Furnace Troubleshooting

[Davehwa]

The stock Dometic furnace in our 2020 Hawk is acting up. It throws a code that points to the limit switch.

Before I go further, I’ll indulge in a small rant:

When troubleshooting, people often blame the electronics for not working properly, when they really need to take a step back and start with the basics. As a lift mechanic at our local ski area, I can’t tell you how many times a $20,000 PLC was blamed, when a simple loose connection was the culprit.

Bill Hewitt at powerstrokehelp.com has a great rant about this sort of thing from a mechanical perspective. Search “loss of power 7 3l powerstroke“ on YouTube. Very entertaining.

There, that wasn’t too bad, was it?

So after blaming the electronics, bypassing the heat exchanger limit switch, and still not getting anywhere, I READ THE MANUAL, which told me to clean the blower once a year.

Huh.

I also remembered to check every component in the circuit. In this case, the limit switch and fly switch are in series.

So after remembering the basics, I found:

The fly switch was all jammed up with lint.

The blower was dusty.

The heat exchanger outlet was a bit plugged, because I cleaned the grille and didn’t put it all back together well enough.

So, I:

Cleaned the fly switch.

Blew out the blower with compressed air.

Straightened out the flexible ducting so the blower doesn’t load up.

Shortened the thermostat cycle time to the minimum.

At this point I’ve gotten a pretty good understanding of how the thing works. We’re currently testing it in a Laramie WY parking lot, and I’m cautiously optimistic.

I talked to a guy at an anonymous RV sales/service store who gave me a few hacks:

Remove the ducting and grille to improve airflow.

Bypass the limit switch.

Has anyone here used these hacks to improve performance in cold conditions?

Does anyone have other ideas?

Thanks

 

[Vic Harder]

I talked to a guy at an anonymous RV sales/service store who gave me a few hacks:

Remove the ducting and grille to improve airflow.

Bypass the limit switch.

Has anyone here used these hacks to improve performance in cold conditions?

Does anyone have other ideas?

Thanks

Great post. From what I've heard here, removing the ducting is a common "fix".

What performance issues are you noticing?

 

[Davehwa]

Hi Vic,

Upon getting in the camper and turning the thermostat on, the furnace would run for a good long while then stop before the thermostat turned off.

Blinking light on circuit board pointed to the limit switch.

Seems like it’s working now

Dave

BTW the crew cab came in handy last night, wife and I slept in the cab and idled the truck

 

[Colorado Mark]

One of the things I did on my Dometic 12K furnace was to replace the rocker switch breaker. It's buried inside the furnace.

If it trips, you have to take the outer cover and exhaust off to get in and flip the switch back on.

Replace it with an automatic breaker rated at the same amperage. You still have another fuse from 4WC inside the camper that has the same rating so you are still double protected.

That and remove the foil ducting.

Having to take the outer cover & exhaust off in the early morning when its cold just to flip that switch on the breaker back on really sucks.

Removing the ducting cured the breaker tripping at high altitudes.

 

[Davehwa]

Thanks guys. Good idea on the breaker. Now that we cleaned her up and straightened out the duct work she ran like a champ last night.

 

[Regular Guy]

I got a new Hawk and since I've read a lot on this forum in advance, I removed the duct while I was still at the dealer. However 2 weeks in I had furnace problems. It wouldn't ignite. The flashing light on the control panel told me air flow problem. I removed the sail switch, it was clean, I could hear it click when I moved the sail, I even checked it with my VOM for continuity. The switch was fine. When I reinstalled the switch still no ignition and the same air flow fault. I even checked for continuity when the switch was reinstalled and the fan was running, the switch appeared fine and based on continuity appeared to be switching when the fan was running.

I got help from a service tech. The problem was the sail switch, it needed to be adjusted i.e. bent. Prior to the adjustment the sail had interference with the fan housing and would not completely switch. It must have been enough to show continuity but not closed enough for current.

The Tech said the trick was check for voltage at the bottom of the sail switch. If you don't have 13v at the bottom of the switch when the fan is running you have a sail switch issue. Either switch is dirty, bad, or just needs an adjustment. Sorry I don't have any good insight on which way to bend it. Trial and error I guess.

 

[J D]

My 6 month old Granbys Dometic furnace won't ignite but the blower does turn on. It did work perfectly until last Thursday night. Today, trying to figure things out it did ignite once but has since failed every time and several times prior to the one success. I've read on this forum about removing the ducting so I did that today but it used to work fine with it in place. Anyway, still no ignition. The owners manual suggests the problem is air in the propane line and to purge the line. Not an easy task when the furnace is the only propane appliance. Can't just turn on anything downstream. I don't think that's the issue and I can't be dismantling the cabinet every time I switch tanks - so, there's no practical way to purge the line. I've cycled the thermostat numerous times trying to work out any air but only once did I hear the click for the ignition.

So this thread and others seem to define switch issues as the most common problem with ignition failure. Primarily the sail switch. I assume one accesses those switches through the outside cover based on the parts diagram. Do you reseal with butyl tape?

Some note flashing lights on control panels and codes identifying problems. The furnace in this camper (model DFSAD12) doesn't have that functionality.

Irks me to have this new camper be a confounding project. I didn't buy a new camper to fix. Such a common problem should be, at the minimum, addressed with a thorough fix it guide.

