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Question DetailsAsked on 7/2/2016

I have 2 gravity furnaces controlled by the same thermostat. The main burner on one won't light. Why would that be?

The pilot lights are on for both furnaces. The other works fine. Any suggestions would be appreciated.

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1 Answer


Here are some linked responses to similar questions with answers, which might help - first one has a bit of a checklist of possible reasons - in addition to the obvious one about checking a valve or power has not been shut off to that one furnace, and that the gas control valve is set fully into the ON position from PILOT - try turning back to PILOT then back to ON (or appropriate similar position titles) -

One other thing I may not have mentioned in other posts - most newer or more recently worked on furnaces have a secondary shutdown sensor - which shuts off the power to the gas controller if it exceeds a certain preset temperature - mounted in the upper firebox area or flue (or both) in forced-air furnaces, cleamped to the outflow main pipe in hydronic/steam heating boilers. In hot air devices commonly looks similar to this - may be metal and plastic or all metal - (the third photo - with part number 610046, though can look quite different - but almost always has one black or red or blue wire both in and out from it -

Overtemp secondary thermostat on boilers commonly looks like this and commonly called an aquastat -

This device commonly intercepts the circuit leading from the control box to the gas controller and prevents current from flowing past it is overtemp - automatically resets (when working properly) when it drops back to acceptable temp. IF it fails, could be cutting the circuit even though not overtemp.

BTW - unless both furnaces feed the same ducting or hydronic heating system (which would be an ODD arrangement and not very energy efficient), having them come off one thermostat is odd and not very effective - if feeding different distribution systems they should run off separate thermostats located to register the temperature in the zone that furnace serves.

Answered 3 years ago by LCD

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