Dirt dobber nest in furnace intake

[toolittletime]

I noticed some dirt dobber activity last month before we left on a trip, around the furnace intake. I took the outside cover off and reached in and remove one of their dirt nest.
fast forward today....ran the heater ....started and put out heat just fine but looked like a bit of blue looking smoke came out the exhaust. Second time it started up it backfired, but started and put out heat.
I think the backfire may be related to me quickly opening and closing the tank valve too quick, but how about the blue looking smoke?
I current have the heater out, and have ran a piece of coax (all I got) inside the heating tubes....got a little more dirt out.....probably 1/4 cup total.
My question is......is there a way to clean out the heating tube completely without completely disassembling the heater unit???
Its a dometic dfmd35121 model
Thoughts suggestions????
Thanks........Tim

[wiredgeorge]

Originally Posted by toolittletime

I noticed some dirt dobber activity last month before we left on a trip, around the furnace intake. I took the outside cover off and reached in and remove one of their dirt nest.
fast forward today....ran the heater ....started and put out heat just fine but looked like a bit of blue looking smoke came out the exhaust. Second time it started up it backfired, but started and put out heat.
I think the backfire may be related to me quickly opening and closing the tank valve too quick, but how about the blue looking smoke?
I current have the heater out, and have ran a piece of coax (all I got) inside the heating tubes....got a little more dirt out.....probably 1/4 cup total.
My question is......is there a way to clean out the heating tube completely without completely disassembling the heater unit???
Its a dometic dfmd35121 model
Thoughts suggestions????
Thanks........Tim

Can't help with your specific question but you may want to check the drain tube on your refrigerator. Dirt daubers love those things as well.

[chuckster57]

If mud dawbers have set up house on the burner/igniter, I don’t know of any other way of cleaning without disassembly. Compressed air might work but I wouldn’t count on it.

[wiredgeorge]

Originally Posted by chuckster57

If mud dawbers have set up house on the burner/igniter, I don’t know of any other way of cleaning without disassembly. Compressed air might work but I wouldn’t count on it.

I clean old/vintage motorcycle carburetors every day and mud dauber brown stuff is almost like cement. Compressed air likely won't get it done. The infestation I deal with isn't as deep in and I use a dental pick and dremel w/wire brush generally. Just squirting with carb cleaner doesn't dent the brown stuff.

[toolittletime]

Originally Posted by wiredgeorge

I clean old/vintage motorcycle carburetors every day and mud dauber brown stuff is almost like cement. Compressed air likely won't get it done. The infestation I deal with isn't as deep in and I use a dental pick and dremel w/wire brush generally. Just squirting with carb cleaner doesn't dent the brown stuff.

I guess I'll dismantle it today and see whats inside......wiredgeorge...just rebuilt the carbs on a bmw airhead....my latest project.

[toolittletime]

Welll...It was'nt as bad as I thought. There are 3 screws that hold the burner in place. You have to unscrew the flared line that goes to the valve first...the burner comes out enough to clean it off with a toothbrush. Did not see any evidence of dobbers near the igniter or burner.
Put it back together...checked for leaks and fired her up.
Seems to be burning clean....hopefully I got all of the dd nest out of the heat ex changer tube. Only problem now.....when it fires up seems like a delay ..after the gas is released into the tube seems like the igniter is delaying spark. No backfires.....but did shoot a small blue flame out the exhaust the first time she lit. Ran it three more times with a rest between and it started and heated fine. 4th time.....small blue flame again out the exhaust.
maybe needs a new igniter when I get home??? That sound right to the more qualified folks here?? Is there a certain gap between the probes on the igniter that I should have set??

Thanks.........Tim

[Roper46]

You might try these screens over the vents. I have used them on my 5er for two summers here in south Georgia with no issues or insects.

[sourdough]

Originally Posted by Roper46

You might try these screens over the vents. I have used them on my 5er for two summers here in south Georgia with no issues or insects.

