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Old 01-21-2021, 11:47 AM   #1
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Suburban Furnace Issues SF-35FQ

I have turned here for help. I have a 2016 Forest River Salem Hemisphere 356QB 5th wheel. We use this unit all the time and in all climates. This past Christmas we took it to Colorado for a ski trip. Well the furnace quit and we froze the camper completely! Furnace function on other trips.

Service tech comes out, changes the board, doesn't fix the problem, but he sends me a $400+ bill anyway. Here is what it does now....it will ignite, burn for a minute or 2 and go out, fan continues to run for a few minutes and then will reignite and burn for a few minutes....this cycling doesn't seem to produce much heat and the RV never really warms up...eventually the furnace will just turn off and I will have to reset it with the thermostat. Wall mounted thermostat (Dometic).

So here is what has been done:
New Dinosaur circuit board, sail switch tested and fine, fan motor works just fine, burner clean no build up no spiderwebs etc., new gas valve assembly, new high limit temp switch. Essentially this furnace is new.

So I have 4 ducts, 2 each coming off the sides of the furnace, no kinks, nice sweeping bends and these ducts are the aluminum foil slinky tube. All ducts are clear as I blew a leaf blower through them. When furnace is on, the air speed at each register (5) is ranging from 3.8 mph to 6.1 mph so I feel that flow is decent.

The furnace will run as intended if the front cover is off and heats the main cabin just fine, but the other 4 rooms suffer with no heat. when the front cover is put back on is when I get the very frequent cycling and poor heating. So this is my deduction.

There is too much heat building up in the furnace and it is not escaping fast enough via the ducting causing the upper limit switch to trip thus shutting down the propane till it cools and then cycles again. I have replaced the upper limit switch twice.

Is there any suggestions from this group as far further steps I can take, mods to make to allow more hot air to exit the furnace while not taking away from the other 4 rooms heat? I am at a loss and looking for help. I know this post was long, but I am struggling to find a viable solution. I hope I have provided enough detail and will provide more if needed.

Thanks in advance for your help. John
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Old 01-21-2021, 01:12 PM   #2
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You say the furnace works reasonably well with the cold air return grill removed. It sounds like you have a air flow restriction to the cold air return. Remove anything sitting in front of the grill such as a cooler, grocery bag or duffel bag with clothes? Do not place any type of air filter in front or behind the grill.

For the other floor/wall registers, do they have adjustable louvers? If so make sure they are open.

You say the air ducts are clean and attached properly so that leaves some kind of obstruction such as a crushed duct or even holes in the tubes from mice.

Buy a endoscope which links to your smart phone, tablet or laptop by bluetooth or wifi to visually inspect the air ducts. You can get one from amazon, harbor freight, northern tool or a local hardware store such as lowes, home depot or menards for around $50 with a 16 to 20 foot reach.

Consult the attached Suburban Service Manual for your SF-35QF furnace.
Attached Files
File Type: pdf Suburban Furnace Service Manual.pdf (4.92 MB, 14 views)
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Old 01-21-2021, 01:23 PM   #3
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You say the furnace works reasonably well with the cold air return grill removed. It sounds like you have a air flow restriction to the cold air return. Remove anything sitting in front of the grill such as a cooler, grocery bag or duffel bag with clothes? Do not place any type of air filter in front or behind the grill.

For the other floor/wall registers, do they have adjustable louvers? If so make sure they are open.

You say the air ducts are clean and attached properly so that leaves some kind of obstruction such as a crushed duct or even holes in the tubes from mice.

Buy a endoscope which links to your smart phone, tablet or laptop by bluetooth or wifi to visually inspect the air ducts. You can get one from amazon, harbor freight, northern tool or a local hardware store such as lowes, home depot or menards for around $50 with a 16 to 20 foot reach.

Consult the attached Suburban Service Manual for your SF-35QF furnace.

^^^^Everything he said plus---- Make sure that the air going into the underheated rooms can also escape. There has to be a gap at top and bottom of the door(s) in order for air to circulate.


FWIW, I had a similar problem with my TT but my furnace is small and only two ducts. The ductwork was terrible. Longest duct had something like 5 90 degree bends, two of them just to go around a small (and useless) panel between furnace and area under sink. I remodeled the ductwork so it now has only 2 90 degree bends and about 4' less "Slinky Tube".

Furnace works great now.

