No heat from furnace

[mel s]

Originally Posted by Mr_Fixit

I didn't realize the fan board went back that far in time. It seems like only yesterday, but then as I age almost everything seems like "only yesterday".
You are correct about the view port. I was speaking in terms of a port that did not require disassembly. I should have made that clearer. I was also visualizing Atwood furnaces so that was just stupid on my part.
On the NT series, just opening the back door where the high limits sits there is one that is easy to see.
Steve

Steve

Just for your information my SF-42 furnace is under my fridge and the whole furnace must be pulled to the inside of my coach...(there is no furnace access door on the outside of my coach).

Mel
'96 Safari

[Mr_Fixit]

If you have a furnace, you have to have a vent that opens to the outside of your camper. It can't vent to the inside. The hot water heater vents to the outside behind the water heater door.

Steve

[mel s]

Originally Posted by Mr_Fixit

If you have a furnace, you have to have a vent that opens to the outside of your camper. It can't vent to the inside. The hot water heater vents to the outside behind the water heater door.

Steve

Once again you are correct.

BTW, the chrome plated "combustion air intake and exaust vent", pictured here:

is the the ONLY thing related to my propane furnace on the outside of my coach.
There is no furnace "access door", (aka: furnace "back" door), on the outside of my coach.

Mel
'96 Safari

[Rmind]

I realized I had some confusion between water heater and furnace.
I have two panel doors outside.

I now realize the top one is for the water heater and this is where I saw the continuous flame.

The bottom one must be the furnace:

Here are the details of the model

When I set the thermostat to heat

After a while, I heard the blower running and heard the burner. I also felt some warm on the exhaust outside. But few minutes later, I couldn't feel any warmth from the outside exhaust and still blowing cold air inside.
Since I was trying to run both the water heater and the furnace at the same time in my first test, I tried this time to only run the furnace but I am getting the same result: not heat.

[Mr_Fixit]

Your furnace is failing to ignite send a signal back to the board that it has flame. The same wire that makes the igniter spark has to tell the board that things are good to go and it is okay to keep propane flowing. If it fails to do so, you will have brief ignition, then the board will shut the system down as it thinks something is the matter.

Since it goes to ignition, but fails to stay lit, most likely the problem is your module board, that thing on the left hand side facing the furnace with the clip or wires on it. It is very simple to pull that board and take it in to a dealer's and ask them to check it for you. You can do all of this yourself, if you are so inclined. Most dealers will not charge to check a board with their tester and will be happy to sell you a new board.

Steve

[Rmind]

I did more tests. When I start the heat on the thermostat, after about 10 sec the blower starts and after 10 more seconds I hear the burner running for 6 seconds then stops. About 13 seconds later, the burner starts again for another 6 seconds, and do a third a final 6 more seconds burn also about 13 seconds after the second burn.
After that I can't hear the burner starting even after many minutes.
I was able to reproduce the entire sequence twice with similar delays a 3 6 seconds burn each time.

[Rmind]

Originally Posted by Mr_Fixit

Your furnace is failing to ignite send a signal back to the board that it has flame. The same wire that makes the igniter spark has to tell the board that things are good to go and it is okay to keep propane flowing. If it fails to do so, you will have brief ignition, then the board will shut the system down as it thinks something is the matter.

Since it goes to ignition, but fails to stay lit, most likely the problem is your module board, that thing on the left hand side facing the furnace with the clip or wires on it. It is very simple to pull that board and take it in to a dealer's and ask them to check it for you. You can do all of this yourself, if you are so inclined. Most dealers will not charge to check a board with their tester and will be happy to sell you a new board.

Steve

I assume this is the board in question:

I will take it out a get it diagnosed. Thanks so much for the help.

[Mr_Fixit]

Originally Posted by mel s

Once again you are correct.

BTW, the chrome plated "combustion air intake and exaust vent", pictured here:

is the the ONLY thing related to my propane furnace on the outside of my coach.
There is no furnace "access door", (aka: furnace "back" door), on the outside of my coach.

