Have an 'old' friedrich furnace (model GU4050NBB) with no electronic controls (which I would be more familiar with). Unit has no inducer, no CO sensors and no flame sensor. Just your basic standing pilot with the electrically controlled gas valve. When the heat call comes, the main burner lights. I can see the fan limit moving as the plenum comes up to temp, but the main burner shuts off before the 'on' limit for the blower. Is this a bad gas valve, poor electrical connections, or something else I'm not familiar with on this old beast?

Usually the fan control also has a limit device built into it. It is possible that the blower motor itself is not working or the fan and limit control is defective. Jump the fan control to see if the blower motor will run or if the thermostat has a fan on-auto setting turn it to on. This way your will know if the blower motor works.

If the blower motor works then you know it is good. Now you will have to test the fan and limit control or replace it since it controls the blower motor. I would also check around the furnace to make sure no other control or thermal disc is causing the problem.

One last item you might try. Many times as controls get old they will loose there setting. Hold the dial and move the fan lever to a cooler temperature. That will cause the blower motor to come on sooner and that might help the situation.

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Originally Posted by hvac1000

Usually the fan control also has a limit device built into it. It is possible that the blower motor itself is not working or the fan and limit control is defective. Jump the fan control to see if the blower motor will run or if the thermostat has a fan on-auto setting turn it to on. This way your will know if the blower motor works.

If the blower motor works then you know it is good. Now you will have to test the fan and limit control or replace it since it controls the blower motor. I would also check around the furnace to make sure no other control or thermal disc is causing the problem.

One last item you might try. Many times as controls get old they will loose there setting. Hold the dial and move the fan lever to a cooler temperature. That will cause the blower motor to come on sooner and that might help the situation.

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Wow thanks for the quick response. I failed to include that the blower does come on, but only after the burner shuts down (the dial on the fan limit continues to climb after the burner shuts down). The fan limit never reaches the high safety limit, because once the blower comes on, I can see it 'dialing' back down until it hits the low setting 'off'. I will try lowering the 'on' setting as you suggested and look closer for another hidden limit. If I find one I'll assume that I have a problem with the heat exchanger that is letting hot air blow on the limit and shutting down the burner, but I don't expect to since the wiring is simple and I didn't see any wires straying off to a hidden disc... :) will get back to you with what I find.

If anyone has played with the wiring of that switch that could also be a problem.

OK - here's where I'm at. The fan limit switch seems to operate, though it was a little 'sticky', it operate smoother now just from holding it and adjusting the settings. Following your advice I lowered the 'on' setting to about 115, it had been at 150. This allows me to get a little more heat out of the furnace, but the burner still only burns for about 45seconds, then shuts down, regardless of whether the blower is in an 'on' cycle or not. There are definitely no other limit devices involved. Any more thoughts?

BTW - the limit switch in question is a honeywell, like the B/W photo you attached...

Eliminate the thermostat control to see if that is causing the problem. R and W are the terminals in the furnace. Disconnect one or the other and then Jump R and W together. This will mean that the burner will be on all the time. If the burner runs all the time while jumped then the thermostat is the problem. Post back with results.

Sorry for the delay. As soon as I read your post I knew you were right. How did I overlook the simplest solution? I guess because I rarely get an easy fix. Anyway a new thermostat solved the problem. I replaced the fan limit too, since I didn't like it's herky-jerky operation. Thank you so much for your advice.

Glad you got it working again.