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Question DetailsAsked on 10/5/2017

Furnace burner will not light

I have an older furnace (25yrs) with a piezoelectric pilot (i.e. not standing pilot). When the thermostat calls for heat the pilot lights, the igniter "clicks" three times but the burner doesn't light every time. It does light once in a while but not every time. Could this be a bad/dirty thermocouple, igniter, or flame rod?

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1 Answer


Yes to all three. Things to look at first, roughly in order I would do them:

0) eyeball (while turning up tempeature dial to cause it to light off) the ignitor to see it is producing a good bright spark - may be the ignitor is going out

1) make sure pilot/ignitor are blown clean and firmly mounted (sometimes mounting screw/bolt rusts away on older furnaces) - ignitor not tilted away from pilot, or pilot not crudded up with accumulation of rust that has fallen from the bottom of the tank or such

2) thermocouples commonly go out about every 5 years or so in water heaters, in my experience - sometimes just cleaning the metal tip with 400-600 grit sandpaper or fine steel wool will solve the problem - otherwise a cheap changeout (if uyou DIY) and at worst you end up with a spare thermocouple to hang up next to the heater for next need

3) when you pull thermocouple out to inspect/clean it, be sure there is nothing blocking the outlet of the pilot tube - rust or carbonized dust accumulation or such - blow air through the tube

4) also check the ignitor connection at the controller

5) when it does fire up (or try to) look to see the main burner flame is strong and blue and ignites across the entire burner (or all tubes) roughly same time, not weak and flickering or totally yellow or taking seconds to move from one tube to another or one side of burner to the far side, which might indicate low gas flow/pressure to the burner, which would usually indicate a failing gas control valve (and if original gas control valve, about 20-25 years is a common life for them).

OK - wait a minute here - you say the pilot lights then the ignitor clicks - other way around, don't you mean ? Gas flow starts to pilot and the piezoelectric ignitor lights the pilot, THEN once the thermocouple says it is hot enough the gas flow to the burner starts (evidently imtermittent on that). This makes it sound like the pilot is lighting reliably, just not the main burner ? If that is the case, then ignitor and pilot sound OK - problem is maybe rust accumulation on the burner keeping the burner gas from reaching the pilot (though would usually eventually ignite with a whoosh or boom as the combustion chamber fills with gas then explodes all at once), pilot or pilot back flame deflector finger tilted or turned wrong so it is not exposing the pilot to the burner gas,

Or the thermocouple (which for this type of installation, with no standing pilot, is more likely than a thermopile), which provides more power to power the ignitor and the LED lights on the gas control valve. The thermopile, when it starts going out, can sometimes allow the pilot gas to flow to light the pilot OK but not put out enough power to turn on the main gas part of the valve. Check for any blinking or pattern of LED lights (if yours has lights - not if original controller/gas control valve) indicating a fault - usually a pattern of so many blinks in a row or short and long intervals as opposed to the normally just one or two blinks every 10-30 seconds, depending on brand. Check LED color/blink codes in your owners manual.

Here are quite a few similar questions with answers which might also (hopefully) help - otherwise a Heating and AC contractor (your Search the List category) is what you need - or if a steam/hydronic heating boiler a Plumbing contractor in some areas/states.

Answered 2 years ago by LCD

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