View Full Version : WIRING DILEMA , Honeywell Thermostat RTH7500D Programmable

Dec 28, 2006, 09:01 PM
I want to replace an Old thermostat mechanical type WHITE-RODGERS 1F56W-361model with the high tech Honeywell RTH7500D Programmable,. read carefully the Owner's Guide but my situation doesn't fit any description.
I have 6 wires on the old thermostat to terminals:
-RH(2 wires,on the same terminal)
The colors on this wires don't think counts as long as I will TAG them properly(and anyway they are only black and white)

THE PROBLEM is that the book for Honeywell says don't connect 2(TWO) wires on the same terminal!

I think the problem came from the installation of the AIR CON. Unit ,later on, approx. after 1 year I moved in the house .
Furnace is air forced type LENNOX 90UGFA2-50-6 AIR CON. CARRIER
Now probably the technician who installed the air con. Unit didn't bother to put a new wire with 5 wires and he used old wires . I checked and on the FURNACE I also have a terminal with 2 wires .
I would greatly appreciate an expert opinion in this mater .
IS IT REALLY WRONG to connect 2(two) wires on the same terminal?:confused:

Dec 28, 2006, 10:03 PM
You did not mention if you had a Heat Pump or not so I am not sure what you are wiring for, but I am going to assume that you have a Gas furnace and just a AC and not a heat pump. To wire for this you actually only need four wires. One for each R-G-W-Y. You say you have two wires on RH and one wire on RG. If you look closely you may see that one of the wires that is on RH is actually the wire that comes from RG.
So if it is the way I say your wiring should be right at the thermostat. You also said that you had two wires on one terminal at the furnace, that is correct to have two wires on the Y terminal at the furnace it is just a connection that runs the Y from the thermostat directly out to the AC when the thermostat calls for cooling, The other wire going to the AC should be on the C terminal. I hope this helps you, if not you may have to check your wires out to make sure that you have the right wire in the right place.

Dec 29, 2006, 07:44 AM
Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. Many have RH and RC to allow separate transformers for heating and cooling. With just one red wire, jumper them. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

You may need the superset wires to the RC (not RG) and the RH. I don't understand why in some cases when they add an A/C, they add a second transformer. If so, that transformer needs to be connected to the RC and to the coil of the relay for the outside unit. Most systems use one transformer with a jumper between RC and RH. I see no reason to run 2 wires from R to RH. It is possible the second one is 2 different wires powering 2 other things such as a humidifier. If they don't disappear into the same cable at both ends, you may want to try to trace them.