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    mhanks's Avatar
    mhanks Posts: 2, Reputation: 1
    New Member

    May 18, 2004, 02:09 PM
    Furnace fan
    I have a forced-air gas furnace w/o AC and a simple dial-type heat-only thermostat.  How do I change my system so that I can manually operate the furnace fan when not calling for heat?

    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member

    May 18, 2004, 03:25 PM
    Furnace fan
    This will be much easier with some furnaces than others. Some furnace controls have a contact, usually a screw, which will operate the furnace fan when ever energized. Pull your thermostat off the wall and look at the wires. Usually they have a bundle of wires with red hot to thermostat, green blower, white heat, yellow air, and blue common. If your thermostat has all the wires, but only the red and white connected to anything, try touching the green to the red, they are only 25 volts. If the blower immediately comes on, it is a simple job from there. Just buy any kind of a cheap little switch, and connect the red wire to it and the current contact of the red, and the green to the other contact. The only thing is, make sure the switch is a maintained contact switch, and not a momentary switch that shuts off as soon as you let go. You can also buy thermostats that have a fan switch built into them. Note, somebody may have cut corners and switched the red and white wires.

    If you only have the red and white wires, go to the furnace. See if there is a contact marked G near where the red and white connect. Still a fairly simple job. Buy some green bell or hook up wire, likely 26 gauge or a smaller number (bigger wire) will do. Run it from the G contact at the furnace to a switch as I described above. If you ever think you may instal air, maybe you should buy the regular thermostat wire and instal it now, just leave the blue and yellow disconnected.

    If you can't find any contacts in the furnace anywhere marked G, it may not be a simple job. Some where, there is a relay controlling the blower, perhaps one for each different speed of the motor. It should have a blue wire going back to the transformer and the white wire connecting to it through a delay timer or temperature switch. They keep the blower from starting up until the burner has a chance to warm the air. Running a green wire from it back to a switch at the thermostat might work. It might also feed back to the gas valve, turning it on. You may need to call on experienced help to make that work.

    Blowers do not draw that much power. You might be able to run a wire from the 115 volt feed to the furnace through a switch to the blower. Connect black to black through the switch, and white to white. The blower motor should have a plate telling the amperage it draws. Make sure the switch and wires are rated for at least that much at 115 volts, and the higher value if 2 are given, 5/10 amps. If you want the switch in the house away from the furnace, you must use suitable wire, NM or BX the same as the rest of the wire in the house. Again, you may need to call on experienced help to make that work.
    mhanks's Avatar
    mhanks Posts: 2, Reputation: 1
    New Member

    May 19, 2004, 11:37 AM
    Furnace fan
    Thanks, Labman, for such a complete and concise instruction. I 'll give it a try.


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