How do RH/RC/R and Y/W relate for HVAC?

Question

I have the following notes for HVAC (that I've compiled from reading through here):

Thermostat wiring:
  C       low voltage power wire that brings power from heating/cooling equipment to thermostat
  RC      controls the cooling (at the transformer) has 24V
  RH      controls the heating (at the transformer) has 24V
  G       comes from relay that turns on fan
  W       comes from relay that turns on heating system (at the relay)
  O/B     controls if heat-pump is in heating/cooling mode
  Y       comes from relay that turns on cooling system (at the relay)

I am programming my own thermostat; so I'm a bit confused how RH/RC/R and W/Y work together.

My thoughts are that RH/RC/R has a "live" 24V signal that gets sent to the thermostat and the thermostat uses the 24V to "switch" W/Y correct?

If what I say is true then a custom thermostat just has to send 24V down W/Y to turn on/off the heating/cooling? So for all intents and purposes I could ignore RC/RH/R?

If I am wrong (I probably am) please do explain how RH/RC/R wires are related to the W/Y.

Thanks!

This answer might be helpful. – Tester101 Apr 28 '16 at 16:37 — Tester101, Apr 28 '16 at 16:37

score 3 · Accepted Answer · edited Apr 13 '17 at 12:22

I'll use this gif from my other answer, to try and describe how the system works. I'll focus only on the heating side, just to keep things simple.

Electricity flowing through an HVAC system

When the thermostat is not calling for heat, the circuit is open and electricity cannot flow. Once the temperature in the room drops below the set point, the switch in the thermostat closes. When the switch is closed, electricity can flow through the control circuit.

Electricity flows from one side of the transformer, out to the thermostat on the R wire. It flows through the switch in the thermostat, and back out along the W wire. It then flows through the coil of the heating relay, and back to the other side of the transformer (C). When electricity flows through the relay coil, it moves the switch in the relay from the normally closed (NC) contact, to the normally open (NO) contact. When this happens, the furnace is signaled to turn on.

As you can see in the above animation, when the thermostat contact is open, electricity does not flow in the control circuit. Once the thermostat closes, the heating relay is pulled closed signalling the furnace to turn on.

At the most basic level, a thermostat is just a switch. So it's not "sending" 24V, it's completing a circuit.

score 0 · Answer 2 · answered Apr 29 '16 at 05:43

If you've ever assembled a PC or have any DC electronics knowledge, the AC can make things confusing. It's educational to pretend that the transformer is a 24 volt DC supply, and that R/RC/RH is +24V, and C is GND.

(not to be confused with the "equipment grounding conductor" - the bare/green wire in 120V wiring.)

+24V "power" goes to the thermostat. GND does not; the passive thermostat doesn't need it. (a smart thermostat does.) If the thermostat sends +24V power down W, G, Y etc. - then the corresponding device will actuate. Each of those devices connects back to GND, completing the circuit.

Of course, this is completely wrong because the power is AC not DC (at least it's wrong half the time LOL). For understanding the basics, the AC is pretty much irrelevant. Some furnaces might even work with actual DC, but I wouldn't bet on it.

How do RH/RC/R and Y/W relate for HVAC?

2 Answers2