DGAA090BDTA ignition irregularity

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Family member has the DGAA090BDTA Coleman Presidential? furnace installed in her mobile home. The unit was installed about 5 years ago. This season upon a call for heat from the tstat, intermittently the ignition created a loud metal type banging as the fuel stream ignites. It got to the point it did it every time, upon which I pulled the gas valve/orifice/burner assy and steel wooled the igniter and flame sensor. The unit functions sequentially correct. And produces proper temp rise in air stream. Gas pressures are ideal(IE, close to the top of the scale at low use hours). The hot surface unit had very little carbon on it, and functions for all practical purposes as designed. This has me stumped. Its as if there is a slight delay, randomly, of some part of the ignition process causing the small explosion into ignition as opposed to a smooth enveloping roll into burner combustion. Yet there are no adjustable parameters. Should add that when normal ignition occurs, have nice steady blue flame. When mini bomb goes off, have a much less defined flame with a quite bit of yellow flame. I found here that the unit uses a concentric supply/vent line but symptoms being intermittent don't lead there. I was surprised to see that the combustion blower motor is not what I have always seen. More or less a small evap type open frame with a plastic blade. Will this slide out through the opening or is some dissasembly necessary. The blower would seem to be the culprit, but first time diagnosing a coleman, and again first seeing this type motor/blade combo for combustion purposes. Does your experience include these blower motors little sleeve bearings dragging the rpm's down just enough to still make the switch but allow the time lag or low draft that would cause a rich ignition and acompanying ignition bang. Appreciate any input. Pat
ps. gas pressure at manifold is ok under all conditions experienced, hsi unit glowing like a refinery flare, flame sensor inspected and cleaned w/stl wool. I think thats everything. Pat
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hvac1000
Posts: 292
Joined: Wed Feb 28, 2007 9:19 am
Location: Cincinnati ohio

You have just about covered everything but the 024-27666-000 Pressure Switch
Also referred to as a combustion air switch or draft proving switch. It could be possible that this switch is sticking in the closed position once in a while allowing the burner to come on before the air is introduced to the combustion chamber. A simple test is to make sure the tubing is clear of obstruction and testing with a meter with pressure to see if the switch makes or see if the switch is stuck in the on position. If stuck in the on position your meter will show a complete circuit on the switch with the furnace and draft motor off.


web link http://www.mobilehomerepair.com/ColemanGas.html

Located left side down the page a bit.

Pressure switch 024-27666-001 this is the same switch as a 024-27666-000 but a newer number
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Robert
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Posts: 6413
Joined: Wed Feb 28, 2007 12:07 am
Location: Tennessee

Hi,

If it sticks open or closed, the burner will not light and diagnostic light will flash a code on circuit board.

Test the pressure switch for .15" wc at time of closing to send power to gas valve.

That will tell what the booster motor is doing, but again, for burner to light, it should be within range.

Take care and best wishes,
Robert
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?
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