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Question DetailsAsked on 2/26/2017

My boiler pilot light continues to go out after having the thermocouple replaced 2 X, and the gas valve as well.

The pilot stays lit anywhere from 20 minutes to several weeks. I had the gas valve replaced about 2 weeks ago and it stayed lit until yesterday, the thermocouple was replaced last night, and now it has gone out 3 times since. The tech found the new gas valve to have a fault, as it won't switch to "off", only pilot or on, but I don't think this has anything to do with the pilot going out. (the valve will be replaced this week again) The pilot flame when lit is blue and consistent. Everything I've researched leads to replacing the thermocouple and gas valve, and we have had both replaced, so now I am at a loss trying to figure out what else could be causing this.

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2 Answers


OK - I think I am about exhausted by all the intermittent operating water heaters and furnaces recently - below are links to a number of previous similar questions with answers which cover most of the possible causes. Unfortunately, because this is a sporadic thing that makes it tough (and expensive) to track down.

In your particular case, while it might be defective thermocouples (some imports are having very high failure rates), more likely a self-resetting overtemp device, an eductor fan/damper or pressure/damper position sensor failure (if high-efficiency induced-draft unit), or backdrafting causing tripping of a firebox overtemp sensor. Check your manual for the types of sensors/safety devices your system has.

Obviously, assuming no sensors have been replaced in this process, only a self-resetting sensor (many types are - see manual) could do this and then have the unit work again without replacing the sensor.

There have also been a lot of comments in professional blogs about water heaters a few years old going out because the oxygen sensor trips out due to inadequate combustion air. Some brands of water heaters, designed by the proverbial 100 monkeys undoubtedly (retrained from typewriteres doing shakespeare to designing consumer appliances), have an air filter or screen on the BOTTOM of it (where it cannot be seen or gotten at) - if this plugs up with lint/dust, unit can shut down after it runs for awhile. Cleaning usually requires compressed air because usuallynot accessibel for brushing.

Backdrafting due to wind, or an improperly configured duct system resulting in boilear/furnace backdrafting into the water heater, can also blow the pilot out - but if the ducting and the boiler/furnace is unchanged, not likely the cause if this is a new problem. Wind correlation with it going out should be relatively easy - determining whether furnace/boiler is causing it would mean shutting that down for a proloinged period to test - not something you can do this time of year probably.

One other possibility - if it is real cold where you are and this is a high-efficiency unit - is the exhaust pipe through the wall frosting up and blocking the exhaust flow.

Two things I would have the plumber do while there (you would have to let him know in advance on the pressure test - might not carry a manometer or ladder with him) would be to check the flue for blockage (either ladder to roof to take off rain cap and shine a light down inside, or partly disassemble where the flue turns vertical near the water heater, whichever is easier). I would also check the gas pressure to be sure you are getting adequate pressure and gas flow - I ran into a head scratcher like this once where it turned out the gas shutoff valve (on the gasline near the water heater) had a loose knob on it - so while the know turned to OFF, then back to on, in turning to on the valve had only turned a bit while the knob turned 90 degrees - so not enough gas was getting through. Course, if when it works the burners ignite fully and burn cleanly and strong all around then this check is not necessary.

One other possibility I have hit a couple of times - many of the thermocouples come with a saran wrap or stick-on thin plastic cover over the "button" on the end of the tubing - he should take out the thermocouple and check that there is nothing like that stuck up in the receptable - also that the contact in the gas valve (which will be a new one now I guess) is intact for the button to make contact with.

Nother VERY common cause of thermocouple issues - many techs do not know the right way to put one in. You do NOT push the screw commector up into the hole with the button end - tht can result in the button end getting hung up in the threads and not reaching fully to the contact, making for intermittent or no thermocouple function. The right way is to slide the connecter nut back down the tubing, straighten the last 4 inches or so of tubing, and gently slide the button end up into the recess on the gas valve, feeling the way to be sure it seats securely all the way up in, not just hung up on the threads. Then, holding the tubing securely up in place, slide the connector nut up and thread it into place, securing firmly but NOT torquing on it. Excessive tightening - reefing on it at ALL - can flatten the button out and cause it to ground out to the housing - fully or intermittently.

Links to previous similar questions with answers :

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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