Gas and Oil Home Heating Furnaces - Have to cycle thermostat to kick the furnace on

Hi there,

We have a central system, with a 3 year old Evcon UGAF100CD furnace. The last few days (since it's gotten nice and chilly out), we've found that the furnace will run anywhere from 3-10 minutes, and then shut off. The thermostat will show for example, a preset of 70 degrees, with an internal temp of 67. It shouldn't shut off yet, slightly annoying.

Here's the really annoying part. Adjusting the thermostat doesn't help. I have to cycle it (turn from heat to off, then after a few seconds, back to on). Then it will fire up again as before, so it doesn't act like a thermistor or a sensor.

I did the lazy tech thing, and didn't bother putting a meter on it, and just assumed the stat was bad. I had a spare, slapped it on, and of course, didn't fix the problem. So, obviously the stat is putting voltage back, but the furnace is shutting off, and needs the action of dropping voltage from the stat to reset whatever is causing the failure.

This was late last night, so I went to bed, am at work today, and need to deal with it tonight. I haven't seen this one before, anybody got any pointers to get me on the right track?

Check to see if your 24 volts across R and W terminals quits, even though it has not reached set point. Never assume: Some dip switch may not be right, cycle time might be too short, anticpator is way off, or whatever.

Check to see if you have a high limit issue.

Check to see if your pressure switch drops out.

Modern gas furnaces, no matter what brand, basically require the same handful of things that can readily be tested, and the problem found - fortunately.

I appreciate the help. After some disasembly, cleaning, and reassembly, it seems to be running fine now. The filters were dirtier than they should have been, so maybe it was that simple. That's what I'm hoping anyway...