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Old 05-24-2020, 03:45 PM   #1
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Dometic furnace quirky issues, 2018 Coleman 318BH

Howdy folks -

We have a new-to-us 2018 Coleman 314BH travel trailer, and at 0230 last night we woke up to a cold camper - seems the furnace would not 'maintain' the set temperature in the camper (70*). Furnace is either a model AFSAD12, AFSD12, AFSD16 or AFSD20. It has a simple Dometic 'soft touch' thermostat (also controls the fan and A/C) - it is about 3" square and about 1" thick. Manually raised the temp all the way to 90* and nothing. Cycled the thermostat off/on etc. and the heater magically came on but I do not specifically know why.

I stayed up for a 1/2 hour and the system seemed to maintain temp for a cycle or two, then after watching the temperature drop to 66* I tried to cycle things again and nothing - manually upped the temp into the 80s and nothing.

Fuse is fine, propane is full. Please help us resolve this issue. Hoping it's some easy to reach gizmo that needs to be adjusted or replaced.

Cheers,
-Sam
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Old 05-24-2020, 04:26 PM   #2
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Hi, did you attempt to place it in AUTO for the fan and HEAT for the selection? Usually mine drops to probably 66 if I have it on 70 then pumps it up to 72 or so then shuts off. I believe it may have a cool down feature on the heater preventing it from coming on within a few minutes of shutting off. Do you hear the fan come on then the roar of the burner? Have you checked to see if there are any obstructions around the burner? Have the little woman move the thermostat to a much higher temp while you go outside and watch the burner. See if there is a light on on the controller.

See what type of furnace you have and we can find the operators manual for it.

They're temperamental and have a mind of their own sometimes...

Welcome to the forum and congrats on the new rig.

By the way, most of us use small ceramic heaters for supplemental heat. uses the campground's electricity instead of our propane.
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Old 05-24-2020, 07:34 PM   #3
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yup it's on [auto] - the issue is the system does not seem to be maintaining a constant temperature - I woke up to a low 50's degree camper with the thermostat set at 70. The sail switch had no hair or other debris in/around it, and no fault LEDs flashing. Manually upping the temp to 90* did nothing - no clicking no attempted starting, just 'nothing'.
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Old 05-26-2020, 03:35 PM   #4
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My brain is working poorly this morning. These are the potential fail points I can remember.
Thermostat miss set or failure
Sail switch failure
Obstructed air in gases out pathway
Igniter failure - after three tries, control board locks out restart attempts. Power off 12v to control board to reset.
Gas valve failure
Control board failure


To all the other "experts" out there, what have I forgotten?
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Old 05-26-2020, 03:47 PM   #5
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thanks for the feedback. I have a bit more info to also pass on - we're fortunate that we are able to contact the previous owner who is very helpful with any tech info we might need (tho not always able to respond quickly, hence my post here), and after hearing about our issues he said he's also had similar issues and suggested removing/cleaning the sail switch. I removed the sail switch hoping to find a bunch of hair or moths or caked up with dust, but no - it looked pretty 'clean', so I didn't expect much to be resolved after screwing it back in. But during the whole next day the system seemed to perform 'normally'.

I did not expect cleaning the sail switch to cure anything. I don't know exactly what it does but the R&R seemed to do the trick; before the system simply would not 'activate' - no clicks, no fan blowing without heat, no igniter clicking - just 'nothing' - there was simply no 'feedback' from the DOMETIC thermostat. There were also no LED indications from the circuit board indicating an error - that part really surprised me - I expected there to be 'some' kind of error indicating the problem but nooooo...

So, we're going to continue to 'deploy' the camper in our back yard for the next several weekends and see how it goes, and we'll get another sail switch to put in the spare hardware drawer...

The camper is still under warranty so yayee for that - we just did not expect to need it for a while, but, here we are and away we go
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Old 05-31-2020, 04:50 AM   #6
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What was the outside temp? If it's really cold, the pressure in the tanks get low and the system won't work.
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Old 05-31-2020, 12:41 PM   #7
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- it hasn't been quite that cold but thanks for the tip! I'm glad you replied because I had some updates - the system finally give us a code - fault 1 - low airflow limit, so we installed a new sail switch and the system seems to have worked good all last night, and we got a second new sail switch as a spare. We're also looking at getting the tune-up kit which will have several new switches and will help maintain the system that much better.

Cheers,
- Sam
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Old 05-31-2020, 04:32 PM   #8
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Years ago I had a toyhauler with a furnace that would intermittently do what you described. Living in Colorado I sometimes camped at 6,000 to 8,000 feet, and sometimes at 10,000 feet and above. The problem mostly occurred at higher altitude and I determined that the reduced air density at high altitude would not move the sail switch enough to activate the gas valve.
My solution was to fasten a strip about the size of two postage stamps of heavy duty aluminum foil to the sail switch arm so as to increase the area of the arm that the blower could blow against. Problem solved.
Those sail switches can be touchy.
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Old 05-31-2020, 05:19 PM   #9
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Sail Switch

The sail switch and paddle on the switch measures the air flow to see if there is enough air to light the burner. The fan will turn on and run for 30 seconds or so to clear the burner chamber. You can hear it run. The switch should turn on if there is enough air flow and allow the gas on and igniter to click. If not the fan will stop and error code should present.

So if switch failed or air way is plugged:
Fan will run
Gas valve will not open
Igniter will not repeatedly click (snap of electric arc not heard)
Fan will shut off
Error code will present

You said fan did not run. I assume that means it did not run even briefly. In this case it could be a loose or corroded connection. Taking it apart and putting it back together may have fixed it.

I wish you good luck and happy trails ahead!
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Old 05-31-2020, 06:00 PM   #10
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thanks folks - what we think was happening was the system was in 'lockout' mode due to a bad sail switch. The furnace would come on the first couple times like it's supposed to, but something was throwing a #1 fault code...which was disabling the system from doing 'anything' - no amount of manual inputs would activate the system in any way...except...for shutting it all off and/or re-setting the switch on the motor, which would apparently clear the code and the cycle would start all over again, working one or twice correctly, throw a code, and lock it out.

It was on all night and the system maintained a nice '70' all night long, and the sail switch was the only thing we changed, so it appears that was it. We presume any faulty switch in the system would cause the same 'lockout' mode, but the #1 fault code (low air flow/limit) indicated the sail switch so that's what we replaced.

- Sam
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