Gas will ignite as normal run for a minute or two then shut off gas and pilot light, then fan will come on. Have changed thermocouple, clean pilot assymbly, blew off heavy dust on fan switch sensors. Problem does not occure when I have access door open and watching flame only when the metal door is closed.
You need combustion air to keep pilot and gas burning. You should have louvers or vents on that door. If you don't, you may have the blower door and combustion air doors switched. I have been on a couple services calls where the customer had reversed the doors without realizing it.
SOURCE: intertherm furnace explosive burner ignition
You absolutely must stay with Therin's recommendation. You do have a block or a cracked heat exchanger and this is nothing to toy with. Carbon Monoxide is very serious and dangerouos. All of your new furnaces should be power ventilated and you need to retire that one.
SOURCE: Rheem gas furnace pilot lights, but no burners or fan
If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.
SOURCE: weak flame in intertherm furnace
Ok, so we can rule out the ignitor. (standing pilot systems can develop problems that will cause such symptoms.)
Check the gas pressure, make sure you are getting 11 to 14 PSI.
You need that after the regulator all the way to the manifold.
Also check for cracks in the heat exchanger, causing the wind from the blower to distort the flame pattern enough to cause improper contact with the flame sensor.
Make sure the burners and flame channels are clean of rust, and dibri build up, allowing the flame to properly contact the sensors.
SOURCE: pilot light wont stay lit.want to clean the orifice
You shut off the gas.
Shut off the power.
Remove the gas valve and burner assembly.
After cleaning and reassembly you must purge the system and use a gas leak detecting soap to verify no leaks before turning on the power.
SOURCE: gas furnace, main burner ignites the other 2 don't. then shutdn.
Between the burners is a small rail in the pressed steel where the flame runs across to light the other burners and it sounds like there is dirt or debris in the step over rails causing the problem. You will need to use some compressed air on those rails to clear them out. Once the rails are clear the other two burners should ignite right away.
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