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jjdahlin · Nov 24, 2008, 05:48 PM

I researched this in this help desk and got some good ideas, but none worked. It sounds similar to a dirty flame sensor.

The furnace is forced air gas from mid-1980s. I do not see a manuf on the labeling, but there is a Honeywell control unit.

If I remove the thermostat and just connect the red and white wires together (3 wire controller) the furnace will start and run (so I think it might not be a flame sensor -- but I may be wrong). I thought this indicated that the thermostat was bad and I picked one up and replaced the old one (Hunter, old and new the same model) and it did not solve the problem. (I replace the thermostat about a year ago when the furnace would not turn on at all -- the furnace repair guy wanted to charge $500 to put a new thermostat in!).

The problem:
Blower turns on, flames come one, runs for a couple of minutes, flame goes off, then the furnace fan kicks on. After than runs for a few seconds and it repeats. I checked the power supply and it is 26V AC.

I was not able to find a flame sensor that looked like anything I found on line. I followed wire from the control unit to try to find one. There is a connection to a device next to the igniter. The connection is not a wire, but a rigid metal tube (I did not take the assembly apart to look at the other end because that looked like a major task). But it did not look like what I saw on line.

There a device mounted to the inner sheet metal that looks like a mechanical temperature limits on it. There are wires from the controller to this unit. I think this is what kicks the fan on after the flame turns off. (Is it possible that this also replaces the flame sensor in older furnaces?)
Thanks for help.
Jeff

MarkwithaK · Nov 24, 2008, 05:53 PM

That device you are describing sounds like it would be your fan cycle/high limit switch. If your furnace is shutting off prematurely then the high limit cold be failing or set incorrectly.

And for what it is worth that rigid metal tube is your pilot tube. The thing it is attached to is your gas valve.

jjdahlin · Nov 25, 2008, 04:54 PM

MarkWithaK,
Could the fan cycle/high limit switch do this?
I was tracking down the flame detector until I found that bypassing the thermostat made it run correctly (just without a set temperature).
Any help would be appreciated.

At this my next step is to continue the flame detector direction and then maybe look at the controller.