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Open Roads Forum  >  Class A Motorhomes  >  Maintenance Issues & Tips

 > Furnace trouble again .....

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xtal_01

Vermont

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Posted: 11/10/19 06:41pm Link  |  Quote  |  Print  |  Notify Moderator

I don't know what I did wrong this time ....

So back to the 1999 28-T Bounder.

I put a new furnace in last year ... second time I did this .. SUBURBAN FURNACE SF-35

I had also found the safety valve was full of oil (caused low flow and furnace not to run) ... fixed.

Tried the furnace a few weeks ago ... started but only after a few tries.

Well the only thing I had not replaced was the regulator.

So I was on here a couple weeks ago and was recommended to get this one: https://www.amazon.com/gp/product/B00KTS0QDY/ref=ppx_yo_dt_b_asin_image_o03_s00?ie=UTF8&psc=1

It came in and looks very close the the original ... fit great!

Tried the furnace .... started right up!

Only "odd" thing I noticed was the regulator "singing" ... noticeable buzzing sound. I don't remember my old one doing this.

The noise "could" also be the flow limiter right before the regulator .... part of a Tee kit I put in years ago so I could feed the RV with an external tank ... the piece that screws into the regulator has what I think is a high flow shut off (think they are called velocity fuses) ... in case a line breaks and there is high flow it shuts off the line.

Hard to tell but I think it is the regulator.

Anyway ..... furnace runs for about 5 minutes then I hear the noise stop from the regulator and the flame go out .... it is no were near temperature so it is not the thermostat ... but the fan keeps running.

Then after a couple minutes, the furnace ignites again! Fan has continued to run during this time.

Then after about 5 minutes (maybe less) .... flame goes out .....

Fan still running .... a couple minutes pass ... furnace ignites again!

It does this consistently over and over again!

So what is going on ???????????????????

My best "guess" is that the over temp switch is tripping

When it cools, the burner comes back on.

I checked the vents in the RV ... clear.

I checked inlet and exhaust for bugs (I have a screen on them) ... blew it out ... clear.

Could this be it ... if the switch trips, will the fan keep running and the furnace start again when it cools ??????

If this is the problem .... why?

Bad switch? Furnace is only a year old and this has never happened before.

Why did it show up when I changed out the regulator? More propane now so it is getting hotter?

Bad regulator giving too much pressure?

I am not sure what to do ... I need to get this fixed in a few weeks .....any suggestions????

Also, if it is the switch, which one is it ... I see a few different ones out there listed for a SF-35

Another "also", I did check the stove burner ... goes super low with no problem ... on high it seems to have a slight pulsation to the flame (just slightly ... high .. a bit lower ...high ... a bit lower) ... not much but something I noticed.

Thanks ..... Mike

goff1256

Montrose Co

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Posted: 11/10/19 06:59pm Link  |  Quote  |  Print  |  Notify Moderator

That’s normal operation so the furnace doesn’t get too hot it has a high temperature limit switch which limits the temperature inside the furnace and they normally cycle like that LP regulators sometimes make a little noise don’t worry about it if you want to check it you have To get a manometer to check the pressure and the flow you can find that information probably in YouTube


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DFord

Near St Louis, MO

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Posted: 11/10/19 07:07pm Link  |  Quote  |  Print  |  Notify Moderator

There will be an 1/8" pipe plug on the gas valve where you can put an adapter to measure the pressure of the gas. Ideal is 11 inch water column but I think the range is 11 to 14 - read your installation manual. It sure sure sounds like the furnace is getting too hot and the limit switch is interrupting the heating cycle. It would be normal for the fan to continue to run and cool down the overheat condition. If you don't have a manometer to measure the gas pressure you can make a "U-Tube Manometer" with a piece of clear plastic hose about 1/4" ID. You fill it with water about 1/2 way. The gas proper pressure will push one side down 5-1/2" while the other side goes up 5-1/2" (for a difference of 11"). There's probably a YouTube video covering making one and using it.


