Adding a Heating Duct and Vent to My Fifth Wheel

[RussOnTheRoad]

This post is about how I added new heating system register and ducting to my 2018 Arctic Fox 29-5T fifth wheel. I was asked in another thread to post this information. Rather than distract from the topic of the other thread I am posing the information here.

I added an additional duct when it became apparent that the ducting in my RV could not expel the hot air from the furnace fast enough. This caused an excessive build-up of hot air in the furnace which in turn caused the burner to short-cycle on-and-off repeatedly which was overworking the high temp limit switch (a safety switch that shuts off the burner when the furnace overheats) and leading to the need for a premature repair. Adding the additional duct allows the furnace to better expel the hot air so that it no longer overheats and no longer short-cycles. A furnace burner should turn on and remain on until the RV reaches the temperature set on the thermostat. If the burner turns on and off repeatedly before the RV comes to the set temp there is a problem. Many RVs have this problem unbeknownst to the owners. It can be caused by a clogged or damaged vent or by the RV manufacturer not providing ample ducting when designing and building the RV (this happens!) or another problem. (Note: a burner can cycle repeatedly while the fan continues to run. If one listens carefully to the furnace it may be possible to hear the burner turn on and off. Placing a quick reading thermometer such as a digital food thermometer over a heating register will show if the burner comes on and off--the temperature will vary maybe 20 to 40 degrees as it cycles on and off.)

My 29-5T has a Suburban SF-35FQ furnace located under the fridge. (This is not a great spot, IMHO, because the fridge above it is trying to keep things cold while the furnace is heating things up from underneath. It would be better to keep refrigerators and furnaces further apart and not placing one above the other especially with the furnace beneath... but I digress.)

The SF-35FQ has seven, 4” circular vent knockouts on its sides to which 4” round, flexible ducting can be attached via a collar which is inserted into the furnace after a knockout is removed. It also has a large rectangular duct on the bottom which is equivalent to four of the 4” round vents.

Suburban requires at least four of the vents be used when installing this furnace. Arctic Fox did that by using the vent on the underside of the furnace, but they also incorporated 90 degree bends into the ducting and these 90 degree bends reduce the airflow and work against the minimum requirement for outlet ducting. Suburban says “Avoid making any sharp turns in the duct system. Sharp turns will increase the static pressure in the plenum area and could cause the furnace to cycle.” That seems to be exactly what was going on in my RV.

I decided the easiest and best fix was to add another duct. In order to do that I removed the grille from in front of the furnace (4 Phillips head screws), then the furnace cover (2 slotted screws). I then pried off one of the knockouts on the left of the furnace using a chisel, small hammer and pliers. For the new vent I cut a hole in the paneling where it was to be placed. For that I covered the area with painter's tape in order to minimize the likelihood of the surface of the wood splintering. Then using a pencil compass I marked a circle on the tape that I would cut out using a rotary tool at 30,000 RPM fitted with a spiral cutting bit designed to cut soft wood. Next, using a 4” hose clamp I attached the collar required to a piece of 4" UL Listed dryer vent ducting good up to 265 degrees and slid the collar with attached ducting into the cabinet housing the furnace, fitting it into the furnace where I had knocked out the vent cover. I attached the vent to the collar before putting the collar in place because I would not have been able to get to the hose clamp with a screwdriver once the collar was inside the cabinet. Then I slid another hose clamp onto the ducting over the free end and slid that end through the hole I had cut into the wood paneling and over the vent. Next, I placed the vent on the paneling, and after drilling pilot holes in the paneling screwed the vent into place after which I slid the hose clamp that was hanging loose on the ducting into position and clamped it down. Finally, I replaced the wood grille.

The addition of the new duct will not only add life to the furnace before requiring repair but because the furnace can now expel the heated air more quickly and the RV heats up much faster.

Parts List:
Suburban 050715 4" Duct Collar
Deflecto Supurr-Flex Aluminum Dryer Vent Duct, Flexible, Fire-Resistant, 4 Inches Dia. x 8 Feet, Silver (F0408B/4)
4" Hose Clamps
Valterra A10-3353VP Rotating/Dampered Heating and A/C Register-4″ ID x 5-3/8″ OD
Painter's tape

Tool List:
1/8" spiral cutting bit for soft wood
Rotary tool (like Black & Decker RTX, Roto Zip or Dremel)
Flat and #2 Phillips bit screwdriver
Pliers
Chisel (may not be necessary)
Small hammer (may not be necessary

[benderdog]

Very interesting. Great job and lots of information thanks for sharing. May try to add one in front off my closet in bedroom.

