Check out some similar questions!

daniel111111 · Jun 1, 2005, 06:50 PM

Hello,
I own a gas furnace that is 7years old its called coleman evcon.I would like to turn on my fan with out turning the heat on.
My furnace has 2 hires (white and black)caming to the thermostat. I understand I need a three wire relay out of the furnace to get my thermostat to turn the fan on with out heat, so how would I install a manual switch to turn
Fan on with out the heat.I don't want to fish the wire up my walls, I would like to install the switch right on my furnaces

I hope some one can help me out.

labman · Jun 1, 2005, 10:03 PM

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. 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.

One of the wires to your thermostat should be connected to a terminal at the furnace labeled R or RC. If the furnace also has a terminal marked G, it is as simple as connecting any switch between the 2 terminals. Without a ''G'' terminal, you will have to poke about the furnace innards and find the relay that controls the fan. Start at the fan motor. There are limit switches that turn the fan on after the gas is on for a while, and keeps it on a while after the gas goes off.

daniel111111 · Jun 2, 2005, 10:03 AM

THANK U SO MUCH FOR REPLYING SO FAST.
Am sorry for not making my letter clear, but my furnuce has only heat ability, its has no A/C. My purpose is to circulate the air in my home.Thank you again.

daniel111111 · Jun 5, 2005, 12:35 PM

Labman Thank you so much for your help

I tried what you said and worked perfect. I connected R,G togeter. U explained it perfect, At first I had NO mentally concept what you wrote, but reading it over I understood it. You understood me perfect . Thanks so much again.

sukosaki · Jun 27, 2007, 10:00 PM

Originally Posted by labman

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. 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.

One of the wires to your thermostat should be connected to a terminal at the furnace labeled R or RC. If the furnace also has a terminal marked G, it is as simple as connecting any switch between the 2 terminals. without a ''G'' terminal, you will have to poke about the furnace innards and find the relay that controls the fan. Start at the fan motor. There are limit switches that turn the fan on after the gas is on for a while, and keeps it on a while after the gas goes off.

I tried the "jumper from R to G" but had no luck. I have the same situation as above. Is there anything else I can try? I have a switch on the furnace to turn it on, programmable thermo with 2 wires to it. Fan auto or on and nothing. When I put to 'heat' it makes a loud buzz (trying to light). Any ideas?

labman · Jun 28, 2007, 05:49 AM

With only 2 wires, the thermostat can't do anything except turn the heat on and off. The is a good discussion of a similar problem at https://www.askmehelpdesk.com/heatin...et-104439.html With only a 2 wire thermostat cable, it is possible your furnace is not set up for A/C and fan only. If so, you would have to track down the coil terminals for the blower relay as in my first post.

If the heat won't come on, see the excellent sticky Northern Heat has at the top of the forum. Work through the sequence of operations. I will not take time to do a poorer job.

sukosaki · Jun 28, 2007, 10:46 AM

I have only 2 wires. Black and white. I will try to wire it to the Rc terminal on the thermo, they are jumpered right now.

I have nothing going to green on thermo or furnace. I thought if I jumpered R to G it would turn on with the furnace switch.

I haven't had any luck with any setting on the therm, On or auto, heat or cool, for JUST FAN. The thermo is a new Honeywell 7day programmable.

Thanks for the help, I'll keep trying.