Mel
'96 Safari

Mel,

I have probably pulled a hundred or more of those furnaces from the inside of the coach. It really gets to be fun when the manufacturer runs plumbing behind the furnace or the access opening to the back of the furnace does not line up with the furnace.

I like Suburban furnaces, but they are a pain to work on. Pulling the furnace to do something as simple as change a board is just nuts in my book.

Steve

[Mr_Fixit]

Originally Posted by Rmind

I assume this is the board in question:

I will take it out a get it diagnosed. Thanks so much for the help.

Yes, that is the board. It is mounted on a plastic mount. Undo the wing nut at the top of the mount (yes it is there) and slide the whole thing out, don't try to take the board off the mount while it is in place. They can test the board on the mount so you don't even have to undo it, until you know it is the board for sure. Just detect the clip (it pulls straight off on that one), and pull off the igniter lead.

Steve

[Rmind]

I found the following in the repair manual for the atwood 8531 furnace that seems to describe my problem and propose remedies

BURNER IGNITES BUT IGNITION SYSTEM "LOCKS OUT" AND TURNS BURNER OFF

--------------------------------------------------------------------------------

CONDITION
SOLUTION


Low gas pressure

Exhuast blocked
Combustion air wheel loose
Electrodes out of adjustment
Electrode defective
Circuit board defective

Flame sensor wire between electrode and
circuit board defective
Air leakage at gaskets
Defective heat exchanger

Set pressure to a minimum of 11"W.C. with all appliances running. Replace regulator if not obtainable.*
Clean Exhaust*
Reposition wheel and tighten
Adjust electrode according to furnace specifications
Replace electrode
Clean plug contacts. If still defective, replace. (Check on board tester when possible).
Replace wire

Replace gasket
Replace heat exchanger

[Rmind]

i found another thread describing exactly my problem
http://www.irv2.com/forums/f54/atwoo...ms-140951.html
the problem in his case was apparently low gas pressure, which he fixed by "replacing the tanks"
but another post suggested:
This is typically a sign of an obstruction in the outside air intake or exhaust, e.g. a wasp or mud dauber nest, spider webs, etc. Try cleaning the intake and exhaust air tubes thoroughly.

I believe your controller board is working fine - it goes through the start-up and ignition process fine. Just can't keep running.
__________________
Gary Brinck
2004 American Tradition; 2007 GMC Acadia
Homebase in the Ocala Nat'l Forest near Ocala, FL

[Rmind]

there was wasp nests in the enclosure of the furnace and various parts of the RV.
i guess my next step should be to check exhaust and air intake. any pointer on how to do that?
after that, i would check the gas pressure. any input on how to do that as well?

[Mr_Fixit]

Originally Posted by Rmind

there was wasp nests in the enclosure of the furnace and various parts of the RV.
i guess my next step should be to check exhaust and air intake. any pointer on how to do that?
after that, i would check the gas pressure. any input on how to do that as well?

The only wasp nests of any real concern are in the tubes that exit on the outside of your RV.

Regarding gas pressure, as I asked in my first response to your post, how does the flame at your cooktop look? Can you light all the burners? Gas pressure is actually checked with what is called a manometer, but you are not likely to own one and I would discourage you from doing anything with your gas lines. At the same time, pressure low enough to stop a furnace from firing is normally very apparent to the human eye. You will get a flame at the cook top, but it is very low.

Since you are worried about gas pressure do this. Light you burners and then fire you furnace. Since you said your furnace does go to ignition, it will require more gas than all the burners on your cook top combined. Does your cook top get very low or go out when the furnace fires? A working regulator should be able to keep up with the demand from your system. Low gas pressure, while possible, is not a common finding with the symptoms as you describe them.

Steve

[mel s]

Rmind
A propane furnace that fires but then refuses to continue to burn, (shuts OFF after a few seconds), can be caused by a faulty or dirty flame sensor.
See: Atwood..RV...the burner shuts off after a few seconds..20-30 seconds
Mel
'96 Safari