Atwood Furnace in Jayco Baja

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pg2676

Member
Joined
Sep 12, 2010
Posts
5
Ok, this one has me stumped...
My Atwood Hydro-Flame 7916-II will start, run, heat the trailer, but once it goes through its heat cycle and shuts down, it COMPLETELY shuts down and must be reset manually in order for it to fire up again.  This is a real pain when you have to get out of bed on a cold night at 3am to restart the furnace!
The error code the board is giving me is "Air Flow/Limit Switch" (one flash).  I have made sure the intake is clear and I have installed a new limit switch and it still gives me problems.  Here's the stupid part....

I pulled the grill cover off yesterday, and detached the board panel from the frame and let it hang still hooked up.  The furnace ran fine all day, starting, heating, shutting down, and restarting the way it's supposed too.  I left it on overnight and it was still running fine today.  I reinstalled the panel and grill cover today and guess what... it's back to its same shut down problem it had before!!

I'm at my wits end with this and I don't want to take it to the local dealer to have them "try" to fix something I'm sure can be solved in my driveway.

Any ideas would be appreciated.

Thanks.
 
Update...  Removed the grill cover and left it on.  Its been running fine since.  WHAT GIVES??
 
Update #2... Running fine all last night and today.  Put the grill cover back on and it shut down on me!  AAAARRRRRRRGGGGHHHH!!
 
pg2676 said:
Update #2... Running fine all last night and today.  Put the grill cover back on and it shut down on me!  AAAARRRRRRRGGGGHHHH!!

You have an air flow problem. The grill is creating just enough restriction that the sail switch is not being activated and the the system shuts down. Check your other output ducts to see whether one or more is crimpled, closed off, or plugged in some way. Is there some sort of filter in the grill cover?

Richard

Edit: I just reread my post and found that I had a brain cramp in the reply above. The sail switch is the in the air intake/exhaust air flow. However, it still appears that the an air flow restriction may still be involved. Does it shut down as soon as the grill is replaced or does it take a few minutes?
 
Thanks for your reply.  Once the grill is replaced, it takes a few minutes for it to shut down.  Usually when it's running fine, once the thermostat reaches the preset temp, the furnace goes into a "cool down" mode where the fan runs but the flame is off.  When it's giving me problems, the thermostat reaches temp and then instead of going in the cool down mode it completely shuts off and I have to restart it manually, giving me the error code at the same time.
I can't see the sail switch being a problem, but how big a deal is it to change?  I can't see it, but I think its mounted toward the back of the furnace. 
It ran completely fine for 2 years and started giving me this problem last season.  There is no grill filter and as far as I can tell, all the ports in and out are clear and clean.
 
pg2676 said:
When it's giving me problems, the thermostat reaches temp and then instead of going in the cool down mode it completely shuts off and I have to restart it manually, giving me the error code at the same time.
I can't see the sail switch being a problem, but how big a deal is it to change?  I can't see it, but I think its mounted toward the back of the furnace.

I am not very familiar with Atwood furnaces. However, it sounds like the high temperature limit switch is being activated due to insufficient air flow through the heat output sided of the furnace. The Atwood control circuit board may be designed to completely shut the furnace down when the high temperature switch is activated. On Suburban furnaces, the furnace can be cycled on and off by the high temperature switch rather than completely shutting down the furnace.

It still appears that air flow on the heat output sided is involved in shutting down the furnace. How have you determined that the air flow through the distribution ducts is OK? If the total air output is borderline, placing the grill on the furnace may be just enough to cause over heating and activation of the high temperature limit switch.

As my edited post indicated, the sail switch is, I believe, in the air combustion side of the furnace. So unless the grill is causing so much air flow restriction that it slows the blower motor down, the sail switch cannot be involved. BTW, the the combustion air flow and the heating air flow are completely separate.

Keep us posted as to what you find out.

Richard
 
The output from the exhaust outside the trailer is fine, no obstructions.  The grill is clear.  Is it possible the limit switch I bought and installed is either the wrong model or faulty?
 
It's a shot in the dark here, but you are looking for driveway fixes and it won't cost anything but your time.  From reading this thread and your replies, I too, think you have an air flow problem.  Personally, I'm leaning towards the problem being tripping the overtemp cutout due to excessive heat.  This would explain why it takes a while before it activates and why you have to reset the furnace to get it going again.

My suggestion is to look at your squirrel cage furnace blower and see if the fins are all caked with dirt.  These fins have a natural curve to them and when they get dirty, the curve disappears due to caked on dirt and the efficiency of the squirrel cage goes way down.  It may be turning, but it isn't moving the air it should.  As RLSharp says, the front grille is probably producing just enough air restriction to cause the overtemp condition.  It's a longshot and access to the blower wheel may make it more than simple to do.  Use some common sense about it.  If the trailer is fairly new, then the likelihood of the blower being caked with dirt is very improbable.  However, if it has a few years on it, then it is worth a look.

Since your problem is one of "marginal" air flow and is remedied by removal of the front cover....could it be that the overtemp switch has "drifted" temperature-wise over time and now trips at a temperature abnormally low?  I know of no way to test it, your only recourse in this situation would be to replace it and see if the problem goes away.  It is in series with the sail switch.  By all means, do not try to defeat it.  It is there for a purpose and it is doing its assigned job.

I assume you have looked for obstructions in the airflow path.
 
Edited...

Here is a link to the service manual for your furnace:
http://bryantrv.com/docs2/docs/hflamefurn04.pdf

If your fan is shutting off as soon as the flame extinguishes, you have a bad time delay relay, which is supposed to continue to supply power to the motor after the circuit board shuts off the flame to cool the furnace.

The overtemp switch is in series with the sail switch contacts.  If the overtemp trips, it looks just like an open sail switch to the control board.

If the time delay isn't working, shutting down the fan immediately leaves a lot of residual heat in the furnace.  It may be enough to trip the overtemp switch when the fan shuts down.  If it doesn't cool down enough to reset before the next time the furnace is turned on you'll get the sail switch error and have to do the hard reset - i.e. turn the power off then on.

Try replacing the time delay relay and see if it solves your problem.
 
pg2676,

The September 2010 edition of FMCA's FamilyMotor Coaching magazine has an excellent article on furnace maintenance. It has a whole section on the troubles that inadequate air flow in RV furnaces can cause. The article also has procedures for cleaning all aspects of a furnace with pictures of what a dirty furnace can look like. Unfortunately, the article is not online yet and you will need find someone with a copy. I do not know whether it is available in newstands but I would think that large libraries might have a copy.

I am betting on an air flow problem or a problem with some component of the air handling system being the cause of your problem.

Richard
 
Just went through the same basic thing with the suburban furnace in my MH and when I pulled it out and disassembled it, I found old yellow jacket nests and remnants of mud dauber nests stuck in the burner side air squirrel cage. It apparently was allowing to much heat to build up and causing the high limit switch to open. 
 
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