Originally Posted by AaronS
I don't have a nanometer so I tried running the system with the gas off so the exhaust blower would run while I put an ohm meter on the vac switch. When I did this the vac switch would close the circuit and not go off, only with the burner on would the vacuum switch shut off. Could a partial clog still cause this.
I suppose there is a narrow window where cold air doesn't give you enough back pressure against a clog but warm air does?
Or the vacuum switch is touchy, giving you a false positive on the question of, "Is there a clog?"
If it's not too burdensome I guess I'd do a visual inspection of the flue system or check the vacuum switch against its specs with a column of water.
Also check that your nominal 24vac does not significantly change during all this.