[RussOnTheRoad]

Originally Posted by benderdog

Very interesting. Great job and lots of information thanks for sharing. May try to add one in front off my closet in bedroom.

This isn't rocket science but there are things that can be done wrong so I think it's a good idea to consult the literature from the furnace mfr. I did not include such info in my post, for example, vent ducts must be a certain distance from the furnace intake in order to avoid the furnace taking in heated air which can cause it to overheat.

There may be additional helpful info on YouTube and other forum posts, perhaps some of it unintentionally showing how not to do something like this.

[kmac7800]

Thank you for the detailed information. The furnace in my rig is right under my fridge also, however I have not noticed the fluctuations.


Regards


//KMac

[RussOnTheRoad]

Originally Posted by kmac7800

Thank you for the detailed information. The furnace in my rig is right under my fridge also, however I have not noticed the fluctuations.


Regards


//KMac

I did not mean to suggest that the repeated on-off-on-off cycling of the furnace in my unit (or anybody else's) would be related to the relative position of the fridge and furnace. I only mentioned that my fridge was above my furnace as an unrelated observation about a design that didn't seem the best idea.

The furnace burner cycling was caused by my furnace not being able to sufficiently expel the hot air it makes which caused the burner to overheat and the high temperature limit switch to activate.

[Ryqster]

Can't say that I have noticed this up to now. I will check this though. Seems like this could be problematic if not addressed.

[RussOnTheRoad]

Originally Posted by Ryqster

Can't say that I have noticed this up to now. I will check this though. Seems like this could be problematic if not addressed.

I have heard from an owner of a 2018 Arctic Fox 32.5M that he is seeing the problem. I have also heard from an owner of an Arctic Fox 29-5T that he hadn't noticed it, but I asked him to take a careful look. I don't mean to single out Arctic Fox because I expect this may be a more widespread issue.

I do hope you will check your 5T and report back whether or not you are seeing the temp swings at the register and notice excessive burner cycling or not.

BTW, your signature file indicates you have a 28-5T. I'm guessing that's a typo... I never heard of a 28-5T. You must have meant 29-5T, no?

[Ryqster]

Typo...oops

[grantrekker]

Great info and timely for my purposes, AND I hope this reply is not viewed as too much of a digression that it needs it's own thread. It seems what you have done is applicable to what I intend to undertake. History: In my 2014 AF 29.5K, the heat unit (appears to be identical to the picture you posted) is under the kitchen sink cabinetry. I noticed that the entire cabinet was warming up so I pulled some drawers and panels and discovered that one of the large knockouts on the top was dislodged and had fallen down into the plenum (?) which seems to surround heat tubes (I now believe you are aware of my lack of education with respect to HVAC). Since this "cabinet warming" happened suddenly on the Alcan Highway and the large knockout was just laying on the tubes in the plenum, I assumed this knockout was intended to remain intact. Now, RV shops in Seward, AK do not exist so I went to the well stocked local hardware store and bought some of the silver ducting tape and some tin and tinsnips and cut a sheet which I superimposed over the open area where the failed knockout was removed from. Problem solved I hope. ??? Now to the issue at present. My observation is that the only heat duct exiting from the plenum originates from the bottom of the plenum area (I think I have that term right) and enters a duct that feeds four vents, two in the lower cabin and two in the upper bedroom cabin and whatever else is shuttled to heat the water and waste tanks. We leave two heat register vents in the upper cabin shut permanently as we enjoy cold sleeping and can tolerate a chilly bathroom and shower area or can open the connecting door to the upper area and the warm "downstairs" air heats the upper room very well. In the process of this on the road repair, I noticed that there are two 4" knockouts that face into the living area (floor level) and thought that I would take one of these out and introduce warm air into the main lower cabin through the cabinet wall which is approx. 1-1/2" from the knockout. This is intended to 1) allow for additional warm air introduction into the lower cabin hopefully resulting in quicker warm-up time, and 2) provide a airflow offset to the heat registers that we keep closed in the upper cabin in the event that there is some back pressure issue created by only using two of the four cabin register outlets in the factory design. First question: Am I on track with this so far? Your post states "...Suburban requires at least four of the vents be used when installing this furnace. Arctic Fox did that by using the vent on the underside of the furnace." Assuming this to be the case for my heating unit and observing that the only exit vent used is the large rectangular down vent that effectively services only two active cabin registers and whatever flow that is directed down into the holding tank areas, I question whether the required outflow is being met after adding the to be installed 4" vent. If all is good to this point, on to the next question.

