furnace not starting

[freezing]

i have a coleman furnace model# DGAA090BDTA and the code light is flashing 3 times saying combustion air switch stuck closed). when i the thermostat tells it to turn on i hear a humming noise that goes on for about 30 seconds till the furnace stops trying to start. So i read about how the valve might be bad so i tested it for resistance and i show that with no vacuum applied it reads open and when i apply vacuum to one of the ports it opens. so after a played around with it i hoked it back up and now i get error code 4 and it doesn't hum anymore unless i unhook one of the wires that gos to the switch. Anyone have any ideas? i'm leaning towards the switch being bad but i can't afford to just throw parts at it.

[Robert]

Could be the pressure switch, the booster motor, air pipe connected to motor, tubes connected to switch,,,,,,,,so check them to see if they seem ok.


Would need to know water column pressure at switch to know if switch is good or not.


Can give parts ordering info when/if needed.



Thanks,
Robert

[freezing]

I think the pressure sensor might be bad. I talked to a friend of mine who is a tech and he had me turn the furnace on, unhook one of the wires from the pressure sensor and then plug it in a few seconds later and it started working! but now its not hiving hot air, just lukewarm. I did take the burner off and i had brushed the carbon deposits off the burner, igniter, and heat sensor. Is it possible that i've messed up the heat sensor so its not putting out enough heat?

[Robert]

What did you clean it with ? Unplugging the wire just resets it and really is not a good indicator of what it is doing.


If everything I named in first reply seems ok and working, then it is most likely the pressure switch.


Thanks,
Robert

[freezing]

i just used some sandpaper to brush off the carbon. would the pressure switch affect the heat output of the furnace. I have it running right now and i have a strong totally blue flame but the heat coming out of the vent is only slightly warm.

[Robert]

Hi,

Use steel wool, not sand paper to clean the flame sensor. The pressure switch is for ensuring the booster motor reaches proper level of speed to clear combustion chamber of any residual gases before allowing current ignition cycle.


Inspect the burner tube and burner orifice for spider=web-bugs-etc.. .


Thanks,
Robert