I've got a power vent gas water heater that has operated perfectly for several years but lately been unable to shut off without a racket.
The problem seems to occur when there are multiple uses for hot water, i.e. weekends.
The vent fan comes on and off rapidly and the solenoid clicks like a machine gun. I can make the unit "fire up" by running hot water until a definite demand is created. I replaced the thermostat and gas control valve. I've checked the air line and then replaced the air pressure switch. I've had the gas company out to check the pressure which was 7" W.C. so now I'm lost.
Any experts out there?
I have contacted the manufacturer, who tried to help via email but gave up.

Try reposting this on the plumbing forum

It really is a relay, not a solenoid. Check the voltage to it when it is clicking. If it is OK, replace the relay. If the voltage is low, follow it back to its source.

Thanks for thoughts-
This weekend I noticed the clacking noise (as the valve and vent were struggling to start) was concurrent with the operation of the gas clothes dryer. Is it possible that the dryer is causing a low pressure situation at the WH?
And equally puzzling why after ten years and replacement of thermostat, gas valve and solenoid, air pressure switch and relay.

Thanks again.