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doxiemom11

Somewhere between MI and TX

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Posted: 11/10/19 07:41pm Link  |  Quote  |  Print  |  Notify Moderator

Ours is an Atwood, but does that so the furnace doesn't overheat. After a short time the burner relights and it will continue to do this until you reach the temp the thermostat is set for. Then it will turn burn off, fans continue to run for a minute, then shut down.

xtal_01

Vermont

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Posted: 11/10/19 07:53pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks .....

You guys read my mind (my wife says it is a short story) .... I was planning on heading to the hardware store tomorrow to make a manometer and check the pressure.

So you guys are saying it is "normal" for the high temps switch to cut in and out?

I thought I read on a dealer side (been searching for info all over the web) the unit should run and stay on until it comes up to temperature? The high temp switch is just that ... a safety switch. Or am I wrong?

The RV is 20 years old so for all I know the old regulator may have been failing for years and maybe I just never saw this problem as the furnace has never gotten enough fuel ... or maybe it is getting too much now ... or ????

One thing that always bothered me ... Bounder cut the furnace case and has the lines coming off the top ... not the ends or side where Suburban has holes for it ... it came this way from the factory! I was always wondering if there is the correct air flow over the heat exchange. I would think there is or everyone would have this problem.

Anyway, I guess I will check the pressure first and go from there.

Thanks .... Mike

DFord

Near St Louis, MO

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Posted: 11/10/19 08:36pm Link  |  Quote  |  Print  |  Notify Moderator

https://www.airxcel.com/rv/suburban/service-support

PLEASE NOTE: Suburban Gas Appliances should be installed and/or serviced by a certified gas technician or authorized service agency. To find an Authorized Service Center near you, please CLICK HERE.
AIRXCEL, INC. - SUBURBAN DIVISION
676 Broadway Street
Dayton, Tennessee 37321
Customer Service

Phone: 423.775.2131

Fax: 423.775.7015

Service Department

Phone: 423.775.2131

Fax: 423.775.7015

xtal_01

Vermont

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Posted: 11/10/19 09:00pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks!

I was just on their web site.

I tried to send an e-mail but it would not go through ... kept getting errors.

I will give them a call tomorrow.

Thanks so much !!!!!!!!!

Mike

xtal_01

Vermont

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Posted: 11/11/19 06:54am Link  |  Quote  |  Print  |  Notify Moderator

Just called the factory ... the unit should not cycle ... the limit switch should not be tripping.

So .... I can think of three problems

1) bad high temp limit switch

2) too high fuel pressure (burning hot as it has too much fuel)

3) too low fuel pressure (burning hot as it is burning very lean)

Do any of these seem plausible?

Again, I did not see this problem before I changed regulators. I am not sure if it a regulator problem or having a new regulator just made an existing problem show up.

Anything else I should look at?

Thanks ..... Mike

Chris Bryant

Arden, North Carolina

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Posted: 11/11/19 07:29am Link  |  Quote  |  Print  |  Notify Moderator

Ducting, hot air recirculating, return air.


-- Chris Bryant
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xtal_01

Vermont

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Posted: 11/11/19 07:41am Link  |  Quote  |  Print  |  Notify Moderator

I completely agree but .....

My problem is this is a 20 year old unit and the problem just started after I changed out the regulator.

My "feeling" was the old regulator was not supplying enough fuel ... the burner sounder like it was pulsing (whoosh ...whoosh ... whoosh) ...

It now sounds much more steady of a flame.

I have not changed any of the duct work in the RV and I checked to see if it was blocked ... it is not.

I did check the outside vents ... I installed a bug screen over them years ago ... even blew air down it .... seems fine.

I bought the unit 10 years ago and we lived full time in it for 3 years while we built a house (not fun here in Vermont).

One thing I did notice ... the factory cut a new hole in the furnace housing to install the ducts. Rather then use the factory cut outs, they cut out the top of the furnace.

It just always seemed odd to me as there is now way for the air to pass by the back of the exchanger ... the blower sends it over the top and out. If the air was exiting the back, it might coll the switch better.

As I said, I have had this unit for 10 years and it has never been a problem before but by putting on a new regulator and possibly putting out more heat, could I have discovered a problem that has been there all along?

Any suggestions on how to fix the problem would be greatly appreciated!

Thanks again .... Mike

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