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Hi,

I am working on a Pres II model 7663B 856 series. When the thermostate is engaged the burner comes on and the unit heats up, the blower comes on and runs for about a minute and then shuts off. The burner remains on. After a while about 6 minutes the blower comes on a runs for a minute and then shuts off. Called a service man, he thought my valve was bad because the burner flame did not appear as intense as it should have. He replaced the valve, no change in flame or blower motor operation.

I read through your forums and discovered that the diagnosis was that my fan/limit switch was bad or I could possibly have an air leak causing the sensor to cool off and turn the blower off. I replaced the fan/limit switch. The blower will run for about 3 to 4 minutes now but then cycle off. I went ahead and replaced the lower limit switch. No change.

I am not sure what to check next. The thermostate reads 25 volts. The thermostate lines to the valve read 25 volts when thermostat is engaged and 0 volts when not. When the blower is not running the fan/limit switch is reading 120 from point to point and 0 when running. I can not get any readings from the lower limit switch from point to point whether running or not. I get 120 when I test each individual point to ground (heater cabinet). I can jumper the fan/limit switch and the blower will run until I remove the jumper.

Could it be that my burner flame is not high enough or hot enough? How can I test to see if cold air is comming from somewhere and cooling the sensors down? Is there a way to adjust the flame of the burner? I am confused and lost. Heelllppp

Check your burner tube and also the burner orifice/spud behind the gas valve for blockages (spider-web-bugs).

Make sure flame is a strong blue and not dancing or wavering.


IF burner is lit, blower should not go off any.


IF it does and fan switch is reading 120vac at that time, then the fan switch has opened.

Since it is new, then I would feel it is opening because it is being cooled down.


Some of the older furnaces had a small sheet metal piece riveted in to protect the fan switch.

You can remove what is necessary to see if you see any rivets or pieces down in bottom that may have broken loose.

OR, if the piece is there, sometimes the fan and limit switches were in wrong places and had to be switched.


Thanks,
Robert

Thank you very much. You are right on the money. It was a spider web and spider in the orfice tube. Cleaned it out and everything is working perfect.y. I have a 110 degrees coming out of the registers.

Thanks again.

Hi,

You're very welcome, happy to help and glad it is working again.


Take care and best wishes,
Robert

Perhaps you paid for a gas valve you didn't need.

Most definitely.


Thanks,
Robert

Dang spiders....

Yep and ants too. They love it in there .


Thanks,
Robert