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01-15-2025, 05:27 PM
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#1
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Senior Member
Join Date: Oct 2022
Posts: 102
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Suburban furnace high limit. Anyone run a higher temp?
So, out of pure desperation, an RV dealer handed me another limit switch. I realize they are 'not' supposed to trip, but I am finding that is common as dirt, and people constantly having issues with them.
My OE switch was a 160F. I could not get a 160 and I am in a bad spot here regarding cold weather. He handed me an orange code switch, which is 225F. They are a dealer and service center and said that is all their ever put in them. I am a bit nervous about that much of a bump! Unfortunately I don't get to pick and choose! I either run that or I have no heat.
Personally, I would probably have preferred a 180-195.
Yes, I realize this is apparently an air flow issue. I have actually open ended 2 of the flex ducts and even added another, which makes 5, 4" ducts, on a 35k unit.
On the 160, I would say it would trip the thermal about 6x/hour. I come from residential HVAC and tripping a limit is not good, but seems these RV furnaces do it on the regular. The dealer was saying they are replacing them on near new equipment.
Hoping to hear some good news. I need sleep bad.
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01-15-2025, 05:55 PM
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#2
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Senior Member
Newmar Owners Club Winnebago Owners Club Freightliner Owners Club
Join Date: Mar 2016
Location: Kingman Az
Posts: 1,745
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I found by sealing all of the ducting to the furnace case incleased the flow to each grill outlet by about 40%. taped over the precuts in the case as well. the case was fit wrong and had a factory obstruction between the floor duct entrance that I removed as well. The factory install was just very poor. the furnace cartridge that slides into the furnace and uses a sheet metal block to seperate the intake from the output, I had to rebend the case to get it to fit correctly to stop the hot air from being drawn into the cold air intake.
__________________
May your black water hose never break!
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01-15-2025, 06:00 PM
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#3
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Senior Member
Join Date: Oct 2022
Posts: 102
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Quote:
Originally Posted by lonfu
I found by sealing all of the ducting to the furnace case incleased the flow to each grill outlet by about 40%. taped over the precuts in the case as well. the case was fit wrong and had a factory obstruction between the floor duct entrance that I removed as well. The factory install was just very poor. the furnace cartridge that slides into the furnace and uses a sheet metal block to seperate the intake from the output, I had to rebend the case to get it to fit correctly to stop the hot air from being drawn into the cold air intake.
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Can you tell me more about that? I am trying to understand better on the case, but yes, recirculation of hot air in the return side is something I have concerns about. Just a very poor design!
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01-16-2025, 09:28 AM
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#4
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Senior Member
Newmar Owners Club Winnebago Owners Club Freightliner Owners Club
Join Date: Mar 2016
Location: Kingman Az
Posts: 1,745
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Quote:
Originally Posted by fastline
Can you tell me more about that? I am trying to understand better on the case, but yes, recirculation of hot air in the return side is something I have concerns about. Just a very poor design!
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Need to seal the inside of the "can" or furnace case to the moho so the ducting doesn't leak. again, the case needs to be bent so the top and bottom seal against the furnace cartridge. I had to put the steel rod under the furnace case to seal the bottom. I don't see any way of sealing the top except just bending the metal does a pretty good job. From these pix you can see how the case was blocking air flow to the floor vents. I removed a section of it to allow full flow and sealed it to the vent entrance. don't block the air intake passages on the blower side of the furnace case. I believe these furnaces are very good but because coach builders poorly mount them in the coaches reduces the efficiency.
__________________
May your black water hose never break!
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01-17-2025, 12:40 AM
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#5
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Senior Member
Join Date: Oct 2022
Posts: 102
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I appreciate it! it was certainly something I was not immediately thinking about but indeed, I got to work doing some sealing. Simply amazed at how poorly these expensive RV devices are built.
But after all the seal work, though I will certainly admit I believe I have better air flow at the vents, which does mean leakage near the furnace, the high vent temps have not really changed. Seeing as high as 170F, which means easily 200F in the box, but I guess that is not stupid insane.
I think with my DC boost regulator mod, I will have actual control of the blower speed, which no OEM actually does, so that will change things quite a bit here.
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01-17-2025, 09:08 AM
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#6
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Senior Member
Join Date: Sep 2018
Location: Bushnell, Fl.
Posts: 1,394
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You are modifying a safety device on a Gas Fired appliance. I would at lease contact Suburban and get their advice on this.
