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Question DetailsAsked on 11/1/2017

Pilot light works and stays on but burners don't lit and I do have all power and new Thermostat

checked all devices on heater and all check out with power

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1 Answer


Not sure if you mean new thermostat for a furnace/boiler, or thermostat repalcement on a water heater - but similar situation so most of below, which assumes a furnace/boiler, applies to water heaters too. Suggested possible issue below are in no particular order.

I am assuming you have the control knob all the way to ON position (may have to depress a stop/lock button or push down on the the control knob to allow it to rotate from PILOT to ON position.

And that you have adequate fuel supply (sometimes wiuth low fuel level in oil or tanked gas supply the main burner will not kick on, or will only receive a trickle of low-pressure gas which does not look like it is burning).

I am also assuming (if you have electronic control unit as most modern ones do) that the LED's are not displaying an error code (either by color of LEDs illuminated or a blink pattern) - check your owners manual (if you have it - or find at manufacturer's website) on what the codes mean.

Otherwise, check owner's manual debugging guide - may be a safety interlock is preventing it from firing up. Could be a sensor tripped out which is not self-resetting, due to overtemp in the firebox, flame rollout, inadequate fresh air supply (blocked filter), blower not kicking on, or if a high efficiency condensing unit perhaps the electrically controlled flue damper is not opening or the eductor fan is not kicking on, which can stop or shut down the main burner.

Could also be, if electronic control, you probably have a thermopile (which provides more power than a thermocouple) rather than thermocouple to detect the pilot flame and provide the power to the control module. Sometimes when these are making poor connection, are grounded out to the appliance frame along their routing, or are getting old and weak, they can allow the pilot to burn but not have enough juice to allow the main gas control valve to open. Thermocouples can rarely do this too - though in my experience their failure usually prevents pilot flame burning too. Thermocouple/thermopile failure is probably one of if not the most common cause of failures like this.

And of course check gas or oil shutoff valve is open - sometimes a slightly open will let the pilot burn but not give enough gas for the main burner part of the control to function, or be putting out so little gas that you can't see the main burner flame. Rare but possible.

Rarely - usually only on quite old units - the gas control valve springs in the gas control unit will get weak and not allow the gas control valve to work right, so it has to be replaced - but most gas control valves outlive the appliacne they are on or are replaced because of valve stem gas leakage before that happens. However, with cheaper import units that can be a factor to consider.

And of course check the thermostat is programmed for proper temp (many factory default to about 60-64 degrees if they go dead or have not been programmed, some to OFF), that the thermostat is actually getting power from the furnace in the leads, and that it is set to HEAT or FURNACE setting, not FAN (in which only the fan will run) or to A/C or COOL. (Or similar titled settings - usually a slide switch on simpler thermostats, programmable buttons or settings on higher-end and remote control electronic ones.)

And that the thermostat, if has separate wires for A/C and furnace as is common, is hooked up right - accidentally hook the furnace leads to the A/C screws on the thermostat and vice versa and while you think it is calling for heat (like probably now) it will actually be calling for A/C operation. Some electronic thermostats are also polarized - right color wire of each pair has to go to the right screws on the back.

Also - with many or most electronic thermostats - batteries have to be in correctly and have a good charge or it will not work - or may revert to the 60-62 degree or OFF default temperature setting, with some models.

I have also heard of electronic thermostats that fried internally because they were connected live to the wiring or the wiring was swapped. If you are home-handy with a volt-ohm meter you should be able to dismount the thermostat and let it hang and play around with settings to find out if it is providing electrical connection between the inbound and outbound leads for the furnace or boiler, at the thermostat, when wired up. Then check same thing at furnace - is it getting a return signal from the thermostat ? Sometimes during thermostat or some zone valve changeouts having the system live can fry boards or valves.

If these suggestions did not solve it or some of them are beyond your electrical testing ability level, then you need a Heating and A/C technician (your Search the List category). If new thermostat was installed by one, presumably call the same one back to make it work at no added charge for you.

You can also find a lot of previous similar questions with answers about possible causes, some in more depth, in the Home > HVAC and Home > Plumbing links, under Browse Projects, at lower left. [Plumbing category for water heaters, but controls for gas/oil water heaters and furnaces are commonly pretty similar].

Answered 2 years ago by LCD

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