From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Atwood Furnace Won't Stay Lit
Date: Wed, 03 Nov 2004 13:05:28 -0500
Message-ID: <eh6io0hu0u4vofbhadsdm7pmf4pksrlrvt@4ax.com>

On Wed, 3 Nov 2004 09:10:43 -0800, "Ulysses" <therealulysses@yahoo.com>
wrote:

>Hi.  I have an Atwood Excalibur 8500 III hydro flame furnace.  It comes on
>and the burner ignites but it won't stay lit.  It "attempts" to light the
>burner several times even though it is actually lit and then it just shuts
>down the gas (propane) and the blower continues to run (until we freeze to
>death, turn it off, or the battery runs down).
>
>According to Atwood's web site there are many things that can cause this
>problem including low gas pressure, bad circuit board, bad flame sensor, too
>much static pressure, not enough static pressure, bad wires, loose blower
>wheel etc.  I found that the blower housing had a missing screw which would
>cause low static pressure.  I replaced the screw and the furnace worked
>fine--for a while.  Then it started doing it's thing again.
>
>I have not tested the circuit board, the gas pressure, or the static
>pressure as of yet mainly because getting all of the needed equipment to do
>this would probably cost as much as a new furnace.  I have checked the sail
>switch, checked the exhaust for obstructions, checked for loose connections
>etc.

I've become somewhat of a Hydro-Flame expert, having replaced just about
everything except the case over the years.

Your symptoms are almost always caused by the flame proving circuit.
Usually because the flame isn't hitting the flame rod well enough.  You
should have a little mica window on the burner where you can look in at
the flame.  The flame needs to touch the rod by at least a quarter inch or
so.

The flame proving circuitry on the control board is rather lame (I
reverse-engineered mine so I could do some repairs) so it might be the
board.  My experience is that if any conducting gunk builds up on the
board then the flame proving circuitry becomes erratic.  Clean the board
with pure grain alcohol, scrubbing the solder side with an acid brush or
equiv and then bake it out at about 150 deg in your oven for a few hours.
If that makes it work then the board is marginal.  I put a conformal
coating (Krylon krystal clear spray) on mine to resurrect it for awhile
but that was a temporary fix.

>
>I would like to know before I yank the thing out and take it apart if there
>is a common problem with these furnaces that can cause this particular
>problem.  I found an aftermarket circuit board but it costs $150 and it says
>that after two failed attempts to light the furnace it shuts down
>permanantly.  Sound rather final.  Also sounds like maybe you need to buy
>another $150 board if it shuts down.  But at least it doesn't keep the
>blower running ;-)

That "permanently" means only until you cycle the thermostat.

If you're looking at the Dinosaur board then you're looking at the best
available.  I finally punted and bought a dino board about 3 years ago.
Best thing I ever did for my furnace.

The dino board can be jumpered to operate as you describe above but its
native mode is MUCH better.  In the native mode it tries ignition 3 times
and shuts down.  About 5 minutes later it tries another cycle of 3.  If
that fails then it retries every hour until the furnace either lights or
someone turns off the thermostat.  This feature has been a godsend to me
for keeping my rig heated when unoccupied.

Probably the most important feature of the dino board is that if ignition
fails, it turns off the fan motor!  Nothing like getting ready to use the
rig only to find that furnace didn't light and the fan ran the house
batteries down.

I'm about to replace my furnace finally, as the firebox walls are getting
thin.  I'll take my dino board with me to the new furnace.

John