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    labman Posts: 10,580, Reputation: 551
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    #1

    Nov 29, 2007, 02:23 PM
    Need major help with OLD furnace
    From a PM:

    ''hello, I have a OLD Siegler Holly V70 forced air gas furnace with a General Controls gas relay, and trying to rewire it so that 1 the auto fan limit works right. And 2 make it so that I can turn on just the fan with the thermostat.

    I believe this is just a 2 wire system because when I started by replacing the thermostat to a digital one with a nonswitch fan control, the wires to the old t-stat were 2 black wires.
    So I pulled some new 18/5 to the furnace.

    This furnace has I said a fan limit which worked only if I pulled the knob to manual. I would not turn on in auto. As well the furnace has a switch on the outside to turn on the blower which is what I used to push out the heat sense the auto did not work.

    Now here is my big problem I foolishly disconnected all the wires so that I could hook up the new thermostat. Not knowing that normally the wires are not just black Im stuck. ALL the wires besides the 2 from the transformer are black ( and the new wires I just pulled) to make matters worse the gas relay is marked as such.
    There are 4 screws in a row from top to bottom facing the gas relay . Above the first two screws is written 24v and above the next two is written Thermo. Now I unscrewed the cap off and only the top screw and the bottom screws have any thing connected to them.

    So as a quick recap. Off the transformer there is a red and white wire leading to the gas relay, a switch that connects to the outside furnace, 2wires going to the fan limit both are black and wires going to the blower. From the fan limit 2 black wires load/line heading to the gas relay. And the 18/5 wires from the thermostat to the furnace/ gas relay area.

    I hope I detailed this out well enough I have not been able to find a wire diagram for this furnace so I hope you can help.
    Thank you''

    Start by reading through the sticky on thermostat wires at https://www.askmehelpdesk.com/heatin...ons-58313.html

    Next identify the wire from the thermostat that is connected to one terminal of the gas valve, the coil of the fan relay, and any other doodads. That is the common, connect it to the blue wire. Connect the other wire to the red. It is the hot to the thermostat. Likely you can use the top 2 terminals marked 24 volts. It should also connect to one side of the limit switch. The wire that was labeled thermostat should connect to the other side of the gas valve. Connect the white wire to that terminal. The manual fan switch should have had the side of the transformer now connected to the red wire and a wire to the fan coil connected to it. Connect the wire that goes to the fan relay coil to the green wire. Leave the yellow to connect to an A/C.

    You should now have blue connected to the transformer, red connected to the other side of the transformer, white connected to the gas valve, green to the fan relay coil, and yellow not connected. At the thermostat, the wires all connect to the first letter of their color except red may need to go to RH, and blue to com.

    Power should flow from the transformer to the limit switch, through the switch when hot, and on to the fan relay and then back to the transformer. If it was rigged before, it could have been because the switch was bad.

    I have posted this to the forum to allow others to see it, and comply with site policy against asking and answering questions by PM or email.

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