As for cold air intake, it might be necessary to remove the grill the factory installed and substitute a piece of heavy duty screen with 1/4" holes for more air flow. If removing it keeps the furnace running there is a definite relationship to grille and operation.
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Old 01-21-2021, 01:40 PM   #4
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Quote:
Originally Posted by Scrapper View Post
You say the furnace works reasonably well with the cold air return grill removed. It sounds like you have a air flow restriction to the cold air return. Remove anything sitting in front of the grill such as a cooler, grocery bag or duffel bag with clothes? Do not place any type of air filter in front or behind the grill.

For the other floor/wall registers, do they have adjustable louvers? If so make sure they are open.

You say the air ducts are clean and attached properly so that leaves some kind of obstruction such as a crushed duct or even holes in the tubes from mice.

Buy a endoscope which links to your smart phone, tablet or laptop by bluetooth or wifi to visually inspect the air ducts. You can get one from amazon, harbor freight, northern tool or a local hardware store such as lowes, home depot or menards for around $50 with a 16 to 20 foot reach.

Consult the attached Suburban Service Manual for your SF-35QF furnace.

Thank you for the reply. I think there is a clarification I need to make that is causing a misunderstanding. The cover I pulled off is not the return air, it is the actual cover to the furnace allowing all heated air to exit the furnace into the main cabin and not going through any ducting. Nothing is in front of or blocking the return and never has been.

Floor registers do not have adjustable louvers and are always open.

I would think a crushed duct would give me poor flow when I measured with a digital air flow meter. If the ducts were all chewed up from mice then that would mean less restriction and the heated air would be flowing out the additional holes.

I will look into a digital probe to look at the ducting. But as I mentioned, I took a leaf blower to all ducts and all seem to be free and clear of any collapse, crushed point, kink or mice ridden.

Thank you again for your reply.
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Old 01-21-2021, 01:46 PM   #5
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^^^^Everything he said plus---- Make sure that the air going into the underheated rooms can also escape. There has to be a gap at top and bottom of the door(s) in order for air to circulate.


FWIW, I had a similar problem with my TT but my furnace is small and only two ducts. The ductwork was terrible. Longest duct had something like 5 90 degree bends, two of them just to go around a small (and useless) panel between furnace and area under sink. I remodeled the ductwork so it now has only 2 90 degree bends and about 4' less "Slinky Tube".

Furnace works great now.

As for cold air intake, it might be necessary to remove the grill the factory installed and substitute a piece of heavy duty screen with 1/4" holes for more air flow. If removing it keeps the furnace running there is a definite relationship to grille and operation.
Thank you for your reply as well. While dealing with this furnace issue, I kept all doors open just to circulate what heat I was getting. So I do not see this as a potential problem.

My ducting coming out of the furnace has to make 3 90 bends just to get under the floor. All ducts run under the floor. As I mentioned when I measured the flow with a digital air flow meter, I had 3 mph to 6 mph at the registers. This seems like acceptable flow and is comparable to friends campers that measured their air flows with the same meter.

I do not see this as an issue with return air either.

Thank you again for your reply.
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Old 01-21-2021, 03:19 PM   #6
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Originally Posted by camplight View Post
Thank you for the reply. I think there is a clarification I need to make that is causing a misunderstanding. The cover I pulled off is not the return air, it is the actual cover to the furnace allowing all heated air to exit the furnace into the main cabin and not going through any ducting. Nothing is in front of or blocking the return and never has been.

Floor registers do not have adjustable louvers and are always open.

I would think a crushed duct would give me poor flow when I measured with a digital air flow meter. If the ducts were all chewed up from mice then that would mean less restriction and the heated air would be flowing out the additional holes.

I will look into a digital probe to look at the ducting. But as I mentioned, I took a leaf blower to all ducts and all seem to be free and clear of any collapse, crushed point, kink or mice ridden.

Thank you again for your reply.
Am I correct in understanding you opened the furnace housing and it runs fine except the heat is dumped in one area?

It seems that I remember another member had similar problems and he found some of the round plates where the ducts connect to the furnace plenum were left in place, cutting off the air from those ducts.

The furnace needs a minimum square inches for cold air to enter and heated air to exit. Start on page 17 with the description on how to determine the numbers needed and the actual numbers are on page 19 in the service manual I attached previously.
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Old 01-21-2021, 03:41 PM   #7
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Am I correct in understanding you opened the furnace housing and it runs fine except the heat is dumped in one area?

It seems that I remember another member had similar problems and he found some of the round plates where the ducts connect to the furnace plenum were left in place, cutting off the air from those ducts.