Regarding installation of the new 4" vent. It appears as though the now removed 4" side vent can easily exit through a 1/8" wood cabinet base (much like the picture you show only my heat unit is under the kitchen cabinet complex and NOT the refer) into the lower cabin area just 1-1/2" away with just the addition of a collar and a vent cover, with maybe some ducting tape. Seems fairly simple. What am I missing? Would using both of the 4" side exits be better instead of just one?

Your observations of and answers to the above are greatly appreciated.

[RussOnTheRoad]

Originally Posted by grantrekker

Great info and timely for my purposes, AND I hope this reply is not viewed as too much of a digression that it needs it's own thread. It seems what you have done is applicable to what I intend to undertake. History: In my 2014 AF 29.5K, the heat unit (appears to be identical to the picture you posted) is under the kitchen sink cabinetry. I noticed that the entire cabinet was warming up so I pulled some drawers and panels and discovered that one of the large knockouts on the top was dislodged and had fallen down into the plenum (?) which seems to surround heat tubes (I now believe you are aware of my lack of education with respect to HVAC). Since this "cabinet warming" happened suddenly on the Alcan Highway and the large knockout was just laying on the tubes in the plenum, I assumed this knockout was intended to remain intact. Now, RV shops in Seward, AK do not exist so I went to the well stocked local hardware store and bought some of the silver ducting tape and some tin and tinsnips and cut a sheet which I superimposed over the open area where the failed knockout was removed from. Problem solved I hope. ??? Now to the issue at present. My observation is that the only heat duct exiting from the plenum originates from the bottom of the plenum area (I think I have that term right) and enters a duct that feeds four vents, two in the lower cabin and two in the upper bedroom cabin and whatever else is shuttled to heat the water and waste tanks. We leave two heat register vents in the upper cabin shut permanently as we enjoy cold sleeping and can tolerate a chilly bathroom and shower area or can open the connecting door to the upper area and the warm "downstairs" air heats the upper room very well. In the process of this on the road repair, I noticed that there are two 4" knockouts that face into the living area (floor level) and thought that I would take one of these out and introduce warm air into the main lower cabin through the cabinet wall which is approx. 1-1/2" from the knockout. This is intended to 1) allow for additional warm air introduction into the lower cabin hopefully resulting in quicker warm-up time, and 2) provide a airflow offset to the heat registers that we keep closed in the upper cabin in the event that there is some back pressure issue created by only using two of the four cabin register outlets in the factory design. First question: Am I on track with this so far? Your post states "...Suburban requires at least four of the vents be used when installing this furnace. Arctic Fox did that by using the vent on the underside of the furnace." Assuming this to be the case for my heating unit and observing that the only exit vent used is the large rectangular down vent that effectively services only two active cabin registers and whatever flow that is directed down into the holding tank areas, I question whether the required outflow is being met after adding the to be installed 4" vent. If all is good to this point, on to the next question.

Regarding installation of the new 4" vent. It appears as though the now removed 4" side vent can easily exit through a 1/8" wood cabinet base (much like the picture you show only my heat unit is under the kitchen cabinet complex and NOT the refer) into the lower cabin area just 1-1/2" away with just the addition of a collar and a vent cover, with maybe some ducting tape. Seems fairly simple. What am I missing? Would using both of the 4" side exits be better instead of just one?

Your observations of and answers to the above are greatly appreciated.

It sounds like you taped some tin on the furnace to cover the knockout that fell out. It may be more likely that the knockout was removed and left inside the furnace by a workman. That's the kind of thing I have come to expect from RV manufacturers. I believe they make duct cover plates for the purpose of covering knockouts that you can use instead of tin and tape. This might be the item:
https://www.rvautoparts.com/050733-S...iABEgI_W_D_BwE

You refer to a knockout on the "top" of the furnace. Did you mean "side"? I'm not sure there are any knockouts on top. Furnaces do differ from one to the next, I'm sure.

I would suggest you determine which furnace you have so that you can be certain of its requirements both in terms of servicing and parts. Manuals may be hard to find because Suburban doesn't seem to like the idea of regular Joes like us getting our hands on them but they can be found if not on the Suburban web site. Try this page/site:
https://techsupport.pdxrvwholesale.c...urban-sf-35fq/

The airflow requirements for ducting are in the installation instructions and are provided in terms of square inches and number of ducts required to be used... at least that's what I recall. I did say that Arctic Fox (more properly Northwood) met the venting requirements in my RV by using the bottom vent but I also said they put 90° bends in the ducting that works against that. Said another way I think they failed to meet the minimum venting requirements.

