No flame to heat water

Hi,
I have a water heater made by A.O. Smith model FPS that will not produce flame to heat the hot water. The pilot stays light, and the blower motor is running, but the flame will not kick in to heat the water. I had already tried vacuum all the rust and dust out of the bottom of the screen thinking it might be an issue with the air flow. But, that didn't make any difference. I took out the Thermocouple and clean it with sand paper but didn't help either. It seems like there was no gas coming out of the burner tube. Any idea what could be the problem here? Please advise. Thanks.

There's a vacuum/pressure switch that must close once the fan comes on. If it's bad or there's a hole or crack in the intake air or exhaust vent line, then that safety switch will prevent the gas from turning on.

How can one tell if the pressure switch is bad? How would you test it?

Look up the owner's manual for your heater on line, or if you're lucky, you still have it. It should give you some trouble-shooting and theory of operation. The switch will likely have some 1/4" spade push-on connectors...you should be able to use a multimeter to check it fairly easily. It probably has a vacuum hose going into it. You could just suck on that to see if the burner comes on. If it does, the switch is okay, but then you'd know you have a flue or intake problem. The hose could be split, too, just like a vacuum hose on your car.

There is a tube that runs from the fan to the vacuum switch and if you apply a vacuum to it (suck on it) and the burner flames on then the switch is most likely good...if the switch is good then it may be the fan is not producing enough vacuum to activate the switch...about how old is this heater...

I tested the voltage going into the solenoid valve and there was nothing. It reads zero. I then tested the voltage going into the Gas Pressure switch and it reads 29.4 volts. Does that mean the Fan Relay is bad? The heater is about 15 years old. I found online service manual for the FPS 40/50 226-227 series but not for this model. The model I have is a FPS 40 224 and I can't seem to find anything on it.

electric

YOu may be reading the voltages wrong. A switch ONLY indicates voltage when it is open, any other time the only way to check voltage is to measure between the terminal and a known ground/neutral/negative pole. If there is a voltage reading across the pressure switch, then it it open, and possibly not functioning. If you tested each terminal you will probably find the input side has voltage, but the outlet to the gas valve does not have any.