The furnace needs a minimum square inches for cold air to enter and heated air to exit. Start on page 17 with the description on how to determine the numbers needed and the actual numbers are on page 19 in the service manual I attached previously.
Correct. All ducting has been disconnected and inspected. No pieces are still in place that would restrict airflow. I have looked at the service manual. That page states for an SF-35/35F Ducts Req'd = 4-4" (check, I have that), no bottom duct, no top duct, L & R Side Duct Sq Inch = 48" (4 - 4" circles actually equals 50.27 sq inches, so plenty there). Return Air Sq Inch = 55" (I well exceed this requirement)

I feel there is a restriction somewhere that isn't allowing the heat out of the furnace and causing a build up of heat.
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Old 01-21-2021, 05:25 PM   #8
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Correct. All ducting has been disconnected and inspected. No pieces are still in place that would restrict airflow. I have looked at the service manual. That page states for an SF-35/35F Ducts Req'd = 4-4" (check, I have that), no bottom duct, no top duct, L & R Side Duct Sq Inch = 48" (4 - 4" circles actually equals 50.27 sq inches, so plenty there). Return Air Sq Inch = 55" (I well exceed this requirement)

I feel there is a restriction somewhere that isn't allowing the heat out of the furnace and causing a build up of heat.
Somewhere under the floor or in the walls there are restrictions, either collapsed tubes or sharp 90* turns which an endoscope will help you locate.

I know you said there was decent air flow when testing the duct tubes with a leaf blower but you have to take into account the leaf blower produces pressure and high volume where the furnace fan produces low to medium volume without pressure. This probably means the leaf blower has enough power to overcome the collapsed duct tube. Also, the leaf blower is forcing air through one duct at a time which helps it overcome any restriction where the furnace blows through all the duct tubes simultaneously.
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Old 01-21-2021, 05:36 PM   #9
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Somewhere under the floor or in the walls there are restrictions, either collapsed tubes or sharp 90* turns which an endoscope will help you locate.

I know you said there was decent air flow when testing the duct tubes with a leaf blower but you have to take into account the leaf blower produces pressure and volume where the furnace fan produces volume without pressure. This probably means the leaf blower has enough power to overcome the collapsed duct tube(s). The leaf blower is forcing air through one duct at a time which also helps it overcome any restriction where the furnace blows through all the duct tubes simultaneously.
Thank you again for your reply. The airflow measurements I took were with the blower of the furnace, not the leaf blower. The leaf blower was only used to push any obstruction out of there was something.

I will look into getting a camera to check the ducts.

Thanks again.
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Old 01-21-2021, 05:40 PM   #10
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Is it possible that heat is being recirculated in the space surrounding the furnace. Leakage at ducts that makes it's way back into the cold air intake?

Any holes and tears in the ducts that may have been overlooked, again dumping heat near the furnace cold air intake?

Have you measured the air temp near the furnace? I know that the area my furnace is installed get quite warm and it wouldn't take more heat to cause the furnace to trip the limit thermostat.

If there is a lot of heat building up due to heat from ducts (even without leaks) then replacing the foil covered slinky ducting with insulated duct might be the solution.
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Old 01-22-2021, 10:09 AM   #11
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Is it possible that heat is being recirculated in the space surrounding the furnace. Leakage at ducts that makes it's way back into the cold air intake?

Any holes and tears in the ducts that may have been overlooked, again dumping heat near the furnace cold air intake?

Have you measured the air temp near the furnace? I know that the area my furnace is installed get quite warm and it wouldn't take more heat to cause the furnace to trip the limit thermostat.

If there is a lot of heat building up due to heat from ducts (even without leaks) then replacing the foil covered slinky ducting with insulated duct might be the solution.
Thank you for the reply. I inspected all ducts and they all seem very intact and minimal heat leakage that could be recirculated. I have not measured the air temp in the area of the intake. I would have to think the intake air would have to be pretty warm / hot for this to be a factor. I will check this out as well.
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Old 01-22-2021, 10:17 AM   #12
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Update - I called Air Excel, they own Suburban. First tip, do not tell them you are an end user, they will not help you. You must tell a little fib and say you are technician. Although I told them I was an end user, I was able to persuade the person to at least ask the tech some questions. I am a mechanical engineer by trade.

They said my ducting is most likely an issue. For each 90 bend in the flex duct, you can lose up to 15% air flow. So just from my furnace to get under the floor, I have 4 - 90 bends in the first 2-3 feet if flex duct. And there are 4 ducts like this coming off the furnace and making their way under the floor. I do not know what other "bends" there are beneath the floor.

Also called a Forest River service center 2 hours away and the tech there told me to check for mud dauber nests in the exhaust and combustion chamber. I had a access to a small camera but it would not make it past the first bend in the exhaust chamber, but did see a mud dauber nest. So I will be taking the furnace out and seeing if I can get the chambers apart to inspect and clean further. My advice, put on the insect screens to keep these pests out of your RV orifices.

