My furnace is overheating and I suspect that the blower motor does not go into high speed. Can anybody say what is the cause or how can I check if it indeed switches to high speed?

Here is the wiring diagram

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and here are the specs. This sticker shows that it has a second speed. I suspect that switching to that second speed is not happening

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the furnace is hotter in the areas marked on this picture. I suspect it might be either the blower at reduced speed or the heat exchanger that is cracked

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  • Increasing the speed of the air moving through the furnace, will likely have the opposite effect than what you're looking for. The air will not have enough time to pick up heat, and the furnace will overheat faster.
    – Tester101
    Nov 19, 2014 at 15:26

1 Answer 1


Most furnaces; yours included it appears, do not change the speed of the motor during normal operation. The motor typically has three or more speeds (high, medium, and low), and is hard wired to use different speeds based on what mode the furnace is in.

For example, in heating mode most blowers run at low or medium speed. While in fan or A/C mode, they run at high speed.

If your furnace is overheating, the first thing to do is to make sure the filter(s) are clean. Also make sure all the registers and returns are fully open, and unobstructed.

If you haven't checked already, check the temperature of the furnace when the limit trips. You could have a bad limit, that is tripping when the furnace is not actually overheated.

  • I have checked the filter (between cold air return and the blower) and it is clean (just replaced in September when the system was in AC mode)
    – MiniMe
    Nov 19, 2014 at 14:08
  • How do I check the temperature of the furnace? What I did was to just keep my hand next to the fan and limit switch. You can hold your hand there but it becomes unbearable if you keep it for longer than 30 seconds let's say. I already suspect that the switch has some problems and I will have to replace it, I noticed that sometimes the switch does not turn the blower off as it should. Right now the furnace is working like this, 30 minutes blower off (cooling cycle) then it takes 1 minute to reach 200F and then again 30 seconds to cool off and so on.
    – MiniMe
    Nov 19, 2014 at 14:37
  • I added more details to this. The problem that I am trying to solve is this. The furnace overheats in some areas and that could be either due to reduced air flow or due to ..cracked heat exchanger(?) I am not sure about the last one since I don't know what is inside but my guess is that the flames or the head might rich the bottom circled area almost directly through the cracks in the heat exchanger. I would be really glad to see a picture of it for this model (UltraMax Olsen HCS2 81AD). I could not find any
    – MiniMe
    Nov 19, 2014 at 14:44
  • You can check the temperature with a thermometer, an infrared thermometer is a good tool for this. When the limit switch trips, the blower should not turn off. It should continue to run, to purge the excess heat. Sounds like a cracked heat exchanger. You should contact a local HVAC company, and have them come take a look. Probably cheaper to replace the furnace, than to swap out the heat exchanger.
    – Tester101
    Nov 19, 2014 at 15:22
  • I think this is where I am going ..to replace it. The thing is old, I am really sorry to see it going because it is simple and if it was not for the blower I could maintain it myself..it is pretty simple
    – MiniMe
    Nov 19, 2014 at 18:15

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