Atwood 8535-III Will Not Relight (Re-Light)

[Goreje]

I have a 1997 Atwood 8535-III furnace (I always connected to Shore power. I?ve never ran the trailer off of battery power) that will operate normal or correctly on the first or initial startup. The problem I have is when the temperature set on the thermostat is reached and the trailer has reach the toasty temp my wife desires, everything is great. The problem comes when the trailer cools to below the set temp on the thermostat. The furnace goes through the normal start start up procedure as the initial startup but will not ignite. Fan runs, igniter goes through it three attempts and then blower just continues to run. I can then manually turn the unit off, wait 3-5 minutes turn the unit back on and furnace will again ignite. Furnace never re-lights after initial cycle.
What would cause the furnace not to ignite after the initial startup cycle?

 

[Gary RV_Wizard]

This can be a tough one to diagnose, but since your fan is running and the DSI sparker clicks, failure to ignite is almost surely an air/fuel mixture problem. Why it only happens on a re-light cycle is a mystery but something I've seen reported here often.  Try cleaning the exhaust/air-intake tubes, making sure there is no rust flakes, insect nests, or other obstructions, no matter how small.

 

[Goreje]

Thanks for the reply. I have done that recently (3wks ago and nothing) but, will try that again and repost.

 

[Gary RV_Wizard]

There is a slight possibility that the DSI sparker get out of alignment after the first heating cycle and the spark isn't close enough to the gas/air stream to ignite, but you say that waiting 3-5 minutes is enough to re-start it.  That doesn't sound like a heat related issue, since the burner has already been off for several minutes when the re-light attempt is made.

 

[rvwanderer1]

thats tough, I keep following the course of the gas,  why wouldnt it start?

I thought maybe the thermostat is bad but if so, it wouldnt send a signal to light...
could it also be telling it to turnoff??????  turn the thermostat up a bit on the second try and see what happens.

Could the igniter be out of sync as the system gets hot?  I dont think so ,  3 minutes isnt enough time  to cool it  ( or is it)

Could the system have picked up air and gotten it into the lines????

tough one for me, an expert may know exactly whats wrong..............