Thanks all so far for all the suggestions and help.
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Old 01-22-2021, 11:14 AM   #13
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Update - I called Air Excel, they own Suburban. First tip, do not tell them you are an end user, they will not help you. You must tell a little fib and say you are technician. Although I told them I was an end user, I was able to persuade the person to at least ask the tech some questions. I am a mechanical engineer by trade.

They said my ducting is most likely an issue. For each 90 bend in the flex duct, you can lose up to 15% air flow. So just from my furnace to get under the floor, I have 4 - 90 bends in the first 2-3 feet if flex duct. And there are 4 ducts like this coming off the furnace and making their way under the floor. I do not know what other "bends" there are beneath the floor.

Also called a Forest River service center 2 hours away and the tech there told me to check for mud dauber nests in the exhaust and combustion chamber. I had a access to a small camera but it would not make it past the first bend in the exhaust chamber, but did see a mud dauber nest. So I will be taking the furnace out and seeing if I can get the chambers apart to inspect and clean further. My advice, put on the insect screens to keep these pests out of your RV orifices.

Thanks all so far for all the suggestions and help.
A blocked/partially blocked furnace exhaust would certainly overheat the furnace.
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Old 01-22-2021, 02:50 PM   #14
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Also called a Forest River service center 2 hours away and the tech there told me to check for mud dauber nests in the exhaust and combustion chamber. I had a access to a small camera but it would not make it past the first bend in the exhaust chamber, but did see a mud dauber nest. So I will be taking the furnace out and seeing if I can get the chambers apart to inspect and clean further. My advice, put on the insect screens to keep these pests out of your RV orifices.
Suburban says to never install those screens because they restrict the air flow too much. If you do use them, remove them before running the furnace and definitely do not have them installed when you go into a dealership for warranty work.

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Old 01-22-2021, 05:15 PM   #15
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Suburban says to never install those screens because they restrict the air flow too much. If you do use them, remove them before running the furnace and definitely do not have them installed when you go into a dealership for warranty work.

Ray
I've had Camco bug screens on my Suburban furnace and water heater for 7 years and never had a problem.
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Old 01-22-2021, 06:24 PM   #16
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I know there have been several different suggestions which could be the problem however, going back to your original post:

Quote:
The furnace will run as intended if the front cover is off and heats the main cabin just fine, but the other 4 rooms suffer with no heat. when the front cover is put back on is when I get the very frequent cycling and poor heating
this makes a really strong case for the issue being in the ductwork. As you were told by the manufacturer, 90 degree bends restrict flow noticeably. Distribution box cover on--furnace problems. Cover off----no problems. To me this is very telling.

Before I opened up the furnace for any further investigation I'd focus my energies on the ductwork. Something tells me you're going to find that there is a hidden restriction that's holding back air from the furnace fan/blower itself.

If possible I'd figure out how to lessen the angle of the bends to more of a sweeping bend than a 90 degree bend. Maybe even see if it's possible to upsize the ducting to a larger size/diameter.
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Old 01-22-2021, 09:47 PM   #17
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Two impellers

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Update - I called Air Excel, they own Suburban. First tip, do not tell them you are an end user, they will not help you. You must tell a little fib and say you are technician. Although I told them I was an end user, I was able to persuade the person to at least ask the tech some questions. I am a mechanical engineer by trade.

They said my ducting is most likely an issue. For each 90 bend in the flex duct, you can lose up to 15% air flow. So just from my furnace to get under the floor, I have 4 - 90 bends in the first 2-3 feet if flex duct. And there are 4 ducts like this coming off the furnace and making their way under the floor. I do not know what other "bends" there are beneath the floor.

Also called a Forest River service center 2 hours away and the tech there told me to check for mud dauber nests in the exhaust and combustion chamber. I had a access to a small camera but it would not make it past the first bend in the exhaust chamber, but did see a mud dauber nest. So I will be taking the furnace out and seeing if I can get the chambers apart to inspect and clean further. My advice, put on the insect screens to keep these pests out of your RV orifices.

Thanks all so far for all the suggestions and help.
The two impellers (inside air and outside air) are on the same motor, one at each end. I was thinking that the outside ports could be clogged (mud daubers are common) and that could reduce the impeller speed.

But Richard (Scrapper) always reminds me that reducing impeller flow leads to higher motor rotation speed.