I would just warn that those screens can cause problems. I did the same thing and placed them on my last trailer. Water heater didn't give me an issue but the first time I tried to use the furnace it would light, start running then quit. Turned out to be the screen causing it to heat up too much and the hi limit switch cut it off. Removed screen, never another issue. I believe the manual will tell you that those exhausts aren't supposed to be blocked in any way. I never thought that screen would do it but it must cause just enough interference/turbulence in air flow to shut it down (at least mine).

[Roper46]

Originally Posted by sourdough

I would just warn that those screens can cause problems. I did the same thing and placed them on my last trailer. Water heater didn't give me an issue but the first time I tried to use the furnace it would light, start running then quit. Turned out to be the screen causing it to heat up too much and the hi limit switch cut it off. Removed screen, never another issue. I believe the manual will tell you that those exhausts aren't supposed to be blocked in any way. I never thought that screen would do it but it must cause just enough interference/turbulence in air flow to shut it down (at least mine).

Thanks for the heads up Danny.

[toolittletime]

Only problem now.....when it fires up seems like a delay ..after the gas is released into the tube seems like the igniter is delaying spark. No backfires.....but did shoot a small blue flame out the exhaust the first time she lit. Ran it three more times with a rest between and it started and heated fine. 4th time.....small blue flame again out the exhaust.
maybe needs a new igniter when I get home??? That sound right to the more qualified folks here?? Is there a certain gap between the probes on the igniter that I should have set??


Re posting the above looking for suggestions??
Like the idea of the screens when not using the equipment, maybe just remove them when camping?
The saga continues.....this morning heater lit beautifully...no shooting flame...no backfire...but after 15 minutes or so the propane floor alarm went off. Turned everything off and reset the alarm.
Any ideas for troubleshooting from this point????


Thinking of just waiting a week until I am home to take the heater out (again) and ream out the heat ex changer tube....maybe use a small plumbing snake...check and adjust the ignition module (maybe replace)..and blow out the burner and propane orifice with compressed air???
Is there something I am missing????

Tim

[JRTJH]

In posts #5 and 6, you said that you dismanteled the furnace, cleaned it and put it back together. Now the LPG alarm goes off ??? I'd strongly suspect that there's a gas leak somewhere in your work. It could be as simple as a loose gas fitting or it could be more complex, like a gasket on the gas manifold...

BUT: Before you go ripping the furnace out again, I'd check all the other "nearby gas sources" like maybe the oven pilot light blew out, a stovetop burner leaking, etc.

AND: Sometimes the LPG will give a "false gas alarm" triggered by a "farting dog" or some cleaning agents, like Windex, etc. So it may not be a "problem" but rather a "supposed to have done that"...

[jasin1]

delayed ignition is nothing to mess around with…the igniter is probably at fixed height from the burner tube…like john said a gas leak or low gas pressure could cause delayed ignition…clogged or rusted burner tube holes could cause it…BUT it is not something to experiment with as a delayed ignition in any gas appliance can cause a violent explosion…i once had my hair singed when a very old gravity furnace delayed and a fireball shot out of the combustion chamber …i heard the gas come on and no ignition and barely was able to say “oh shhhh” before the fireball….

(Customer upstairs) … “is everything ok??”……..(me downstairs)…. “yEss…eVerYthinG IS fInE…(shaking and smoking ) …iM a pROfEsSIonAl!..


take the furnace to your rv dealer and drop it off for them to fix is my suggestion

[travelin texans]

Originally Posted by toolittletime

Only problem now.....when it fires up seems like a delay ..after the gas is released into the tube seems like the igniter is delaying spark. No backfires.....but did shoot a small blue flame out the exhaust the first time she lit. Ran it three more times with a rest between and it started and heated fine. 4th time.....small blue flame again out the exhaust.
maybe needs a new igniter when I get home??? That sound right to the more qualified folks here?? Is there a certain gap between the probes on the igniter that I should have set??


Re posting the above looking for suggestions??
Like the idea of the screens when not using the equipment, maybe just remove them when camping?
The saga continues.....this morning heater lit beautifully...no shooting flame...no backfire...but after 15 minutes or so the propane floor alarm went off. Turned everything off and reset the alarm.
Any ideas for troubleshooting from this point????


