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Atwood Furnace


scubadave

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Atwood Hydro Flame 8940 in a fifth wheel. Turns on OK, fan starts, gas ignition occurs. Stays lite for about 5 seconds and then goes out, lites again in about 10 seconds. Continues this cycle until turn the stat down. Does not provide enough heat to maintain temp. Any ideas on what the problem may be?

 

Thanks

 

Dave

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If the sail switch was bad, it would never allow the burner to ignite. The following is from the Atwood manual, and this is assuming the blower never turns off, and the burner is igniting, then shuts off, then ignites again, over and over again:

 

LIMITING = BURNER CYCLING ON AND OFF - BLOWER RUNS CONSTANTLY WITH THERMOSTAT ON
Restricted or insufficient discharge ducting ------------a. Ducting must meet furnace’s minimum requirements.*
b. No excess ducting or unnecessary bends.*
c. All closeable registers must be fully open and unrestricted.*
Furnace over fired ----------------------------------------------Set gas pressure to a minimum of 11˝ W.C. with all appliances
running replace regulator, if not obtainable. Also, check main
burner orifice, it must comply with furnace’s specifications.*
Restricted return air supply ----------------------------------Make sure return air meets furnace minimum requirements.*
Defective limit switch ------------------------------------------Replace limit switch

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I tend to think like post #3, except that if the burner actually lights but only burns for 5 seconds, that isn't long enough to trip the high limit thermostat, if it is working properly. If it is failing, that clearly would give that symptom but I'd first time the burn of propane to see just how long it really is. It might also be very low propane pressure or if actually 10 seconds, it may not be sensing the heat from the burning propane. In most furnaces, the spark will repeat about 3 to 5 times and then the control goes into lock-out and must be turned off and back on to start over.

 

How long does the 5 sec./10 sec. sequence continue? Does the spark continue to fire even when the propane is burning? Am I correct in gathering that you have watched this with the outside access open and someone inside turning the furnace on while you observe? If you haven't done this, I suggest that you do so just to make sure that you are accurate in what is taking place.

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It could also be the flame proving circuit. If the flame rod is out of alignmet, or if the flame is not touching the flame rod strongly or if the flame rod is dirty you will get these symptoms. I have also found that a poor ground from the circuit board can cause this as the flame is rectified through the flame to ground.

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I had the same problem with an Atwood 8535----I cannot find schematic for 8940--..I agree with other posts. Unfortunately they didn't work for me. -

 

However your description of problem leads me to believe there could be four solutions. Going on your description of events.

 

1--blocked air flow in exhaust-easy fix.--wasps or others bugs

 

2--check all connection for proper contact, I used famous wiggle test.

 

3--Atwood had two types of electrodes for ignition---If you have two wires going to electrode---one is ignition--other is temperature sensor (that tell circuit board there is a flame).

 

New models incorporate both ignition and temperature in one electrode..an one wire--if probe doesn't detect--- flame shuts off---and restarts---in mine three times and locks out for an hour---unless you turn it off. Temperature probe is one you have to physically look at (far as my experience goes).

 

4--circuit board--last thing I would replace...

 

Course you checked all vent inside to ensure they are not blocked..

 

After three days of working on mine, I found the wire to igniter had high open--(which is also temperature sensor). Would ignite, but didn't detect flame. Not hard to work on, just use common sense and go slow.

Good luck--Trucken

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Thanks for all the info. I thought nothing had changed since it ran on. But as I was going through the steps in 57becky's post I found 2 of my registers had the plastic shutters had dropped off the metal register cover. Removed these and no love. Timed the cycle and it was 2 seconds of burning and 16 seconds of not burning. It did this for over 15 minutes until I shut it off. Next put a fresh tank of propane on the primary side of the rig and switched over to it. The second tank is on the opposite side of the rig. This got things running properly. So a new red regulator on the secondary side should solve the problem. Will put one on tomorrow.

 

Dave

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Next put a fresh tank of propane on the primary side of the rig and switched over to it. The second tank is on the opposite side of the rig. This got things running properly. So a new red regulator on the secondary side should solve the problem. Will put one on tomorrow.

And there is the advantage to having two bottles in separate locations, each with a regulator of it's own. Being one of those who has always had RV's with only one propane pressure regulator, I usually don't think of the test you just did. I would draw the same conclusion that you did when trying another propane source. It might be wise to have both regulators tested for proper propane pressure supplied to the inlet of the water heater, just as a precaution. I like to have mine tested anytime I have it in for service as that is one of those things that most of us do not carry the equipment to do.

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