Tim
__________________
2015 Silverado 3500HD CC DRW Duramax
2006 Hitchhiker Champagne
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01-17-2025, 10:13 AM
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#7
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Senior Member
Join Date: Nov 2015
Posts: 28,351
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Limit switch temp trip rating is based on Heat Exchanger design, required airflow, orifice/burner and required discharge/return sq. ft.
A 235*F Limit switch for a NT-35 model should not be used in a 160*F Limit switch rated SF-35 model
That is a recipe for disaster!
Suburban builds models that are then Installed by a 'floor jockey' that may not have an understanding of the need for correct, proper installations. May have not even seen a 'Installation Manual' that Suburban, Atwood, Dometic etc have on each model
*Piece mill work is not conductive to taking the required TIME to do the install correctly
Slap them in, get on to next one in line and go home early if you get all assigned units for the day
lonfu has shown you how poorly some installations are and how he has corrected it
Sorry you are having limit temp trip issues but using a limit switch rated at 75*F HIGHER then OEM is not the way to resolve the issue
Here is the Suburban Master Manual
https://techsupport.pdxrvwholesale.c...ing-Manual.pdf
Here is a 160*F Limit Switch for $4.30
https://pdxrvwholesale.com/products/...tch-160-232504
Here is Suburban part numbers for CORE Replacement (if needed)
https://suburbanrv.com/files/product...t%20122221.pdf
__________________
I took my Medication today. HAVE YOU?
Dodge 3500 w/Tractor Motor
US NAVY---USS Decatur DDG-31
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01-17-2025, 10:45 AM
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#8
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Senior Member
Join Date: Jul 2010
Posts: 5,693
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ionfu,
I see you've installed a furnace filter. Have you noticed any measurable reduction in air flow?
__________________
Tom
2025 BTCruiser 5255
2021 Jeep Gladiator Sport Willys
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01-18-2025, 06:35 AM
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#9
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Senior Member
Newmar Owners Club Winnebago Owners Club Freightliner Owners Club
Join Date: Mar 2016
Location: Kingman Az
Posts: 1,745
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Quote:
Originally Posted by wildtoad
ionfu,
I see you've installed a furnace filter. Have you noticed any measurable reduction in air flow?
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Ok, so after working on various styles of heating units including heat pumps I have found that air flow is what usually ends up the main problem in almost all designs and brands. This is especially true in RV furnaces. the problem most common is the installation.  I don't have a fancy air flow meter!!! I use my face  I use the farthest vent, and guestimate the distance from the vent to where I can feel the air flow on my face. (yes, I know, weird!)  My hands are not sensitive enough.  on my front vent I had to get down on my hands and knees put my face about 18" from the vent to feel the air flow. after making the ducting and case corrections I can now feel a stronger flow on my face when standing, about 6 feet high with the filter in place!  Sadly the cabinet that surrounds my furnace is not air tight, but it is about 95% blocked to cause the intake air to flow through the filter. I used a common 12x24" filter for shop air filter machine. I live in the desert so I just need to catch the larger dust particles to keep the robot vac and wife happy!  In short, it does not reduce the air flow noticeably. If you install one and you notice that it affects the air flow then I'd skip it as you don't want your furnace to over heat and catch fire.
__________________
May your black water hose never break!
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01-18-2025, 06:46 AM
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#10
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Senior Member
Newmar Owners Club Winnebago Owners Club Freightliner Owners Club
Join Date: Mar 2016
Location: Kingman Az
Posts: 1,745
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Quote:
Originally Posted by Old-Biscuit
Limit switch temp trip rating is based on Heat Exchanger design, required airflow, orifice/burner and required discharge/return sq. ft.
A 235*F Limit switch for a NT-35 model should not be used in a 160*F Limit switch rated SF-35 model
That is a recipe for disaster!