Thinking of just waiting a week until I am home to take the heater out (again) and ream out the heat ex changer tube....maybe use a small plumbing snake...check and adjust the ignition module (maybe replace)..and blow out the burner and propane orifice with compressed air???
Is there something I am missing????

Tim

During your furnace troubleshooting running the furnace quite a bit if the rv isn't plugged into shore power &/or you have a weak battery the LP/CO detector will chirp indicating low voltage.

[toolittletime]

Thanks for all the comments...


Ok let me address the suggestions one at a time:
Have been plugged into shore power all along....batteries are charged.
No leaks at stove or other locations.....no farting dogs or humans.
I did clean the windows yesterday with windex.
Not sure what "gasket on gas manifold" is? Nothing on the schematic?

No clogged or rusted burners...I eyeballed it when I had it apart.
Before I would take it to the local idiots at any nearby rv dealer and pay them the 180.00 an hour, I would just buy a new furnace.
Any other suggestions???
Tim

[toolittletime]

Should have said:
Before paying them 180 per hour to do "guess and check" I can buy a lot of parts

[jasin1]

when you started the furnace was it on your workbench or in the rv? and did it have the exhaust cap installed?…if not it could have been pulling too much combustion air in and lifting the gas off the burner before it hit the igniter


wasn’t sure if it was still under warranty

and for the record i never advocate working on gas appliances or line voltage electric unless your either licensed or experienced or both

[toolittletime]

Just found a good video on adjusting the igniter, and burner.
https://youtu.be/AB69oqtSiwc


Not my exact model, but good information.

[toolittletime]

Originally Posted by jasin1

when you started the furnace was it on your workbench or in the rv? and did it have the exhaust cap installed?…if not it could have been pulling too much combustion air in and lifting the gas off the burner before it hit the igniter


wasn’t sure if it was still under warranty

and for the record i never advocate working on gas appliances or line voltage electric unless your either licensed or experienced or both

Was in the rv...we are on a trip.
It was completely reassembled....yes exhaust cap was on.

It is a pain to get to, as I have to remove it through a hole I cut in the pantry. Other access is through the under belly storage, and have to remove all of the carpeted vertical walls more of a pita.
Cut the access hole 2 years ago to be able to replace the furnace control board after having 2 techs tell me 5 or 6 different things it could be...and of course paying them a trip charge. New control board fixed it (suggested here on this forum) and has been working great for 2 years, until I noticed the dd's trying to build a nest in the exhaust tube.

I think I knew more about rv's than either one of these dudes.
2018 unit....not under warranty.

[jasin1]

Originally Posted by toolittletime

Was in the rv...we are on a trip.
It was completely reassembled....yes exhaust cap was on.

It is a pain to get to, as I have to remove it through a hole I cut in the pantry. Other access is through the under belly storage, and have to remove all of the carpeted vertical walls more of a pita.
Cut the access hole 2 years ago to be able to replace the furnace control board after having 2 techs tell me 5 or 6 different things it could be...and of course paying them a trip charge. New control board fixed it (suggested here on this forum) and has been working great for 2 years, until I noticed the dd's trying to build a nest in the exhaust tube.

I think I knew more about rv's than either one of these dudes.
2018 unit....not under warranty.

well good luck…sounds like you are narrowing it down

[toolittletime]

Update:
After I got home , I removed the heater and got it on the workbench. I removed the burner and igniter and cleaned thoroughly. I ran a small plumbing snake up into the heating tubes as far as it would go from one end, then the other. I blew it out with my my leaf blower and a lot of crap came out....small pieces of dd nest and a lot of soot.....don't stand in front of it while blowing. I also blew it out from the other direction and swapped up several times until nothing came out.
I re installed the burner and adjusted the gap on the igniter. I sealed around the cover plate, as it has a gasket and also had been sealed with sealant.
Re installed into unit....checked for propane leaks and fired her up.

No bang, no odd smell, no nothing except good clean heat. I ran it through the heat cycle several times after letting it cool.
Calling it good!!!
Thanks for all the suggestions.
Tim