How much vent vacuum needed?

[Renhoek]

Hi,


I want to test my 10-year-old State/AO Smith hot water heater's vent vacuum. The smoke test I did with a piece of incense makes me think it's not drawing well enough to prevent overheating the TCO. The burner & pilot keep going out after a short time.

Can I use a car carburetor vacuum gauge (which should be easy to find locally) or do I have to use an air vacuum gauge specifically for a HWH? (I have no idea where to get one locally, or whether they'd sell to a "non-plumber".)

Is there a general range of vacuum that my heater vent exhaust have -- or is it specific to my heater? I saw a YouTube video that shows a reading of .02" as being acceptable.

Thanks,

RH in CA.

 

[mbetter95]

A T&P valve is to releve pressure from the system. Your response is leaning towards a back flow reason. Am I missing something?

 

[johnjh2o]

Your problem may be caused by a dirty combustion air filter.
http://www.ehow.com/how_8540338_clean-smith-promax-water-heater.html