Suburban builds models that are then Installed by a 'floor jockey' that may not have an understanding of the need for correct, proper installations. May have not even seen a 'Installation Manual' that Suburban, Atwood, Dometic etc have on each model
*Piece mill work is not conductive to taking the required TIME to do the install correctly
Slap them in, get on to next one in line and go home early if you get all assigned units for the day
lonfu has shown you how poorly some installations are and how he has corrected it
Sorry you are having limit temp trip issues but using a limit switch rated at 75*F HIGHER then OEM is not the way to resolve the issue
Here is the Suburban Master Manual
https://techsupport.pdxrvwholesale.c...ing-Manual.pdf
Here is a 160*F Limit Switch for $4.30
https://pdxrvwholesale.com/products/...tch-160-232504
Here is Suburban part numbers for CORE Replacement (if needed)
https://suburbanrv.com/files/product...t%20122221.pdf
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I agree with most of what you have written, and as an official IRV2 arm chair furnace non-expert can say that you don't want to over heat your furnace...
what I noticed is that the Fan motor is larger on the SF-42. Fire box and most other parts are pretty close to the same parts. My current unit has a "OEM limit switch" for the SF-42 series; it has a 195-degree open limit and a 155-degree closed limit. Not sure if you would call that a 155 degree switch or a 195 degree switch. But that is the one that is called for  Sounds like the dealer is having problems with defective parts. I wouldn't be afraid to use the higher temp switch as the 160 degree switch is most likely a top limit of 200 degrees...that said, I would test it with a lighter, temp gun and ohm meter to see exactly what temp it opens at... If I wanted to further upgrade the furnace in my 01 DSDP I would look at increasing the fan motor RPM to push more air through the fire box and into the coach. Like I mentioned before, these furnaces are very good, the installs are very bado.  But, what can you expect for RV furnace that is heating a leaky cardboard box.
__________________
May your black water hose never break!
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01-18-2025, 12:50 PM
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#11
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Senior Member
Join Date: Nov 2015
Posts: 28,351
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Limit switches are rated on temp they trip at
195*F trips OPEN when that temp is reached
160*F trips OPEN when that temp is reached
235*F trips OPEN when that temp is reached......75*F higher then what OPs 160*F is rated for
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I took my Medication today. HAVE YOU?
Dodge 3500 w/Tractor Motor
US NAVY---USS Decatur DDG-31
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01-19-2025, 12:01 AM
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#12
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Senior Member
Join Date: Oct 2022
Posts: 102
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Quote:
Originally Posted by Old-Biscuit
Limit switches are rated on temp they trip at
195*F trips OPEN when that temp is reached
160*F trips OPEN when that temp is reached
235*F trips OPEN when that temp is reached......75*F higher then what OPs 160*F is rated for
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Actually the switches are labeled like any other switch. an "L160-40" is a 160F cutout, and 120F cut it.
my replacement was actually a 225F. HOWEVER, that limit switch does not dictate the running temp. I am pretty much running about 145-160F right now. It has tapped 170F briefly.
It will probably get me through a tough spot without setting my hair on fire, but I do want to evaluate more. I am working on a module to increase blower speed as I feel that is the right direction to go, and I think Lonfu agrees. These systems just can't push much of anything in terms of duct pressure. Couple that with anemic POS designs and there is the recipe.
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01-19-2025, 09:11 AM
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#13
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Senior Member
Newmar Owners Club Winnebago Owners Club Freightliner Owners Club
Join Date: Mar 2016
Location: Kingman Az
Posts: 1,745
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Quote:
Originally Posted by fastline
Actually the switches are labeled like any other switch. an "L160-40" is a 160F cutout, and 120F cut it.
my replacement was actually a 225F. HOWEVER, that limit switch does not dictate the running temp. I am pretty much running about 145-160F right now. It has tapped 170F briefly.
It will probably get me through a tough spot without setting my hair on fire, but I do want to evaluate more. I am working on a module to increase blower speed as I feel that is the right direction to go, and I think Lonfu agrees. These systems just can't push much of anything in terms of duct pressure. Couple that with anemic POS designs and there is the recipe.
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I would be afraid to increase the voltage to the furnace motor because the style of bearings that is used in the motor. They are just solid copper bearings and slowly wear out, meaning the longer they run at one time the sooner they wear out. there is no oil lubercation of any kind. I've tried lubing the bearings at each end but it just calms the noise they make when worn for a short time. I wouldn't be afraid to put a additional fan in the ducting system to help boost and move more air through the ducting. That said, some moho's use ducting to the basement to keep the lower compartments from freezing, they have a boost fan in the ducting. I removed my basement ducting due to the smell that kept coming up from through the basement ducting from the wetbay into the coach when moving. I found it helped increase the ducting air flow into the coach. I now use a 110v thermostatic heat lamp in the basement and I insulated all the water pipes in the wetbay. the biggest increase in air flow came when I sealed the furnace case in the above pix.
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