Atwood Furnace

Atwood Furnace

? on: Today at 02:14:44 PM ?

Hi,
When searching the internet looking for information on an Atwood furnace i stumbled across The RV Forum Community which is perfect since, although i am currently looking for help with a park model furnace, we plan on getting a motorhome when we retire in 2047 days ... not that we are counting! I am sure that we will be frequent Forum visitors!
What i am looking for is someone knowledgeable about Atwood 8940 III furnaces in a park model. This is an AC only propane furnace. Our park model is a 2011.
Here is the problem ... about a month ago we came to our cabin (park model) for the weekend and found the place near freezing. There had been an early cold snap here in Washington and we set the thermostat at a low setting when we leave to go to the city for the week but it obviously had not come on for some time.
We have a Honeywell digital programmable thermostat.
At that time i switched the t-stat from 'heat' to 'off' and back to 'heat' and it came on. I just thought that perhaps there was some sort of power surge that had occurred and caused a glitch but then it happened a few more times throughout the rest of the weekend.
I would either have to turn the t-stat switch to 'off' and then back to 'heat' or turn it down below room temp and then back up. Sometimes i would have to use the latter method once or three times before it would come on.
Once running it sounds and works fine.
I called the local heating guy and he checked the wiring and replaced the Dinosaur circuit board thinking that there was an intermittent internal fault.
The following weekend the same thing was happening.
He returned and replaced the thermostat with a Honeywell digital non-programmable thermostat.
This weekend the same thing is happening!
Like before the furnace will operate perfectly for hours, sometimes a day, at a time but then it will just not come on.
I should add that once a heating cycle begins it will operate correctly until the t-stat is satisfied ... not once have i seen the furnace quit mid-cycle.
Not only is this starting to get costly it is un-nerving not knowing if we will have heat or not when we arrive and the weather will continue to get colder.
Please help someone! Thanks.
Don & Katherine

Don & Katherine
I suspect that the spark sense electrode is failing:
https://tinyurl.com/ujgdsnl

Other than using 120vac for fan motor, the cycle in the AC version is pretty much identical to the DC.  From your description, the furnace goes into lockout, so turning the t-stat off/on resets it.  Lockout occurs when it fails to re-light for a new cycle or if flame sensor fails to detect the flame and the safety circuit shuts it down.

The 8900-III AC model is covered in the Atwood Hydroflame Service Manual. Get it free at  http://bryantrv.com/docs2/docs/hflamefurn04.pdf

Thanks for the quick replies Gary and Mel.
Both of your ideas are valid ... except that when the thermostat calls for heat occasionally other than 'heat on' being displayed on the thermostat nothing else happens ... the blower motor does not start up so it doesn't even enter the phase where the furnace attempts to ignite.
I don't think that i made that clear in my initial post. Sorry.
I am suspecting that perhaps the wiring between the thermostat and the furnace is somehow compromised. I did notice that at the thermostat the wire is 18/2 and at the furnace it magically became 22/5 so i know that there is a transition somewhere along the way. I suspect that a faulty connection may cause an intermittent problem like i have.
When i return to the cabin i will run a temporary 18/2 thermostat wire and test it for the weekend. If it works (which i REALLY hope it will) then i will figure out how to run it to the original t-stat location.
If it doesn't work i know for certain that the problem lies within the furnace.
I would then triple check all of the connections within the furnace and since the circuit board is new and the motor has been tested it kind of narrows it down to perhaps a faulty time delay switch ... ???
Ughh...

Definitely run the temporary bypass wire - I'll bet it works. It sure sounds to me like some other piece of gear is in the control path. Chances are good it's going through the a/c zone control module. You need to bypass that.

Hi Gary,
Merry Christmas first of all!
Well, unfortunately I still have an intermittent furnace problem.
Since my last post I ran a temporary 18/2 wire from the furnace.
As before it worked for perhaps a day and then when it called for heat ... nothing.
The blower does not start therefore the heat cycle does not even begin.
As before the furnace will usually start if you turn the thermostat from 'heat to 'off' and then back to 'heat or if you turn the temperature below room temperature and then back up. Sometimes you have to do this a couple of times.
I think that this might be a key piece of information ... that the furnace has to receive a new signal to heat before it will start.
I have done a lot of research and have thought perhaps it is a faulty time delay switch but I believe that the switch is integrated with the circuit board and that has already been replaced.
Now I am thinking would it be a faulty sail switch remaining closed after the previous cycle thus not allowing a new call for heat to go through?
I am running out of ideas.
Help!

