Atwood furnace

[49ford]

I have a trailer with a Atwood 7920-II furnace. I have worked it down to the point it gives a code 3 for ignition lockout fault. Does anybody know what that means or what might be causing it? I am guessing it is an igniter problem.
It previously gave a code 1, which was limit switch or air flow problem. It turned out to be bugs inside the chamber I knocked loose by smacking the housing. Now the fan runs, but makes an low pitched click every so often then shuts off.

[2DHoop]

Glad to hear about a problem getting fixed and an education along the way.

I've had ants, birds, wasps and moles rob my wallet and fry my nerves many times.

[CavemanCharlie]

49ford wrote:
I got the furnace working. After failing to take it completely out of the RV. I started taking every screw I could reach out of the furnace. On the right side of the furnace is a small spiral housing that eventually came off and out. It had the fan motor and squirrel cage fan and a small blower fan on it. On taking it out, I saw a mud dauber nest completely blocking the inlet. Cleaned it all out, checked the sail switch while I was in there. Put together and it worked.

Cautions. Don't try to pull the unit out from the inside by cutting around the exhaust flange. You will have stuff to fix.
Put a BUG SCREEN on the furnace outlet, not door screen, a slightly larger mesh screen for the RV furnace.

I hate mud daubers! But I know a heck of a lot more about furnaces than I did a month ago.

Thanks for letting us know that you found the problem.

[49ford]

I got the furnace working. After failing to take it completely out of the RV. I started taking every screw I could reach out of the furnace. On the right side of the furnace is a small spiral housing that eventually came off and out. It had the fan motor and squirrel cage fan and a small blower fan on it. On taking it out, I saw a mud dauber nest completely blocking the inlet. Cleaned it all out, checked the sail switch while I was in there. Put together and it worked.

Cautions. Don't try to pull the unit out from the inside by cutting around the exhaust flange. You will have stuff to fix.
Put a BUG SCREEN on the furnace outlet, not door screen, a slightly larger mesh screen for the RV furnace.

I hate mud daubers! But I know a heck of a lot more about furnaces than I did a month ago.

[Passin_Thru]

Is it the Solenoid. They get weak and kick off.

[49ford]

Still not lighting.
I ordered a manometer to check gas pressure. Got impatient and bought a gas regulator and installed. Nice fat blue stovetop flame but no furnace lighting.
I checked ignitor gap, 1/8 inch not 1/4, shined up probes. I even took a video of the thing arcing. If I can figure out how to get it off my phone.
Not recommended, but, with ignitor out I turned on furnace long enough to get my ear down by the unit and you could literally hear the gas going into the burner. I only ran it several seconds, not a full cycle, and then turned it off.

[LVJJJ]

Could just be that its an Atwood

[CavemanCharlie]

Did you ever get the problem fixed ?

[49ford]

I pulled burner out, cleaned out chamber area and round tube beside it of ladybugs. I checked burner, it was clean. I pulled orifice out of gas valve unit, it was clean. I applied 12 volts to solenoid and it snapped and let air go through. I had circuit board tested, it was ok. There were no visible cracks on igniter porcelain, it had a brown area on it.
Now what?

[49ford]

Ok. Monday and I got the ignitor out. Its gray not sooty. It has a gap about 1/4 inch between electrodes. Is that right? I messed up and lost part of white hi temp gasket so am dead in the water for now. Looking in the chamber I saw nothing. No mud wasps or critters. I really didn't want to but is it necessary to completely dismantle the gas chamber assembly to fix these things?

[49ford]

thanks for explanation. It'll be Mon afternoon before I can get to the rv to work on it.
thanks again

[Old-Biscuit]

Ignition lock out..flame failure

Fan comes on and purges combustion chamber....at same time it is supplying room air flow. That causes sail switch to make up (air flow moves 'paddle' which closes micro switch)....DC Voltage then goes thru sail switch to/thru limit switch and to control board.
When control board gets DC voltage it then sends DC Voltage to 'spark electrode' circuit and to gas valve solenoids.....gas valve opens for 6-8 seconds allowing gas flow to spark electrode.
High voltage spark ignites MAIN FLAME which then generates a miliamp signal (flame ionization/rectification), This miliamp signal goes back to control board to 'PROVE' flame lit. No signal/control board drops DC to gas valve shutting down gas flow and Ignition Lock Out (3 attempts)


You had 'airflow' issues due to low airflow/sail switch not making up......kinda cleaned obstruction so airflow increased/sail switch makes up

Now you have flame failure due to:
*Spark electrode dirty/soot-carbon .....high voltage can cause spark but that miliamp signal can not over come the resistance and go back to control board via spark electrode circuit
*Spark electrode not in direct path of main flame...must be 'engulfed' to generate miliamp signal path
*Ceramic insulator on electrode is cracked---miliamp signal goes to ground
*Spark electrode high voltage wire connection is dirty/corroded/loose...miliamp signal can't flow back to control board
*Main flame lights off but flame is obstructed due to dirty/clogged burner slots


Need to clean up burner area, spark electrode/circuit and combustion-exhaust chamber

http://bryantrv.com/docs2/docs/hflamefurn04.pdf

[rrupert]

Two things to check is the supply voltage and the LP gas pressure. If you can hear the igniter sparking it is probably not that or the control board. I had that problem just last month and it turned out to be the pressure regulator at the tanks. Oil from the gas can deteriorate the diaphram and mess up the pressure. I had the same problem a few years ago with the fridge. It was igniting the flame but the flame was too low to fully heat the chimney.