I have a Tempstar Model NTC6100KFG1 gas furnace. Furnace was working fine through the summer blowing the AC. It's been off for a month or two as we've been changing seasons here. When I went to turn on the heat, I found that it would immediately blow the low voltage fuse on the control board.

I replaced the original control board with a Honeywell ST9120U1011 universal control board about two years ago after the original one blew.

Every troubleshooting tip I found said to disconnect the W (heat relay) and R (24V supply) terminals from the thermostat and jumper them on the control board. If the fuse didn't blow, it meant the thermostat wire was shorting somewhere. If it did blow, it meant the control board was bad.

Well, the fuse blew. I'm having a hard time believing that just the heating section of the control board went bad after two year while not being used all summer. Before I go and buy a new one, is there anything else I should check? For instance the igniters or the gas valve? If so, what is the proper procedure for checking these?

Edit: If anyone knows where to find a schematic for this board, that would also be very helpful.

Following the advice of @longneck, I disconnected all the components that make up the heating circuit and reconnected them one by one. The furnace has two igniters as shown below. If I connect only one of the igniters, the control board is fine. But if I connect both of them, the fuse blows. Each igniter is measuring approximately 3Ω. Since they work individually, I don't think there is a short to chassis. Could they be shorting together? What should I check next?

pilot light chamber

control board

  • Including the make and model of the furnace might be helpful.
    – Tester101
    Nov 12, 2013 at 20:23
  • Did you figure out what caused the original board to die, or did you simply replace it? Did you make any changes before the fuse started to blow?
    – Tester101
    Nov 12, 2013 at 20:37
  • @Tester101 The furnace is a Tempstar. I have yet to find a model number marked on it. At the time, I did know what the problem was when I originally replaced the board. I have since forgotten. I replaced the blower motor earlier this year because it bound up. But as I've said, the AC works fine. When a "cool" request comes from the thermostat, the blower kicks right on. It's when I switched over to "heat" that the fuse began to blow. Switching back to "cool" still works fine. Nov 12, 2013 at 21:05
  • The model number should be on the nameplate, which may be inside the unit. Also, it may not be that surprising that the cool works when the heat doesn't. If it's a furnace with an separate A\C unit, the only thing in the furnace that is on with the A\C is the blower.
    – Tester101
    Nov 13, 2013 at 11:44
  • 1
    @longneck Yes I did but I forgot to follow up here. I replaced the igniters and the control board but neither was the problem. It turns out I pierced the wire to one of the pressure sensors with a bolt when I replaced the blower motor. I assumed that the pressure sensors would be active for heating or cooling but apparently they are only energized during the heating cycle. Your answer was very helpful in the diagnosis so I'm marking as correct. Jan 15, 2014 at 13:54

2 Answers 2


Since you mention an igniter I'm going to assume a gas furnace. So start by turning off the gas. Then disconnect everything else other than the power in and the thermostat wiring.

Now turn on the thermostat. If the fuse doesn't blow, reconnect one thing (like the igniter) and try again. Repeat until something blows the fuse.

Since you have isolated the problem to the igniters, I would try just replacing one or both of them. That's probably cheap to do. If that doesn't work, then you're probably looking at replacing the control board.

  • I have followed your instructions and found the components that cause the short. But I don't fully understand the result or where to go next. Please see my updates to the question. Nov 14, 2013 at 14:48

Bad grounds on ignitors are gremlins. Check your ignitor grounds.

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