 

[PaulT]

If you have had the propane turned off at the tank, be sure to turn it back on slowly so the safety valve doesn't get you, Light the cook top to ensure that propane is flowing and distributed through the lines before trying to start the furnace. Turn off the cooktop and retry the furnace. Good luck.

Paul

 

[Wallowa]

If you have had the propane turned off at the tank, be sure to turn it back on slowly so the safety valve doesn't get you, Light the cook top to ensure that propane is flowing and distributed through the lines before trying to start the furnace. Turn off the cooktop and retry the furnace. Good luck.

Paul

JD's only propane appliance is the frunace/heater...no cook stove.

"Not an easy task when the furnace is the only propane appliance."

 

[PaulT]

Sorry. My bad for missing that clearly stated condition.

It would seem that this may be a recurring issue under these conditions. Perhaps having a propane quick connector installed such that a portable propane appliance like a Little Red Fireplace or a gas grill could be attached to simplify purging the air from the lines for each time the furnace may be needed.

Again, sorry, I overlooked that detail in your configuration.

Paul

 

[Wallowa]

Sorry. My bad for missing that clearly stated condition.

It would seem that this may be a recurring issue under these conditions. Perhaps having a propane quick connector installed such that a portable propane appliance like a Little Red Fireplace or a gas grill could be attached to simplify purging the air from the lines for each time the furnace may be needed.

Again, sorry, I overlooked that detail in your configuration.

Paul

Paul,

No need for "sorry"...your advice is good advice.

 

[J D]

Thanks Paul, and no problem.

I'm pretty certain (though not positive) it's not an air purge issue. Since the furnace is still under warranty I'm going to have Camping World take a look at it.

I'll look into adding a propane appliance but I very much am at odds with that. I chose the shell model for it's simplicity and would like to keep it that way.

 

[J D]

Well, I'm a bit more educated now thanks to Brenda at FWC. She sent some great information! First is this:

FURNACE TROUBLESHOOTING CODES AND POSSIBLE MEANINGS

STEADY ON- NO FLASHING: This is a circuit board fault. No other issues need to be present for this fault code to show up.

1 FLASH, 3 SECOND PAUSE: This can be the limit switch or, airflow problems. In most cases, this is a sail switch problem. This can be caused from 2 different areas. 1) Low voltage = fan will turn at a lower speed thus causing
the sail switch to not depress and not allow the ignitor and gas valve to open or, 2) The sail switch has debris on the switch, keeping it from depressing and allowing the ignitor and gas valve to turn on.
If the furnace fires up momentarily and then shuts down right away, this will be the limit switch. What the furnace is doing is, reading the flame temperature inside of the combustion chamber. Sometimes the flame will be too hot and,
this will shut down the furnace. Restarting the furnace usually helps but, in most cases where this is the problem, the switch needs to be replaced. This item looks like a button the size of a nickel with two wires attached to it.

2 FLASHES, 3 SECOND PAUSE: Flame sense fault code. This is part of the ignitor electrode assembly. This senses if there is flame present or not. If there is no flame present, it sends a signal to the board to shut all power down.

3 FLASHES, 3 SECOND PAUSE: This is the lockout fault code. More than likely this code is flashing due to the unit being turned on and off too many times in a short period of time, usually when the customer is having a hard time getting the furnace
lit. The system needs to go through a cool down phase before the system gets restarted again and, if this sequence is not followed, a lockout will ensue. This code can be erased by pushing in the kill switch AND, hitting the reset button behind the main
external furnace door then, pulling out the kill switch again to get the system to fire up. This code usually pops up when the propane is not of a good quality and the furnace is hard to start.

Second is this:

Youtube video from FWC called: Sail Switch Cleaning or Replacement on Dometic Atwood RV Furnace

I'll inspect the switch. Brenda will send me a switch if I can't get the one on the camper to work after cleaning.

 

[Wallowa]

Wow! Thanks...two questions [my ignorance] where are these flashing light codes displayed? And what is the "kill switch"? Main FWC power switch?

Thanks for references....will store them away!

Phil

 

[J D]

Hey Phil

I just learned this stuff today. Take a look at the video I noted. In that video the guy from FWC will cover most of the stuff. I can't recall if he pointed out the light that will flash codes. The little red light is right on the circuit board. He does point out the switch on the furnace, located just under the circuit board.

On FWC website under the "support" tab are some links for videos. The one on propane tanks is good and pertinent to this topic. Someone mentioned earlier in this thread about turning the tank on slowly. The video explains that more thoroughly.

What I did after gaining some education today (I knew nothing about any of this this morning): I turned off the valve at the propane tank then unscrewed the hose. I turned the main power switch off to the camper (the pull/push switch inside the battery compartment). I took the four screws out of the furnace cover plate and removed the cover. Then I reattached the propane hose to the tank and very slowly re-opened the valve on the tank. Then turned the power back on. With everything turned back on I turned the thermostat up and the blower came on but the furnace didn't ignite. I went out to look inside at the furnace access. The red light was doing a single flash with 3 second interval. From the sheet that Brenda sent, the code indicated a problem with the sail switch. I turned the main push/pull camper power switch back off then removed the sail switch (after watching the video). It looked fine, I didn't see any debris or anything. I blew it out gently with the air compressor just in case, then re-installed the switch. Then turned the main power back on and moved the thermostat to try things again. This time the furnace fired.

Maybe there was something affecting the sail switch that I didn't see. Maybe they're finicky enough that removing and re-installing can be a temporary fix. Maybe there's half a dozen other maybe's.