need help with carrier furnace

TJ1960

I have a carrier furnace 9200 weathermaster. This is a condensing furnase.
I am having problems with the flame. When I pull off the cover to the burners the ignator lite and gas goes on, every thing works.When I have cover on ignator lites, but no flame. My burners do not lite. I am geting power to the pressure switch and the gas valve is turning on, but burner doesn't lite. I think I am getting too much neg pressure in the burn box.
I have check all my hoses for plugs, I pulled off the inducer motor to clean it (looked ok) I replaced the pressure switch and I pulled both the intake and exhaust pvc pipes to check for blockage. I also pulled the condensor trap and flushed it. That was also ok. THis has an evaporator at the top. Should I pull the cover and clean it? Am I missing something easy?
Thanks
trichar3@rochester.r.com

rakes9720

Do you mean the burner cover, or the main furnace door?

TJ1960

Thanks for the reply
The main burner door. It measures 4" x 10 " or something like that. As long as I leave that off everything works fine. If I put that one the ignator gets hot and the gas valve turns on burn the burner doesn't light. I was think there is too much neg air blowing the gas around and stopping it from ignating. Any thoughts would be helpful.

TJ1960

Is this problem to tough that noone replys or does my breath stink

rakes9720

Sorry to keep you waiting. There is a small pressure reference tube running from the burner box to the gas valve and pressure switches (probably on the right side). Check to make sure it is clear. This makes sure that the gas pressure and vent pressure are measured with respect to the burners. Blockages prevent the proper pressures when the cover is on.
Also check the combustion air intake PVC pipe is clear all the way through. You can check this by first making sure the burner cover is on, then turn on the furnace so the inducer is running. Go out side to the vent pipes and make sure air is blowing out one of them and sucking in the other one. A blockage prevents air from getting to the burners. No air = no flame.

There is also a combustion air "restriction" disk that should be installed before the burners to prevent too much air from entering. See the instruction manual.

TJ1960

Thanks for the reply.
I pulled off the pressure tubes and blow into them, no blockage. I also took off both the intake pve and the outtake and made sure there are no blockage. I did not seen a disk.
Anything else I might have missed.
I pulled off the inducer motor, everthing looks ok. I have a new air filter in. When I do a circuit board test everything checks out.
When I do put the burner cover on, it sounds like the gas is there (and there is power to the valve) but it does not ignate. Is it possible I have too much neg. air pressure?
Anything else you might think off will be helpful, I am out of ideas.
thanks

rakes9720

Can you measure the gas pressure? If it is very low, the negative air pressure might be able to suck the flame out, but the negative pressure is not wrong. It is determined by the inducer blower, motor rpm, and heat exchangers. Gas pressure, on the other hand, can be adjusted, but it must be set exactly according to the installation manual. Too high, and you overheat the heat exchangers, too low and you cause condensation in them.

With the cover off:
Do the flames look smooth, do they stay against the burner without lifting off, are the flames the same diameter as the burner or are they thin?

Try turning it on with the cover off, and then put the cover back on.

rakes9720

Do you have the exact model number of the furnace?

TJ1960

I will get the model number. I have been shoveling snow all day.
I should mention the furnace worked fine until my mom's sump pump quit working. The basement had 5 inches of water. I replaced the filter and nothing else seemed wet. It might be a coincidence.

rakes9720

Water would do it if it got to the control board. If any part of it was wet, seemingly random problems can occur. It's still strange that it only works with the cover off and the board is more than 5 inches off the ground, but check for signs of water.