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mikestock's avatar
mikestock
Explorer
Nov 12, 2020

Need help with a SF 30 Suburban Furnace

My original problem was the usual story. The fan would come on and the furnace would fail to light and the fan would then shut down. I, standing outside could hear the switch and smell gas but the spark, I couldn't hear due to fan noise and it was failing to light the gas. I brought the furnace home and checked the over-temperature thermostat and sail switch. They were good. I hooked up a 12 volt battery and opened up the combustion chamber to check to see if I was getting a good spark, which I was.

Took it back and reinstalled it and the fan no longer came on when I called for gas heat. I could, however, use the external 12v battery to supply power through the green and yellow wires while jumping the two blue wires together. With the gas hooked up, the furnace came on and burned, just like it should. I had the internal thermostat out of the circuit and the only hook up to the rv was the gas source. I hooked internal wires back, just like they were originally but nothing would happen.

I'm guessing that I have a thermostat problem. I may have caused it when I back fed power by accident, blowing the 2 amp glass fuse in the thermostat.

Lacking any expert suggestions I guess I will go spend $80 for a replacement thermostat. This one is a 3 way, AC, heat pump or gas furnace. The ac and heat pump functions still work.
  • mikestock's avatar
    mikestock
    Explorer
    Nov 12, 2020
    Old-Biscuit wrote:
    If furnace does NOT...check if the on/off CB switch is tripped///push button reset


    Are you saying this switch is a breaker? Mine is on.

    I don't have two blue wires at the thermostat. I do have the two blue wires on the heater that make up when the over-heat thermostat is closed and sail switch closes. As I said originally, when the two blue wires from the heater are tied together and 12vdc are supplied by an external battery to the red (+12v) and yellow (-12v), hooked up the gas line and the heater started and ran. When I matched up the wires as they were, originally (see last post) matched, nothing happens.

    The other 2 functions of the thermostat, ac and heat pump, work correctly.

    I don't have a clue where the relay is located where the tiny wires from the thermostat transfer to the heavier wires to the furnace.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Nov 12, 2020
    enblethen wrote:
    It is the two blue wires that come from the thermostat.
    Red and yellow is the 12 volt DC supply.


    Red is 12VDC + (positive)
    Yellow is 12VDC - (negative)

    Red supplies 12VDC+ to the thermostat via the On/Off CB Switch (local/on furnace frame) (Blue) and DC Power to 'power' circuit on board

    Thermostat closes and sends DC+ back to circuit board (Blue) and sail switch/limit switch circuit

    Connecting the 2 Blues wires at thermostat makes up 'run' circuit...bypassing thermostat
    If Furnace runs (Fan/Flame/Heat) then thermostat is problem

    If furnace does NOT...check if the on/off CB switch is tripped///push button reset
  • mikestock's avatar
    mikestock
    Explorer
    Nov 12, 2020
    enblethen wrote:
    It is the two blue wires that come from the thermostat.
    Red and yellow is the 12 volt DC supply.


    The wires from the thermostat are black, green, yellow and white.

    The blue wires are both on the heater side. They are blue, blue, red and yellow.

    Thermostat__________Heater
    black------------------------red
    green------------------------blue
    yellow-----------------------blue
    white------------------------yellow
  • enblethen's avatar
    enblethen
    Nomad
    Nov 12, 2020
    It is the two blue wires that come from the thermostat.
    Red and yellow is the 12 volt DC supply.
  • mikestock's avatar
    mikestock
    Explorer
    Nov 12, 2020
    enblethen wrote:
    Locate the two wires feeding the furnace either at the thermostat or in the furnace. Connect them together. The furnace should start and then produce heat. If this happens, separate the two wires. The furnace should go into shut down mode and turn off.
    If this happens, the problem is in the thermostat.
    Make and model of thermostat? Number of wires in thermostat? Does it control the A/C?


    Here on p.35 is the schematic of my unit. The wires coming down from the thermostat are bk, gn, y, wh and they connect to r, bl, bl, and y, in order, on the furnace.

    I have no idea which pair of the bk, gn, y, wh to tie together. I see 6 possible pairs and don't know what further damage could be done by picking the wrong pair.

    I do know that when powering the furnace with an external battery that the red and yellow wires power it up with the two blues tied together. Gas connected and the furnace lights and runs.
  • enblethen's avatar
    enblethen
    Nomad
    Nov 12, 2020
    Locate the two wires feeding the furnace either at the thermostat or in the furnace. Connect them together. The furnace should start and then produce heat. If this happens, separate the two wires. The furnace should go into shut down mode and turn off.
    If this happens, the problem is in the thermostat.
    Make and model of thermostat? Number of wires in thermostat? Does it control the A/C?

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