Water Heat Vent Fan runs constantly

VIN442 · #1 Nov 7, 2006, 09:03 AM

I have a 75 Gallon Natural Gas State Select Water Heater. Model # PR675NRVTZ. It has a fan on top that vents the gas to the outside. Normally, this fan only runs when the water is being heated. Now, the fan runs constantly eben when the water is not being heated. I do not have the wiring diagram, however I found a "switch" that if I unplug it will turn off the fan. The switch has 2 terminals. It has 4 screws that bolt it to what I believ is the gas valve. I ohm'd the switch out and it reads 0.00 (closed). Before I go and replace it, does anyone know if that switch is supposed to read 0.00 only when triggered and open line when not? If so, what triggers that switch and how can I test that other part? Thanks, Vin

labman · Nov 7, 2006, 09:32 AM

You may be ohming the wrong switch. There could be a limit switch further into the combustion area that is giving a false over temperature reading. Or a relay could be stuck. Do some more checking before replacing a part that may be doing its job.

VIN442 · Nov 7, 2006, 12:58 PM

Quote:

Originally Posted by labman

You may be ohming the wrong switch. There could be a limit switch further into the combustion area that is giving a false over temperature reading. Or a relay could be stuck. Do some more checking before replacing a part that may be doing its job.

Labman, There is one other sensor at the top of the unit in the "Gas Hat" where the fan is. It also ohm'd out at 0.00. I unplugged that sensor and the fan keeps running. Any idea what that reading is supposed to be? Thanks again, Vin

labman · Nov 7, 2006, 01:36 PM

If it is a switch, it should either be zero or open. Thermisters are becoming more common. I asked the moderator to move this to H&C where our top man will see it.

VIN442 · Nov 7, 2006, 06:59 PM

Quote:

Originally Posted by labman

If it is a switch, it should either be zero or open. Thermisters are becoming more common. I asked the moderator to move this to H&C where our top man will see it.

Labman, I read this posting (http://www.askmehelpdesk.com/plumbin...ter-33192.html) and lightly tapped the switch that was shown and the fan went off as it should. I then ran the hot water, waited and the flame and the fan came on and then went off as it should. I would surmise now that the solenoid is defective. Thank you for your help, Vin

NorthernHeat · Nov 7, 2006, 07:13 PM

Yes, it sounds like the blower relay stuck closed. This is caused when the relay closes the is an electrical arc which pits the contacts and can cause them to weld them selves closed. When you tapped it it become unwelded. Now that the contacts have become so pitted and worn this will happen more frequently. It is better to just go ahead andget it replaced. It is used so a low voltage demand can be sent to the aquastat and gas valve, and to close the circuit for the blower which is 120 volts. If this helps you narrow down where it might be located for sure.