coleman 7956c856 pilot lights but burner doesn't

[Randy_Hardee]

I have a coleman 7956c856 i think i have run out of things to check for on this furnace. I have checked voltage at the transformer and have 28 volts, 24 volts is going to the gas control when thermostat calls for heat and 0 volts when no heat is called for. I thought the problem was with the gas control but I have just replaced it and get same results. I have checked the limit switches for continuity and they check fine. I did get the burners to light twice but havent lit since. The first time they lit the gas control was making a loud electrical humming so i shut it off with the thermostat and then turned it back on 5 seconds later and it was working fine. My mistake was to shut it off again. It has not worked since. Is this a faulty gas control valve or is this something i have just missed? There is no problem with the pilot staying lit (It is a standing pilot). The model of gas control is 7200ercs or 7956-336 or 7a3-c4f-025? Luckily it is a mild 70 degree January but i dont believe the nice weather will last for very long.

[Robert]

Hi Randy,

Remove and visually inspect the burner orifice behind the gas valve.


Spiders love it in there.


Also check burner tube for blockages.


IF ALL is clear and you have full voltage to GV on a call for heat, you'll have to get the manifold gas pressure tested.



Thanks,
Robert

[Randy_Hardee]

Thanks,
It may have been spiders in there i took the gas control off and looked inside and didnt see anything and put it all back together and i had heat...for about 45 seconds and then it shut off. I turned the thermostat off and then back on again and noticed no sound at all...not even from the damper motor/switch, I believe thats what its called (little round thing that says SYNCHRO). So I took it out and found the wires burnt and the plastic from the switch is melted in to the damper causing the damper to be stuck open (no sound from it opening/closing anymore). Looks to me that the exhaust was exiting through the intake. What does this mean? What have I done?

[Robert]

Need to have a Tech inspect the roof-jack thoroughly.

Also have him/her check what you did. The orifice is behind the gas valve and checking it shouldn't interfere with other problem.


Due to it being a gas furnace and me not being there to visually see, best to have a Tech come in.


This will ensure safety and lessen any damage done from inexperience.


What you have now may have been problem all along.



Thanks,
Robert