Dr-Fix-It! Notebook Archive:
"Thermostats for Electricians..."
Last Wednesday Jerry, an electrician , reluctantly asked me for some advice on how to fix the fan-coil unit in his home. The unit was energizing both the heating and the air conditioning at the same time. Jerry asked me if I thought it might be the thermostat causing the problem.
I replied that the first thing I would check would be the thermostat wiring. To me, it sounded like the white and yellow wires were shorted. From the look in his eyes, I could tell he did not understand. But, when I took a few moments to explain the thermostat circuit and the color codes, Jerry immediately saw that a short circuit was a distinct possibility.
Electricians operate under different rules than control technicians. To an electrician, a white wire is 120 volt neutral, green is ground and yellow could be a 277/480 volt hot leg. These are colors specified by the National Electrical Code. To a control technician,white is heating, green is fan and yellow is cooling. To add to the fun, there is no real standard
So, I thought it might be helpful to publish an article describing, as I did with Jerry, how a thermostat is wired in common electrical terms.
Here we go:
Think of an analog thermostat as a gang of switches which are all wired in parallel to the red lead. The red is the hot lead feeding 24VAC from the fan/coil unit to the thermostat. The red wire connects to the thermostat on the 'R' terminal. 'Rh' is the hot terminal for heating and 'Rc'is the hot terminal for cooling. If the heating and cooling units have separate 24VAC transformers, then using the 'Rh'and 'Rc' terminals will isolate the circuits. If there is only one transformer, then you can simply jumper the 'Rh' and 'Rc' as if they were just an 'R' terminal. If there is a need for a 24VAC neutral to the thermostat, it is frequently the brown wire but is usually labeled 'COM' or 'C'.
As climate conditions change, the various switches in the thermostat make contact and energize various switch legs which control different functions of the heating and cooling. Again: The analog thermostat is a group of switches connecting a hot leg to various switch legs. The neutral is required only to operate certain electronic thermostats. All non-electronic thermostats and some electronic thermostats do not need a neutral.
The thermostat switch legs are identified by color code. (This code is used for the most part but it is not a standard like NEC color codes.)
'R' Terminal: Red wire 24VAC hot lead from Unit
'G' Terminal: Green wire energizes Fan relay
'Y1' Terminal: Yellow wire energizes First Stage Cooling
'Y2' Terminal: Blue wire energizes Second Stage Cooling
'W1' Terminal: White wire energizes First Stage Heating
'W2' Terminal: Black wire energizes Second Stage Heating
'O' Terminal: Orange wire energizes Heat pump Reversing Relay During Cooling
'B' Terminal: Orange wire energizes Heat pump Reversing Relay During Heating
'X' Terminal: No Color usually optional LED indicator
By the way, Jerry reported back to me the next day. It turns out that his wife had unknowingly driven a screw into the wire when she was hanging a picture on the drywall. Jerry said he had been awarded a lot of "points" by his wife because he diagnosed and solved the mystery so quickly...