8351-III furnace, block exhaust & it will ignite

[Ramblin'Jay]

My furnace stopped working - I can hear it try to ignite, and sometimes it does, but immediately goes out.
I cleaned all the connections. I blew compressed air thru the propane line. I replaced the orifice; the burner is fine.
The valve that lets propane thru seems to open, and when it does I can blow thru it w/o issue.
I can smell propane; so it's making it thru. (I have no way to test the pressure/volume of the propane)
While I was giving up; I stumbled and leaned against the exhaust blocking it -- and the furnace ignited.
I repeated this with a block of wood - mostly blocking the exhaust, started the furnace, and boom - it ignites every time.
If I slide the wood away slowly to slowly open the exhaust, after about a 1/2 inch, the furnace quits.

Any ideas?

 

[TonyL]

Best guess as you say it fires when flue is restricted is wrong propane to air ratio. You say you replaced the orifice (jet) so I would be looking at restrictions in the fuel supply or in the regulator. Are the other propane appliances working correctly?
TonyL

 

[Ramblin'Jay]

Hi, thanks for the reply.
Yes, fridge works, water heater works, stove works. But this might be just in my head, the water heater sounds louder, but otherwise everything seems normal.

I did use an air compressor to blow air thru the line. But don't know how well it came out the other end.

So maybe I should check the line again, and replace the regulator?

 

[Gary RV_Wizard]

Those furnaces are very sensitive to air/fuel ratio and any sort of debris in the air intake & exhaust tubes will prevent reliable ignition. Common problems are rust flakes, insect nests/webs, and mud dauber egg cases. You can try vacuuming the tubes, but the best method is to take them out and clean.

It doesn't sound like you have a gas feed problem, so I'd leave the gas line & valve alone. They seldom give trouble.

I've written a Furnace article for the RV Forum RESOURCES library - see https://www.rvforum.net/resources/furnace-troubleshooting.225/

 

[Ramblin'Jay]

Sorry for the dumb question -- but: So take the burner assembly out and vacuum out the void that it's in? it looks completely clean; but I can do that.
thanks.