I'd like to wire my blower motor to my Honeywell CT2800 programmable thermostat so that I can use the manual blower switch. However, I am a bit confused as to how to do this. The furnace is a Clare Brothers MBG120. The 120VAC comes into the furnace and the white wire goes to the 120VAC to 24VAC transformer as well as to the furnace blower. The black wire goes to some type of switch that has a dial on it and a knob that you can pull out and run the fan manually. Limit switch I am guessing. Out of that switch comes a red wire that goes directly to the fan and a white wire that goes to the other side of the transformer. From the transformer, there are two wires and they go to a valve assembly on the gas inlet. On that valve assembly are the terminals TH, COIL, and TR. One of the wires goes to coil and the other one goes to TR. On the same terminals, the thermostat is wired and a yellow wire goes to the TH and a red wire goes to the coil. Up at the thermostat, the yellow wire goes to the terminal marked R and the red wire goes to the terminal marked W. There is a G in the thermostat, but there isn't anywhere on the furnace. I think that I might have to install a relay, but I am confused since the fan motor doesn’t have one already and runs strictly off 120VAC. Can anyone shed some light on this? Thanks very much.

That box with the dial and the white knob to turn on the blower is called a fan/limit switch, and unfortunately the only way to turn the fan on is by pulling the white knob. With these older furnaces there isn't many options you can do to improve the functionality of the furnace, since they are very basic. Connecting a programmable thermostat up to an old Clare furnace is a waste of time.

Brian Belanger
Gas Technician
The Stove Store, Spencerville Ont. Canada

Thanks for the reply. I actually have the thermostat wired up and working for heat. I was looking for a way that I could hook up the fan to the thermostat so that I could turn on the fan by itself for summer cooling. I opened up a hole in the air intake for the furnace so that it would take the cool air out of the basement and put it upstairs. Right now, I just pull the white knob and it comes on. It works fine, but it's a pain in the arse to go up and down the stairs. Was hoping I could wire it to the thermostat. But if that's not possible, would it be possible to put in a on/off switch (near the thermostat) for the fan and bypass the limit switch? If so, how would I do this? Thanks again.

Yes you will need a relay with a 24 volt AC coil and at least one set of contacts rated at 120 volt AC 15 or 20 amps. The contact rating must meet or exceed the amp draw of the fan motor.

Then you will need another wire brought up to the stat to connect to G. You can run a #18-2 thermostat cable up to the stat.

Cap one wire at both ends as a spare, use one ,connected to G, and the other end connect to one side of the coil of the new relay. The other side of the coil should go to TR. I believe this is the common for the 24 volt .

Before connecting to the relay, You should check with a meter to measure 24 volts between the wire from G, with the stat in "fan manual" position, and TR.

On the 120 volt contacts , you need to cut these two wires into the 120 volt feed to the fan limit switch, thus switching the feed to the fan.

Any questions get back with them, and be to to have 24 volts across G wire and TR, if not you will not have 24 volts at the relay coil.

Just make sure you don't touch the limit switch... it is a safety device!

How many wires do you have now going from the furnace to the thermostat, also how many terminals do you have on the low voltage terminal board at the furnace. Most furnaces manafactured in the last ten to fifteen years were made so that centeral air could be added at little cost. So tell me about the wires and the terminals