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Old 10-15-2014, 01:45 PM   #1
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Default Attwood 7900 furnace

i JUST BOUGHT A 2003 HI LO (2203T). THE FURNACE SHOWS NO SIGN OF EVER BEING USED. (NO HEAT MARKS IN FIRE BOX). WHEN I TURN THE THERMOSTAT ON THE FAN RUNS FOR ABOUT 30 SECONDS AND STOPS. THE RED LIGHT BLINKS ONCE EVERY 3 SECONDS WHICH INDICATES A "AIR FLOW LIMIT FAULT. ANY SUGGESTIONS ON FIXES?
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Old 10-15-2014, 05:58 PM   #2
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Originally Posted by red barron View Post
i JUST BOUGHT A 2003 HI LO (2203T). THE FURNACE SHOWS NO SIGN OF EVER BEING USED. (NO HEAT MARKS IN FIRE BOX). WHEN I TURN THE THERMOSTAT ON THE FAN RUNS FOR ABOUT 30 SECONDS AND STOPS. THE RED LIGHT BLINKS ONCE EVERY 3 SECONDS WHICH INDICATES A "AIR FLOW LIMIT FAULT. ANY SUGGESTIONS ON FIXES?
For some reason, the furnace burner is not lighting, or, the flame sensor is not registering that the furnace has lit. I believe the system will cut off all gas to the furnace if this is the case. If it's the flame sensor, it may just need to be cleaned off (this happened in a house we have as a rental unit - the burner would light and then would shutoff). The fix the technician applied was to wipe the sensor off on his pants. Since then, it's worked fine.

However, your burner may not be lighting, possibly due to something blocking the gas inlet or the air inlet to the burners. The only way I know of to check for this is to have the cover off on the outside of the trailer so you can see what is happening. I've never had a problem with the furnace in my trailer, so I'm just offering the path I'd initially follow.

Maybe, there is some troubleshooting procedure in the furnace manual, if you can find it or have it? If you don't, check for one online.

- Jack
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Old 10-16-2014, 01:27 AM   #3
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Did you turn on the gas valve next to the heater inside the trlr. Propane tanks turned on and gas in the tanks?
It will take the heater awhile to purge the air before the system may get gas. Light the stove first to help the purge.

Have you checked the air intake from the outside?

Gas will not continue running into the system if no flame is present. But will purge the system on a limited basis whenever the furnace tries to light.
So a few cycles may be needed.
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Old 10-16-2014, 10:23 AM   #4
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When you turn on the furnace the fan starts and must run for a short time to purg any gas that might be in the furnace. The air moving thru the furnace by the fan closes a sail switch in order for the ignition and then the gas to be turned on. If the sail switch does not detect enough air and does not close ,(turnon) then the system shuts off. This is the reason for the light blinking every 3 sec. Others are correct that you may have to cycle it several times to purge the air out of the gas lines. If the furnace has never run then maybe the fan never comes up to enough speed to close the sail switch. Try some oil on the motor. Maybe the gas valve is stuck shut from never being used. If the sail switch and the Hi temp switch are closed then you should hear the igniter arcing and then the gas valve should open and the furnace should light. Bee nest or dead bugs in the burner and combustion chamber will prevent the gas from lighting. Hope this helps. Let us know what you find.
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Old 10-24-2014, 11:44 AM   #5
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Unhappy No heat

Thanks for the help. I decided to remove the entire unit since I could find no good diagram on it . I soon realized that the sail switch on my unit can not be accessed with the furnace in place. I had exhausted the simple fixes. On the bench I removed the fan and squirrel cage and found the sail switch. It is broken due to a tire blowing and striking the underside of the furnace. I identified the part with the help of an online parts list as #36133. When searching in internet I saw that the part has been replaced by 36680. The replacement part is shaped completely different. The instruction indicate that the shape of the sail is determined by the air flow requirement of the model so now I must contact Atwood to be sure I get the correct part. FYI many parts are available on EBAY
Any idea how dangerous it is to bypass the sail switch?
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Old 10-24-2014, 12:00 PM   #6
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I would not do it.
It is a safety issue and was designed to be there.

Also install a CO2 alarm.

Carbon monoxide is not a friendly camper!

just my .02

steve
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Old 10-24-2014, 12:26 PM   #7
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I'm not trying to be a "Know-it-all" but CO2 is carbon dioxide, CO is carbon monoxide. And yes, don't mess with the furnace, and do get a carbon monoxide(CO)/smoke detector. The cost of being safe is much better than the alternatives. If you don't have an LP leak detector get one of those also.
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Old 10-24-2014, 01:49 PM   #8
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My faux pas
I should have asked the wife.
RichR is correct, it is a CO detector.
steve
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Old 12-14-2014, 03:46 PM   #9
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Talking Furnace problem solved

I did confirm that the new sail switch has been redesigned to a different shape but does the same job. So I bought it on Ebay. Removing the entire furnace was not as difficult as I expected and it is not possible to replace the sail switch without removing it. The furnace is now working very well. Thanks to all for the suggestions.
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