Aruuuuuuuuuuuugh! As furnaces become more complex, they become a bigger challenge to the DIY, even the HVAC technician. The old limit switches had a bimetalic spring or snap disk that closed or opened contacts. If those are really thermocouples, you would need to connect a sensitive volt meter across the right contacts on the circuit board to measure the voltage. You would then need to know the typical voltage at the temperature. Best you may be able to do is to go to where they connect to the board, and check for conductivity with one lead disconnected. If the leads don't seem to disconnect, they could be thermisters, no easier to check. Thermisters and thermocouples are less likely to go bad than limit switches. May be time to gamble on a new board or a visit from a technician.

Hello All,

I am having the same issue... but rather go on with a number of repeat questions, I will enclose a picture of my furnace. I have been reading the replies, but it's easier for me to have a picture to refer to:

http://tinypic.com/hvx6vm.jpg

As I mentioned before, I had a similar issue as the blower doesn't kick on in "Auto" and I had to run the blower at "Fan On" mode. I had a burnt out relay on the control board which powers up the low speed blower during "Heat" mode. I replaced the control board and works like new. Please check for voltage reading. You shall have 120 V AC between the heat tab and the neutral.

Good Luck. :)

Hwuaijuc,

You mentioned that your furnace is 20 years old. As it has a standing pilot (the little flame), would make your furnace very inefficient. Probably less than 80%.

Contact your local HVAC guy and have them bid a new furnace for you. There are some out there that are over 96% efficient.

In our first house, we had one of the old large frame furnaces (30" by 60"!! ) and was at most 60% efficient. We went to a 92% efficient furnace and wow, what a difference. Not as drafty and it really helped the old pocketbook.

ModerDaze, looking at your furnace pic, it seems a little newer than hwuaijuc's but not by much. There is rust on it and that is never a good sign. You would probably want to get a bid on a new system as well.

The only drawback is that as Mr. Labman (typed with proper deference) stated:

Quote:

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Aruuuuuuuuuuuugh! As furnaces become more complex, they become a bigger challenge to the DIY, even the HVAC technician.

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Like most people here, I love to fix things rather than replace them as I am too cheap. Plus, I would rather do it myself than hire out. But, when things are that old and they are starting to break down, I would rather pay the big $ and get a new, extremely efficient unit.

There are a lot of good manufacturers out there. I have had good luck with Carrier and Bryant. My current system is a Bryant and has variable stage blowers AND burners so it only uses as much gas as needed to keep the temp where we want it.

Good Luck, Dan

My furnace does not work on auto fan but will work when fan is on.

Usually the older furnaces have what is called a fan and limit control that only controls the blower in the heat mode. Some of them look like this.

My furnace the t,stat calls for heat the burner lights the fan won't run is this the high limit?

Post a separate thread for your new question.

My furnace blower fan will not start in auto, and eventually the flame is shut off by a high limit on the squirrel cage. The safety is working so that's good.

Now I know that it should start, as I can see the fan control temp dial turning on the L4064B2954 Honeywell combination fan blower limit switch, and it definitely getting well past that setpoint. When I see that, I simply start the fan in manual, and then pull it out into auto, and it stays running. It shuts down fine.

I had to change out a bad blower motor a few days ago, and this is when the auto began to fail. Is it possible the bad motor damaged this combination fan limit switch, and is there a way for me to repair the switch some how?

I can buy one, but they are hard to find. 3-4 days to deliver from Thunder Bay.

Just looking for any kind of advice that's out there. My experience tells me it's pooched, but I just want a quick fix before Christmas.

Thanks, Albert

At least you can still get heat while waiting to receive a new fan limit switch. Buy a new fan limit switch and keep the old one just in case the new one malfunctions in a different way.

For example no blower at all when gas comes on and then blower comes on after gas turns off upon hitting upper limit. Which is what happened with my Rheem Furnace.

Best,