Functions of thermostat terminals

[Stephen Tashiro]

Is there a standard convention for what the output terminals of a thermostat do? - in particular Honeywell thermostats.

The installation manual for the Honeywell RTH1100 thermostat shows its terminals are named Rh, Rc, W, Y, G. However, the wiring instructions tell what color wires to connect to the terminals, not what the terminals do.

I assume W is common, and G is ground.

I think Rc and Rh have to do with running the system being controlled on high or low. I think these outputs are either off or 24 V. I don't know if both outputs can be on at the same time.

I don't know what the Y terminal does.

[Bill Dufour]

https://www.ifixit.com/News/30317/what-all-those-letters-mean-on-your-thermostats-wiring

[Frank Pratt]

Rc - 24V hot for cooling circuit
Rh - 24V hot for heating circuit
W1, W2, W3 - heating stages 1, 2 & 3
Y1, Y2, Y3 - cooling stages 1, 2, 3
G - fan
C - 24V common

[Tom M King]

Turn the power off to the unit before changing a thermostat. Not knowing what type of system you have, a lot of them have a fuse in the outside unit that is easily blown fumbling with the thermostat wires while they're hot.

[Stan Coryell]

Frank has it right. I just wanted to add that Rc and Rh are jumped together unless you have two different systems controled by the same thermostat. Example: A hot water boiler for heating and a separate air handler for cooling.

[Frank Pratt]

Frank has it right. I just wanted to add that Rc and Rh are jumped together unless you have two different systems controled by the same thermostat. Example: A hot water boiler for heating and a separate air handler for cooling.

And some condensing units have their own control power supply, in which case the Rc & Rh jumper must be removed.

[Stan Coryell]

I've never seen one like that. How is the air handler wired?

[Frank Pratt]

I've never seen one like that. How is the air handler wired?

Condensing units with their own control power seem to becoming more common. The air handler, or furnace, is wired the same, just with the Rc & Rh split.

[Stan Coryell]

Condensing units with their own control power seem to becoming more common. The air handler, or furnace, is wired the same, just with the Rc & Rh split.

Do you have a single line or a model number? I'm curious as to how or why they split y up.

I just put in a 2 stage Goodman heater for my parents. It uses a single stage stat. The second stage is controlled by an algorithm. They seem happy with it.

[Frank Pratt]

The thermostat will have a jumper between Rc & Rh that can be removed if needed.

[Stan Coryell]

Frank,

Do they split Y up and share common? It seems more complicated than necessary. What brands or models did you run into this? I'm just curious.