Let us know what you finally find.
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Old 01-27-2021, 06:57 PM   #18
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For sure you have an air flow issue. Commercial HVAC is my career. Tell me I'm wrong but I bet if you put a thermometer in a supply grille the temp just keeps climbing to 140, then the high limit opens and the flame goes out correct? Lack of air flow can be on either side of the heat exchanger, opening up the return will help, adding or straightening out the supply duct will help, perhaps a combination of the two. Blowers are rated at a specific CFM vs static pressure, these little forward curved fans don't produce much static. As an experiment find the one outlet that seems to not blow much and connect a duct directly to that collar and dump it out into the room (short as possible). Then remeasure your temp. rise. I've fixed a couple heating systems by simply adding another supply register close to the furnace, there should be an unused knockout or two in the supply plenum, or cut your own. Any 4" start collar will work to connect the flex to the plenum. We get next to no air in our bedroom and bathroom, but that's OK as I don't like heat when I'm sleeping anyway.
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Old 01-27-2021, 07:40 PM   #19
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Hmm...

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For sure you have an air flow issue. Commercial HVAC is my career. Tell me I'm wrong but I bet if you put a thermometer in a supply grille the temp just keeps climbing to 140, then the high limit opens and the flame goes out correct? Lack of air flow can be on either side of the heat exchanger, opening up the return will help, adding or straightening out the supply duct will help, perhaps a combination of the two. Blowers are rated at a specific CFM vs static pressure, these little forward curved fans don't produce much static. As an experiment find the one outlet that seems to not blow much and connect a duct directly to that collar and dump it out into the room (short as possible). Then remeasure your temp. rise. I've fixed a couple heating systems by simply adding another supply register close to the furnace, there should be an unused knockout or two in the supply plenum, or cut your own. Any 4" start collar will work to connect the flex to the plenum. We get next to no air in our bedroom and bathroom, but that's OK as I don't like heat when I'm sleeping anyway.
Hmm... We get way too much heat in the bedroom. It's on a 3-foot duct direct to the furnace. All the others are daisy-chained on a second duct, I think. This supposition is based on placement. The bedroom is at one end of a 38-foot trailer. The furnace is immediately outside the bedroom. All the other vents are in a single line, going the other way from the furnace. The trailer is insulated below, with a vapor barrier beneath, so I'd have to open it up in ways that I'd rather not, to see whether it really is a single duct. I've got a 10-meter borescope--maybe I will play with that.

You can easily feel the airflow at the bedroom vent. The flow is successively weaker as you go down the line, heading away from the furnace. I imagine the entire system is done in 4" round, not successively smaller sizes as my sticks-and-bricks house. I am, of course, reluctant to damper the bedroom vent for just this reason; fear of overheating and hitting the limit switch.

Any advice for me, Nayther?
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Old 01-28-2021, 11:16 AM   #20
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Originally Posted by nayther View Post
For sure you have an air flow issue. Commercial HVAC is my career. Tell me I'm wrong but I bet if you put a thermometer in a supply grille the temp just keeps climbing to 140, then the high limit opens and the flame goes out correct? Lack of air flow can be on either side of the heat exchanger, opening up the return will help, adding or straightening out the supply duct will help, perhaps a combination of the two. Blowers are rated at a specific CFM vs static pressure, these little forward curved fans don't produce much static. As an experiment find the one outlet that seems to not blow much and connect a duct directly to that collar and dump it out into the room (short as possible). Then remeasure your temp. rise. I've fixed a couple heating systems by simply adding another supply register close to the furnace, there should be an unused knockout or two in the supply plenum, or cut your own. Any 4" start collar will work to connect the flex to the plenum. We get next to no air in our bedroom and bathroom, but that's OK as I don't like heat when I'm sleeping anyway.
Sorry all. For some reason, I stopped getting notifications. At this point, I have no update. I have had other issues come up in life that have taken a higher priority.

I do however have a new flexible camera that I plan to send through the duct work and look at bends, possible collapses...etc. Just in the first 3 feet of duct work on each one (there are 4 ducts) there are 4 - 90 bends just getting the duct under the floor. To modify this, I would need to cut more of the subfloor. A project that I will tackle at a later date. Right now the camper has been placed in storage in my shop. I will revisit this issue in a few weeks hopefully.

I am fairly confident as well that it is a flow issue. But why did it "used" to work? I am stumped by this. I will be using the new flexible camera to check deaper in the chambers for mud dauber nests too...I can do this from the ports on the outside.

The register in the floor right near the furnace, shortest run, shows surface temps of that register getting up to 120! The air temp was hotter.

I appreciate the help of all people commenting and offering suggestions. I will update in a few weeks hopefully. Take care!
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2011 Dodge Ram Crew Cab 2500 6.7L 4x4
2016 Salem Hemisphere 356QB
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