Q&A: How do you determine the common wire in a 24 volt HVAC thermostat?
Question by ikerol: How do you determine the common wire in a 24 volt HVAC thermostat?
Best answer:
Answer by Kirk D
Usually it’s the white wire… In a standard heat/cool thermostat you have four wires…
red is 24volts conected to RH & RC
white is common connected to W
blue or green is fan connected to G
yellow is airconditioner connected to Y
What do you think? Answer below!
October 28th, 2011 at 4:40 pm
If the thermostat was installed to code specifications, the white wire is your common, green ground, red heat, blue cooling.
October 28th, 2011 at 5:11 pm
the common is white, but there are usually only 2 wires in a thermostat, one way in and one way out.
doesn’t matter which is which.
the wires in thermostats are bell wires and not full voltage electrical wiring.
the 24volts are taking a little bit from the forced air unit to regulate
October 28th, 2011 at 5:36 pm
Its the white wire thats if someone has’nt been working on it already. [ The wiring system]
October 28th, 2011 at 5:42 pm
Ok , well some interesting answers. I wonder why you might be looking for the commom. Untill recent , most thermostats did not have a common wire present. Now days with the new big screen back-lit displays, we often need to supply a common to the thermostat because they need to consume power. Although there are some minor variations on colors the following are close and cover most applications.
{Terminal name}, {color}, {function}
(R), Red, hot side of transformer.
(C) Common side of transformer (See B)
(Y), Yellow, Compressor activity (cooling or cooling and heating on a heat pump).
(W), White, Heat (gas burner, oil burner , electric heat, (auxiliary heat on a heat pump including defrost output from the outdoor unit to activate electric heat and turn on the AUX. heat lamp).
(G), Green, furnace blower fan. (needed for air conditioning, heat pumps and some electric furnaces). NOTE: on most thermostats the “G” and “Y” are connected together at all times when the fan switch is in the “Auto” mode, If you sully R to G then you will most likely energize the outdoor unit contactor!
(O), Orange , Energize to cool (used for reversing valve on heat pumps)
(B), Blue or Orange, Energize to heat (used on some systems, Rheem/Ruud is notorious for this).
(B) or (X), Blue, brown or Black, common side of transformer. Needed on some electronic thermostats or if you have indicator lamps. Do not confuse with (B) Reversing valve (energize to heat) above. York and Trane like to use (B) as common.
(E), blue, pink, gray or tan, emergency heat relay on a heat pump. Active all the time when selected, usually not used.
(T), Tan or Gray, outdoor anticipator reset.
Used on GE/Trane/American Standard and some Carrier Products.
(W2), Pink or other color, second stage of heat (may be same as (W) on heat pump or fossil fuel system). Note: some thermostats require a jumper from W1 to Y for heat pump operation.
(Y2), Blue or Pink, second compressor stage.
(L), Blue, brown, tan or gray service indicator lamp.
Numbers, see manufacture’s diagrams but can be stages of electric heat especially on York/Borg Warner/Coleman-Evcon, Frasier Johnson/ Air-pro.
(X2), Second stage of Electric Heat on GE/Trane/American Standard. See link above. Also used as indicator lamp or misc. contacts on other systems.
And lastly the answer to your question should be ” the wire that goes to the 24 volt transformer , opposite the red wire, of course on the 24 volt side.
October 28th, 2011 at 6:16 pm
These responses are a little off. The white wire is the switched wire for 1st stage heat. The red is the 24v lead. The common is usually blue in a gas furnace / ac setup, or brown in a heat pump setup. It is needed for digital thermostats without batteries, or ones that need a constant 24v lead.
October 28th, 2011 at 6:50 pm
Common will come from secondary on the transformer and go to the c on the t-stat.