Hi All - I have a '94 Interthem forced air furnace that's acting up. Forgive the lengthy explanation, but I've checked quite a bit already. The problem is, the blower fan only runs for a minute or so at a time, then quits. The Tstat is good - it does the same thing with the wires crossed. The burner lights, and continues to burn constantly when heat is called for, even after the fan cuts off. Here's my theory - the burner flame is not getting hot enough, and the low limit cuts off the fan when it cools down too much, then turns it back on shortly when it re-heats. It seems like the air gets cooler just before the fan cuts off. The burner flame seems lazy - it's blue with yellow tips, and just seems to flow out of the burner. It's quiet, and has no shape. There are no air obstructions, either intake or exhaust, and I've oiled the fan motor and it doesn't seem to run hot. My question is, should I, or can I, adjust the pressure on the gas valve - at least to troubleshoot?
Thanks in advance,
Wayne

I would change the fan limit before I go messing with gas pressure

On the name tag of the furnace you will find the proper gas pressure (usually 3.5 in. water column for natural gas and 10 in. for propane) this is checked with a manometer.To determine if it is the fan switch you could check it with a volt meter or block off some of the air entering the blower and see if it stays running. Mike

Will the blower motor come back on after the furnace heats back up and if so what is the temperature in your house or at the return air? Your temp act switch will cut out(turn blower off) around 90 degrees and cut in(turn blower on) about 115 or 120. That's the temp inside at the heat exchanger.

The blower disc or tstat might be out of calibration allowing the fan to come on to soon. When this happens the fan will cool the furnace down then turn off only to come back on a minute or two later. If this is in a mobile home replace the blower thermostat switch.

For parts or other questions on mobile home units see below.

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You'll never believe what I found -
First, thanks to all of you for your input. Last night I really started thinking about the problem, and everything led me back to that lazy flame on the burner. Before ordering a new gas valve & limit switches, I decided to yank the burner & clean everything. Burner looked good, so I got really brave and pulled the orifice assembly out of the gas valve. From the back side, I could see what looked like a small piece of cotton in the tube. So I grab a small plastic tube off a can of WD-40 & start poking through the orifice. It was just poking small holes, so I flip it around and give it a good blow from the orifice side. Out pops a nice fat spider and egg sac! Dropped right in my hand & scared the pants off me. He was quite fresh, but quite dead as well - talk about head in the oven. Anyway, after re-assembling everything & spraying for leaks, the furnace fired like new, stayed running, and works just fine. Whoda thunk.
Thanks again for all your responses -
Wayne

Yep spiders get into the pilot and burners all the time.

Several times I have found flys in the pilot, once the pilot goes out for some reason flys lay their eggs in the pilot (standing pilot) I guess they like the raw gas, This just goes to prove a little perserverance pays off.