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AaronS · 10-22-2012, 07:19 PM

Help, it seems my heater malfunctioning. The exhaust blower will turn on then the burners will light and run for two minutes, then the vacuum switch will shut it off for about ten seconds and from then on it cycles for a minute on ten seconds off the entire time the heater is running.

Wuzzat? · 10-22-2012, 07:25 PM

You have an oscillator.
I'd think the control board would detect this abnormal condition and shut everything down.

The flue may be partially blocked. If you have a flexible tube filled with water and some fittings you can check for excessive back pressure if you have specs on this.

You may be able to clear it by pulling or pushing a "ball" of hardware cloth through the pipes.

AaronS · 10-22-2012, 07:46 PM

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.

Wuzzat? · 10-23-2012, 01:03 PM

Quote:

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.

Good clue.

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.

AaronS · 10-23-2012, 09:44 PM

Well I checked the flu as best I could and didn't see any obstructions. Also sat a volt meter on the vacuum switch leads and had a steady 24v. How would I go about doing a water column test on the vacuum switch because at this point I think it is either that or the blower isn't spinning fast enough.

Wuzzat? · 10-24-2012, 10:56 AM

24vac +/- 20% may be normal, depending on the load on the transformer at the time.

You need to pry the spec's from the switch maker (the OEM) and then duplicate however many inches of water column is supposed to actuate the switch.

http://www.efunda.com/formulae/fluids/manometer.cfm

I can help you step through the calcs. on this. Any clear flexible tube will do, for pressure or vacuum.