Is there a relay between the control board and the fan motor?

Normal sequence:

1.  Call for heat
2.  Fan motor on
3.  Air from fan trips the sail switch
4.  Sail switch enables gas flow and ignition sequence.

If the motor doesn?t start there is a possibility the motor or relay is bad or the relay that controls it is bad.  Find the relay (if it has one) and tap it.  If it?s removable take it out and  check all the connections for corrosion.

Hi,
I believe that the fan relay is integrated into the circuit board.
It is a Dinosaur board and was replaced when the heating guy first came out.
When the problem was not cured it was replaced again as a precaution and I was told it was the thermostat.
The thermostat was replaced.
The problem was not cured.
I then ran a temporary 18/2 wire from the furnace to the new thermostat thinking that perhaps the wire was compromised.
Still the problem persists.
The furnace will often run fine for hours ... sometimes an entire day ... and then you will  hear the thermostat 'click' and the fan does not come on.
When the fan does come on everything seems to work fine ... the blower comes on and after about 45 secs the burner fires up and when the t-stat is satisfied the burner shuts off and the blower fan runs for another minute or so and turns off.
Thanks for your thoughts and ideas.

Have you gone to all the components in the system and tapped on them lightly with a hammer? 

Where are you getting power from?  Pedestal, etc?  Have you tracked incoming power to see if it is varying?

Is there anything you can do to cause the problem to occur?

Before I spent any money on new boards or anything, I'd take every blade connection in the furnace apart, scuff the blades with sandpaper or steel wool and hook 'em back up.  I'd also squeeze the female connectors lightly with a pair of pliers to tighten them up. 
Intermittent problems like this could be due to a corroded connection.

If the +12v "heat needed" signal from the thermostat reaches the fan control, then failure to start the fan has to be something along the lines of what Muddypaws described.  A corroded connection, faulty fan control, or maybe even a bad fan motor. 

Nothing is going to happen if the fan doesn't start, so that's where you need to focus your efforts.

I have seen those arched blade connectors loose their arch. The arch can be reestablished with a dental pick.

Richard

Possible you have a bad fan motor. It is not uncommon to have a motor developed a dead spot whether it be electrical or mechanical.

During a failed fan start, try to verify that power is or is not being delivered to the fan motor.

First of all ... all of you have been awesome suggesting all sorts of tips. Thank s to all ... there is definitely a helpful and knowledgeable bunch of people on this forum.
Due to a nasty cold and work we haven't been down lately but when we arrived last night the thermostat was calling for heat but the furnace was not starting again.
All I had to do to get it to start was to switch the thermostat from 'heat' to 'off' and then back to 'heat' and away it goes. It did not start once during the night and again once this afternoon.
Luckily when we leave now we have an electric heater set on low in anticipation that the furnace will fail. 
We now have a foot of snow and the thought of sitting out in the cold checking and cleaning electrical connections isn't very appealing but that will be my next step once the weather breaks.
I like the idea to check all connections and to check for power to the fan motor when the thermostat is calling for heat but nothing is happening.
Hopefully the weather will cooperate soon and I can continue troubleshooting.
Thanks again.

.

You need to go outside.  when it does not start, DO NOT TURN T_Stat off.. Remove the outside access panel  This way you can observe it in action.

There is a switch (really a circuit breaker) atop the blower housing,, cycle that to restart.

now observe does it start promptly. Does it start 3 times then stop.. Does it fail to light.

Report findings


Possible issues
Low battery voltage or blower speed (1st can cause the 2nd)  It will not try to light
No gas  You will hear the CLICK as the valve opens and the SNAP SNAP SNAP of the ignightor but no gas
(Out of gas. clogged orifice. bad solenoid)
Spark gap too wide Gas but no spark sometrimes on the 2nd try it willl spark for reasons .. Well it is science)
Lights. goes out x3  Bad flame sensor (Thermocuople) bad control board (minw was)

Of if the blower fails to start. that can be a wiring issue. or bad blower