Air Conditioning Heating Cycle & Thermostat

The information shared here is designed for aircon technicians or at least someone who’s quite technically inclined.

With a gas-fired heating cycle, the gas combustion is easy to see, and there are some obvious things to check, like the gas supply, if it is not working. Most gas burners have some sort of combustion air fan – a small fan, not to be confused with the room air fan, which is in the main air-stream – and if this is not working, then the controls will not allow gas to flow. You should understand that the process of lighting the gas requires a sequence of various functions, and, if any does not happen, the unit controls will not allow the operation to go further for safety reasons. For this very rationale, you should not attempt to bypass any of the checks if they fail, but find the cause first, and the self-diagnostics built in to virtually every unit give you a head start in this.

There’s not so much to go wrong with electric heat – this is just like an electric fire in the air-stream. Checking for voltage at the heater will narrow down the fault, and bear in mind that there is a safety cut-out if the heater gets too hot, but otherwise voltage means heat with a functioning heater.

Finally is the heat pump. There may also be auxiliary electric heat in this model, so be clear which cycle is not working. If the heat pump is not producing heat when on that cycle, a common problem is the reversing valve sticking. This is not surprising, as the valve stays in the same position all cooling season long, so may not be inclined to move when called upon. If you find that you are getting cooled air when it should be heated, this is almost certainly the cause.

Typical Thermostat

This picture shows a typical electronic thermostat with the cover removed. The option switches allow for use with a heat pump system, and choice of temperature display. Note the fuse on the right.

The terminal strip in this thermostat includes RH, RC, W, Y, B, O and G. You can see that in this particular application RH and RC have jumper wires to combine them.

There is no official standard for colours or terminals, and you should always consult the equipment supplier’s information. However, there is some commonality in practice, and the following list gives generally used functions. Note that all colours are not used in all cases.

• R Red – 24 VAC power
• RH Red – 24 VAC power for heat
• RC Red – 24 VAC power for cooling
• W White – heater control line
• Y Yellow – cooler compressor (or first stage heating on a heat pump)
• B Blue – heating changeover, or common transformer
• O Orange – heating changeover, or damper control
• G Green – fan switch

Other terminals that may be found:

• C Black – 24 VAC common power
• W2 Various – 2nd stage heating
• Y2 Blue or orange – 2nd stage cooling
• E Various – emergency heat, disables heat pump and uses auxiliary heat
• O Various – reversing valve cooling
• X Various – 24 VAC common, or emergency heat relay
• X2 Various – 2nd stage heating

Information courtesy from Mike Newberry from the Aircon Repair