State Industries 75 gal gas water heater, blower motor never shuts off

[Snoggo]

My water gets heated just fine, no problems in that department. But the powered vent motor never shuts off. I have tried turning off the whole system for a few days, and disconnecting the air pressure tube and blowing into it, but no go - but 5 minutes after I turn on the water heater, that blower motor comes on and stays on.

It did this a year ago, and after a week of turning it off every night, it started working properly. Six months ago it came back and I haven't been able to get it to shut off since.

As I said, the unit is working and running properly, aside from this annoyance -- and I'm sure it is costing me money, I know it is giving me a headache, and it will ultimately burn out the motor before its time.

Please help!

I asked the company for a manual, and they emailed it to me, but it does not include a detailed wiring diagram. I tried to upload it here, but the website says it is too large. I have checked other forums, and the general consensus is it is probably a sensor of thermostat problem - but obviously not one that causes the whole system to shut off, just one that causes the motor to fail to disengage. I would really really really like to DIY, replace whatever sensor or temperature gauge is faulty, but I just can't find one. Looking at the unit, I don't see anything to replace on the outside. It is over 20 years old, and every plumber I have called says just buy a new one... but I call me cheap, call me a DIYer, call me what you like, but I really don't want to replace a working water heater because some $3 sensor is failing to disengage the motor.

Here are all the details from the unit itself:
Automatic Storage Water Heater, suitable for water (potable) heating and space heating
Model number: PRV 75 NRVt1
natural gas
75,000 input
MFD. by State Industries, Inc., Ashland City, TN USA
Capacity: 75 US gal

Please offer your suggestions! It has been 25 years since I used a multimeter, but if I need to check voltages or whatever, I will brush off my old skills and try to do it. I just don't know what or where to measure.

If all else fails, I suppose I will buy a fancy new tankless system. But I don't want to give up on the old system without a fight. Thanks in advance.

 

[Phog]

I am not a plumber. Perhaps some plumbers here have seen similar issues and can chime in with help, but my guess is that this is something where most would just install a new unit, either under warranty or at owners expense (for older tanks like yours).

Without a wiring diagram you're going to have an extremely difficult time fixing this issue. I agree with the other people you've spoken with who suspect a flue thermostat is giving a false reading. But where exactly this sensor is located, what type of sensor it is (thermocouple, RTD, etc), and where it connects in the wiring harness are all information you don't have. It also might not be the sensor itself but rather a corroded pin in a connector that has high contact resistance.

If it makes you feel any better, 20yrs is about double the average life expectancy of a gas tank. Maybe you don't mind playing the lotto of "will this fail during Christmas Eve dinner with out-of-town family staying at the house all week", and i sympathize with your instinct to repair what's still working... But it really is probably time to think about your next water heater, even if you do get this one fixed. Good luck.

 

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[Plumber69]

There may be a limit switch thats stuck open

 

[Master Plumber Mark]

If the unit is 20 years old you just need to buy a new one...

replacement parts are probably not available