Furnace issues

[kk5fe]

Hello all! I have a Suburban SF-35FQ in my Puma 31FKRK. I tried to light it the other night and it clicked, fired and then immediately shut off the gas.
I removed it and took the burner out and found a mud dauber bee had made a nice mud nest inside the burner. I cleaned it out and blew everything else out and inspected..... didn’t find anything else wrong. I connected it to power and checked the fans which ran fine.
I installed the unit back in and turned on the furnace. Came on and lite on the first try. Warmed up..... but it seems I still have an issue.
It runs for a few minutes then the limit switch clicks... gas gets turned off but main fan runs.
Less then a minute later the gas is opened again and the unit fires and starts warming.... then it starts all over with shutting the gas off.

My first thought was it was something I did so I removed the unit and checked. Nothing wrong that I could see.
At this point I think it’s an air flow issue. I don’t think it’s a limit switch because it’s supposed to cut off around 170 degrees? I have a heavy duty thermometer and stuck the probe in the furnace next to the limit switch. It got well over 220 degrees.
But if I take the front panel off and run it.... it runs fine so it seems it’s over heating.
I checked the floor ducts with a long fish tape with a rag zip tied to the end. I could easily push the entire way through the ducts. The only other odd thing I see is the slot in the furnace that’s supposed to sit over the hole in the floor vent is about an inch off. They cut it like that from the factory. I’m wondering if that’s enough to slow the air? I can’t move the furnace cage to match the hole because it’s against the wall. . I’d have to cut the floor to match the cage.

The camper is about a year and a half old. I live in south Louisiana and really don’t use the furnace but maybe a few times a year. I’m really wondering if it always did it and I just never noticed it.

Anyway after a long post.... does anyone have anything other ideas of what I can check or look for?

[5picker]

Did it work OK in the past? (before mud daubers?)

[kk5fe]

Originally Posted by 5picker

Did it work OK in the past? (before mud daubers?)

Yes in the past it seemed to run fine. But like I said I’m wondering if I just didn’t notice that it was cutting on and off. The fan continues to run.
I’m just doubting myself now.... it may have run perfectly before.

[myxkp]

Another nest beyond the first one?
Do both burners look the same when running? If one is restricted it may start out okay, then show signs of backing up and rolling out toward the front.

[kk5fe]

Originally Posted by myxkp

Another nest beyond the first one?
Do both burners look the same when running? If one is restricted it may start out okay, then show signs of backing up and rolling out toward the front.

Hello and thanks for the reply. In my furnace there is only one burner. The burner chamber (I’m sure it has a name but I’m not sure of it) is clear now. I snaked something through it and blew compressed air. When it turns on there is a lot of air that comes out the exhaust port on the outside of the camper. The air flow I mentioned is on the inside of the camper. The squirrel cage fan that blows air down into the under floor vents is putting out a considerable amount of air.
I think my next step is to get a camera of some sort to see inside of the duct. I’ve snaked through the ducts but maybe there is a partial blockage?

[myxkp]

Great, the burner seems clear. Next the ducts. They're likely in a row so a small flashlight at one end and a mirror at the other should do the trick. You can shorten the view by starting at one end and checking each supply working back to the other end.
I installed a filter on mine so the blower wheel stays clean; a dirty blower wheel can greatly reduce airflow.

[kk5fe]

An update to this. I forgot I had a small GoPro type camera that can project to my phone. So I put it in the ducts at each floor vent location with a flashlight.
Perfectly clear from one end to the other. With the exception of building saw dust which I vacuumed it was very clean.
So I’m back at the furnace end thinking it may be an alignment problem with the hole cut in the floor.
If that’s the case then it was like that from day one.
Go me for not noticing it... but I can count on one hand the times I’ve used it.
So.... back at it.

[kk5fe]

Wait..... forgot to put this in my original long post. At some point early this year I replaced the gas regulator. The original one had the manual switch break. I replaced it with one I bought at a camping place for two tanks similar to what I had.
But I read where someone had an issue where their new regulator was putting out to much gas and was causing the heater to overhear?
Is that even a thing that can happen? I don’t think I used the furnace since I replaced the regulator.

