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Thermostat signals and wiring
From Transwiki
[edit] Thermostat signals and color code
[edit] Lack of standards makes this interesting.
While there isn't an official standard for thermostat circuit wiring colors, there is a general pattern. Your best bet is to see the manufacturer's documentation (both thermostat and HVAC).
 Thermostat Schematic |
[edit] Wire colors
If you only have 4 wires - Red, Yellow, White and Blue coming out of the wall - that is phone wire and it isn't good because it is too thin for the current. If there is a short it is possible it could cause a fire. Also the colors are not standard.
If you need an inexpensive meter to work with thermostat wiring see M300
We also have meters that can display thermocouple temperature Process Control Meters
| Older systems and base signals on newer ones | |||
|---|---|---|---|
| Terminal | Probable wire Color | Signal | Description |
| C | Black | 24 Vac Common | From one side of the 24Vac transformer (Think of this as 24Vac neutral |
| R or V | Red | 24 Vac Power to be switched | From other side of the 24Vac transformer (Think of this as 24Vac L1) |
| Rh or 4 | Red | 24 Vac Heat call switch power | Same as R, but dedicated to the Heat call switch |
| Rc | Red | 24 Vac Cooling call switch power | Same as R, but dedicated to the cooling call switch |
| G | Green | Fan | Fan switch on thermostat - is connected to R when Fan /Auto switch is in the fan position |
| W | White | Heating call | Is connected to R or Rh when themostat calls for heat. (Might be jumpered to Y on a heat pump. On others might be second stage heating |
| Y | Yellow | Cooling call | Is connected to R or Rc when themostat calls for cooling. Also Cooling or 1st stage heating on a heat pump Most often connected to G when Fan switch is set to auto |
| New signals - for heat pumps and staged systems | |||
| Terminal | Probable wire Color | Signal | Description |
| Y2 | Blue or Orange | 2nd stage Cooling | |
| W2 | Varies | Second Stage Heating | First stage Auxiliary heating on a heat Pump |
| E | Varies, blue, pink, gray, tan | Emergency heat relay on a heat pump. Active all the time when selected, usually not used. | Disable the heat pump and turn on first stage Aux heating |
| O | Varies, Orange | Reversing valve | Energize to cool (Changes from heat to cool on heat pumps) |
| B | Varies, blue, black brown, orange | Sometimes common side of transformer. Needed on some electronic thermostats or if you have indicator lamps OR Reversing valve (energize to heat) above. York and Trane sometimes use (B) as common. | Might be heating changeover or on older stuff common of transformer. |
| X | Varies | SEE NOTES BELOW | Might be common but sometimes emergency heat relay |
| X2 | Varies | Second stage heating or indicator lights on some thermostats | Might be emergency heat relay |
| T | Varies, Tan or Gray | Outdoor anticipator reset | Used on GE/Trane/American Standard and some Carrier Products. |
| L | Varies | Service Light | |
