagazine March 2025

Tech News

until after five minutes have passed because there is a cooling delay of 5 minutes between the last time cooling was energized or when power to the thermostat was lost. This is a safety feature to allow the refrigerant pressures across the compressor to equalize before the thermostat tells the compressor to turn back on.

below. The 24v power on both the Y and G terminals means that the furnace must turn the fan on at its highest setting. In the furnace, the Y and C terminals of the circuit board should have wiring connected which runs out to the outdoor unit. When Y is powered with 24v and the circuit is completed with the connected C wire, the contactor is powered, which allows the outdoor compressor and fan to turn on.

Now switch the probe that was on the yellow wire to the green wire. You will notice that the multimeter again displays 0.0Ω. This is because during air conditioning mode, Rc connects to both Y and G.

The Thermostat Common Wire

Realize that you don't technically need batteries in a thermostat in a real life scenario if the thermostat has a C terminal. 24v power from the furnace can power the digital display on the thermostat. Remember that C is connected to the common side of the 24v transformer in the gas furnace so that when R and C are connected at the thermostat, there is a completed circuit to power the thermostat face. If there are batteries installed in this case, they are used as a backup to hold the programming in a programmable thermostat. Thermostats may come equipped with the capability of being powered with either batteries or hard powered with 24v. This is done in case the existing building's thermostat wiring does not have enough wires to incorporate a common. This would likely be the case if the old thermostat being replaced was a mechanical mercury thermostat instead of a digital one.

In a real life application, this means that the red 24v power wire from the furnace is now connected to the Yellow Y terminal and the Green G terminal in the thermostat, which sends 24v power to both the Y and G terminals at the furnace control board. See the pic