[myxkp]

Certainly overfiring can occur when the lp pressure is too high. Most of the gas valves I have work on have a plug in a port that can be removed to allow a pressure meter to be attached. I haven't
Looked to see if this is the case with LP gas valves on RV equipment.
Can you post a picture of your gas valve?

[nayther]

have you placed a thermometer in a supply duct? if it's cycling on the high limit (which sounds like the case) then it's getting too hot. If the discharge of the furnace is into a hole in the floor, and it's not lined up, then you're short circuiting some of the air back into the return, this air is hot and then gets hotter hence tripping the limit. Try just blocking off the 1" of gap in the floor, may not have to open up the other side. I've fixed a few friends' furnaces by adding another outlet.

[kk5fe]

Originally Posted by nayther

have you placed a thermometer in a supply duct? if it's cycling on the high limit (which sounds like the case) then it's getting too hot. If the discharge of the furnace is into a hole in the floor, and it's not lined up, then you're short circuiting some of the air back into the return, this air is hot and then gets hotter hence tripping the limit. Try just blocking off the 1" of gap in the floor, may not have to open up the other side. I've fixed a few friends' furnaces by adding another outlet.

Hello! Yes I did. I have sort of a heavy duty thermometer and stuck the lead inside the box. It was creeping over 220 and had already cut off.
I really think that’s my next step.... I need to enlarge the hole that the unit sits on top of. Side to side it’s ok, but front to back it’s almost an inch or a bit more over. I might be able to move the unit but it needs to come forward that much and I’m afraid it will pul the exhaust pipes further out of where they go through the wall of the camper.

[nayther]

Originally Posted by kk5fe

Hello! Yes I did. I have sort of a heavy duty thermometer and stuck the lead inside the box. It was creeping over 220 and had already cut off.
I really think that’s my next step.... I need to enlarge the hole that the unit sits on top of. Side to side it’s ok, but front to back it’s almost an inch or a bit more over. I might be able to move the unit but it needs to come forward that much and I’m afraid it will pul the exhaust pipes further out of where they go through the wall of the camper.

yes, get as much air as you can move across the heat exchanger and not "short cycle". That's what we call it when air bypasses the room and goes directly back to the furnace.

[5picker]

Just as a side note... out of the 10 or so R/Vs we've owned over the 45+ years, I'd guess at least 3 of them short cycled (against the hi-limit switch) when running and none of them presented a problem long term.

Yes, under ideal conditions the furnace should continue to burn/run until the temperature is satisfied but ideal conditions are not always built into a R/V.

I'm not saying you don't have an issue... just saying I've seen it happen.

[kk5fe]

Originally Posted by myxkp

Certainly overfiring can occur when the lp pressure is too high. Most of the gas valves I have work on have a plug in a port that can be removed to allow a pressure meter to be attached. I haven't
Looked to see if this is the case with LP gas valves on RV equipment.
Can you post a picture of your gas valve?

Here is a photo of the regulator.
Thanks!

[kk5fe]

Originally Posted by 5picker

Just as a side note... out of the 10 or so R/Vs we've owned over the 45+ years, I'd guess at least 3 of them short cycled (against the hi-limit switch) when running and none of them presented a problem long term.

Yes, under ideal conditions the furnace should continue to burn/run until the temperature is satisfied but ideal conditions are not always built into a R/V.

I'm not saying you don't have an issue... just saying I've seen it happen.

Totally understand and thanks for the input. I’ve read where some folks have this problem. I’d like to do everything I can to remedy it. But in the end I just don’t use it that much to worry about it.

[myxkp]

Originally Posted by kk5fe

Here is a photo of the regulator.
Thanks!

That's a great shot of the regulator but what I need is a picture of the gas valve. There may be another point at which you can put a pressure meter in the line.

[kk5fe]

Originally Posted by myxkp

That's a great shot of the regulator but what I need is a picture of the gas valve. There may be another point at which you can put a pressure meter in the line.

You did say that didn’t you.... but I thought that was the same thing. I don’t know what it is then. The regulator is the only thing I see.

[myxkp]

You're very first picture is a burner sitting on top of the whole furnace assembly and the gas valve is what feeds the lp fuel into the burner