Probably the pressure switch, or a stuck relay.
Check the hoses for obstructions and moisture.
Try tapping on the pressure switch while running on to see if stuck. Never blow into the switch, as it will surely ruin it or at least knock it off setting. If you had a digital manometer, it could be checked, but price on a new switch isn't bad.

Thanks. I ordered a new pressure switch, should be here on Thursday. I also tried tapping on the switch this morning and it kept running.

"I assume there is some sort of switch or sensor that detects when the burner fires up, thus allowing power to the vent."

Close. When the thermostat calls for heat the blower kicks on. When the blower creates a draft that a sensor picks up the burner kicks on. Reverse this scenario for the shut down. Burner off, then blower off.

Got it, thanks.

I have located & downloaded the complete service manual in PDF format. If you would like a copy send me an email and I will return it.

From examining the wiring diagram it shows that the fan motor is controlled by a gas pressure switch on the back of the thermostat unit. When the thermostat calls for heat the gas solenoid opens sending gas to the burner. The gas pressure switch senses the gas pressure to the burner and turns the fan on.

You state that your fan is not turning off. I would suspect that the "Normally Open" gas pressure switch is stuck in the "Normally Closed" position.

You may find that a couple light taps on the switch would release it and turn the motor off, however in most instances when these type of switches are sticking it is indicating that the internal contacts are pitted from arching and it would be best to change the switch.

Great, thanks. I was able to download a copy of the manual. I tried tapping on the air pressure switch without any luck. When I went home, I lit the pilot and turned the vent on and the blower ran all night. I will have to look for the gas pressure switch, not sure where it is located. That may be a job for a plumber I think. New air pressure switch will be here tomorrow, so I will replace that and see what happens.

I suppose I could just shut the fan off at night, but I am one of those nervous nellys and am afraid the basement may collect gas, so I have been just turning off the gas after we take showers in the morning and re-light the pilot in the evening.

Other possible items are stuck relay as stated before or a failing thermostatic gas valve not entirely closing and leaking off enough pressure to keep switch closed keeping fan running.

Thanks.

Just had the plumber come by. He looked at it and said it is more than likely the gas switch, but he is not sure his boss will want to mess with a Kenmore unit. I wouldn't know until Tuesday. They would install the part if I were to get it right from Sears. It looks like the burner and gas valve are all 1 assembly.

Since I am 1 month over my warrenty period, I will need to pay for everything. He mentioned getting a new heater, and was really pushing a Bradford White unit, 50 gallon power vent for $1200 delivered, $1700 installed. I thought that was a little crazy, even if I am going to install it myself.

I have a few options, but if the part and burner assembly is probably a good few hundred dollars and my unit is now out of warrenty, do I just buy a new heater? I have some number crunching to do, but putting that aside, what are some decent name brands you guys recommend? Is BW a good name? How are Rheem units? I believe this unit cost me under $800, but that was 6 years ago.

He also mentioned a RINNAI tankless installed for $2000, but then the tax credit is 30% through 2010, so another $600 back. $1400 installed.

I'm not real familiar with this power vent unit... it just seems a little bit extreme that a broken switch would equal a new unit.

If you are springing for a new water heater, consider the tankless. I'm not sure how we get to $2000 when you are replacing a power vent unit... the electricity and such is right there already. I think because there is a tax credit, people are jacking up the price.

Anyway, the tankless unit will save you about 38% on your water heating. A tankless is a good alternative to a power vent unit.

Thanks. Yeah, I think I am going to call Sears first and see if I can get some idea over the phone, but I am sure they will insist someone come out to look at it for $150.

I really do not know anything about tankless, so I have some reading to do. I assume it still needs some sort of power vent to eliminate the gases?

Yes, it uses a fan to vent the gasses. It's all contained in a small unit.

I have a Rheem unit that was $1000 with the vent kit. I was looking at a power vent when it became clear that the tankless was a much better way to go.

Thanks. Do you have it maintained yearly to make sure there is no scale build up? Sorry for sounding silly, just trying to read up on some pros/cons and that was a con I saw.

I don't have hard water. I've had it a couple of years and I probably should run vinegar through it... It's working great though.

By the same token, you are supposed to drain your regular tank heater once a year also.

Good point, drain tank once a year.

Well, I had the blower off starting yesterday afternoon and did not turn it on until this morning when the wife wanted to take a shower. Fan went on and so did the burner. It ran for 15 mins or so and then shut off. WTF? It is back to normal now. Must be something sticking, but it cycled again when I took a shower and went off.

I may have dodged a bullet for another few days, maybe weeks which will give me time to learn about the tankless units.

Look at the cost benefit of going tankless, the cost will be much higher and payback may be years coming. May need a larger gas line, will need a new vent stack, etc. For the two of us the cost for tankless was not justified.

I installed a 50 Gallon Bradford White, the tank cost was approximately $400. Normally with labor in the area it costs around $800 to install. Power vents not normally used where I reside. Guess with power vent cost to install $1000 assuming electrical is near. Tankless quotes here are between $3000 to $4000. Most gas lines need to be changed to accommodate gas volume requirements and the vent needs to be changed to handle the increased heat.

Not a fan of Sears or their water heaters, Bradford White was rated the best by Consumers Reports. I am very satisfied with Bradford White.

Looks like turning on and off reset the blower.

My two cents.

Thanks.

Yeah, still working great, but who knows for how long.Those are great points about gas line size, will have to look into that. If we end up replacing and sticking with a tank, it is good to know that the plumber recommended BW and you state they were #1 by CR.

"Look at the cost benefit of going tankless, the cost will be much higher and payback may be years coming. "

Wow, I just love the generalizations and the FUD.

Did you miss the part where the OP stated that he got a quote for the tankless, installed, and it was $300 cheaper than the power-vent unit after the tax credit? Presumably that installed price includes the gas line and the vent.

A situation where the owner is considering a power-vent unit is an ideal candidate for a tankless.

I knew that I would get a sharply worded feedback, guess this forum belongs to a select few. Tankless is not the answer to all applications.

stash-hdy,

It's not that the forum belongs to a few, it's that you ignored all of the facts and posted FUD instead. That should warrant a sharp response.

There are times when I have said to people don't use a tankless. This isn't one of those times. This is an ideal case to consider a tankless.

pdog,

Did you need to replace the tank? Just curious.

Actually, it has been running normal for the past month until yesterday. Kept running in the morning, so I hit the fan shut off switch. I will click it back on this morning. This generally fixes the fan run on for a while. Still not sure what is causing it, but am trying to get the most time out of this since we don't really have the $$$ to replace it this moment.

Hi pdog,

I have a problem with my Kenmore water heater, Power Miser 8 and the problem seems the same as you have described in your posts. The fan was always on for a couple of days before I realized it. I googled and found your original post. I read these posts and checked the air pressure switch and air tube. They looked normal. Then it turned back to normal and stayed for normal a about a week. But now the fan stays running again. My guess is the Gas pressure switch does not work well.
My question is: did you fix your water heater and what is the problem or you did replace it.
Any help would be greatly appreciated.

John