No Heat on Gas Furnace

jtherens

Older Lennox furnace will not heat (installed around 1976). Pilot light is lit, tried to replace thermostat, no circuit boards in system. Transformer is putting out 24v. The furnace does have 120v too. I can turn on the fan on the thermostat by selecting fan only.



Scratching my head and getting colder!



Jeff

Paul S_3

fan limit

Check the high limit..... Maybe it's stuck In the open position.... Our check for 24 v at the gas valve.... Maybe gas valve is at fault .... Paul S

jtherens

limit and gas valve

I check the limit switch with a ohm meter and found the left side open and the right side closed so from what i am reading that is okay. If the gas valve was a problem, would i still get my pilot light to stay lit like it is? and do you know how i can test the valve?



btw, if i turn the limit switch by hand, the fan turns on but no fire other than the pilot light.

Paul S_3

check

To see where you are losing your 24 v. ... spill switch? .... You said you have 24 v at the transformer. ... The same way you checked it is the way you check the two wires at the gas valve and yes if your gas valve is bad the pilot may still stay lit. .... The problem lies in your low voltage. ... Paul s

Paul S_3

all

We have on this furnace. ....I may be wrong unless you post pics. .... fan center. .. which operates correctly because the fan came on when thermostat was set to "fan on"..... fan limit seems fine because when furnace is cold left side of limit which is the heating fan speed circuit is in the open position where it should be and the right side of limit is in the closed position. .... The fan came on when you hit the button on the fan limit because all that does is Manually make a switch to energize the fan. .... check to see what safety is open. ... Roll out switch. .... spill switch etc. .... bypass thermostat manually. .... And when you check for 24 v at gas valve make sure both wires are on gas valve coil so you can make sure that a common is not lost Paul s