Registers need to be placed a minimum of 18" from the return air opening. From the installation instructions for the Suburban SF-20FQ • SF-25FQ • SF-30FQ • SF-35FQ • SF-42FQ: "No outlet register is to be placed within 18” of the return air opening. Any register installed at 18” should never be adjusted to blow the outlet air toward the return air opening. If a register is installed on a wwall [sic] 90 degrees to the return air, it can be less than the 18”. They don't want hot air coming back into the furnace because this can cause it to overheat. You can get a copy of the instructions at the page/site I mentioned above.

I found all the parts I used on Amazon. BTW, the vent register I used protrudes several inches on the rear to allow for attaching 4" flexible ducting. I don't think you could use it if the furnace is only 1.5" from the wall on which you plan to mount the register. You may need to use a knockout on another side of the furnace with some flexible ducting.

HTH.

[grantrekker]

Originally Posted by RussOnTheRoad

It sounds like you taped some tin on the furnace to cover the knockout that fell out. It may be more likely that the knockout was removed and left inside the furnace by a workman. That's the kind of thing I have come to expect from RV manufacturers. I believe they make duct cover plates for the purpose of covering knockouts that you can use instead of tin and tape. This might be the item:
https://www.rvautoparts.com/050733-S...iABEgI_W_D_BwE

You refer to a knockout on the "top" of the furnace. Did you mean "side"? I'm not sure there are any knockouts on top. Furnaces do differ from one to the next, I'm sure.

I would suggest you determine which furnace you have so that you can be certain of its requirements both in terms of servicing and parts. Manuals may be hard to find because Suburban doesn't seem to like the idea of regular Joes like us getting our hands on them but they can be found if not on the Suburban web site. Try this page/site:
https://techsupport.pdxrvwholesale.c...urban-sf-35fq/

The airflow requirements for ducting are in the installation instructions and are provided in terms of square inches and number of ducts required to be used... at least that's what I recall. I did say that Arctic Fox (more properly Northwood) met the venting requirements in my RV by using the bottom vent but I also said they put 90° bends in the ducting that works against that. Said another way I think they failed to meet the minimum venting requirements.

Registers need to be placed a minimum of 18" from the return air opening. From the installation instructions for the Suburban SF-20FQ • SF-25FQ • SF-30FQ • SF-35FQ • SF-42FQ: "No outlet register is to be placed within 18” of the return air opening. Any register installed at 18” should never be adjusted to blow the outlet air toward the return air opening. If a register is installed on a wwall [sic] 90 degrees to the return air, it can be less than the 18”. They don't want hot air coming back into the furnace because this can cause it to overheat. You can get a copy of the instructions at the page/site I mentioned above.

I found all the parts I used on Amazon. BTW, the vent register I used protrudes several inches on the rear to allow for attaching 4" flexible ducting. I don't think you could use it if the furnace is only 1.5" from the wall on which you plan to mount the register. You may need to use a knockout on another side of the furnace with some flexible ducting.

HTH.

Thnak you RussOnTheRoad for your most informative and timely reply. I will reply to your comments:

1) there was a large retangular knockout on the top of the unit and is in the same place that your picture shows the indented area or what I assumed to be a knock out as it was still hanging down into the plenum by a shred of a tab

2) I will measure the now large opening on the top and order the cover you suggested, That is a much more elegant covering than my tin & tape on-the-road solution

3) I do need to find the model number on this unit. Am afraid it is on the top and if so would entail destruction of the cabinet to some degree to view.

4) Airflow requirement as you state "...for ducting are in the installation instructions and are provided in terms of square inches and number of ducts required to be used.... As best as I can determine, the only outlet from the plenum that was used was what I assume to be downward and into the basement area. Further it seems to be a pretty straight shot for the ducting without any severe bends. However, if the manual describes the airflow requirements in terms of output from the plenum, this would be simply one large downward knockout into the main lateral vent line with the four cabin registers and whatever holding tank exits exist. Wouldn't covering/shutting 2 of the factory cabin registers then restrict the airflow at the end distribution point to the same effect as shutting it down at the plenum (except considering the downflow directed to the holding tank areas)? I have effectively closed down 32s.i. of flow so reasoned that opening up one 4" hole withing 1-1/2" of the plenum would be replacing the outflow by only about 12.6s.i. So I might be shy by quite a bit for a one to one replacement of the closed vents if that is to be considered.
5) Regarding placement of the new 4" outlet, it is in a "wall", can be directed with the register cover and will be about 30" from the air return at the end of the bar area. Will have to check the manual when I find the model etc, and confirm this works.
6) Thanks to your research and part numbers, I have ordered the parts on Amazon and I read the collar depth as 1.1" although am not sure as it gives product dimensions as 4 x 1.1 x 5 inches and I'm not sure how to interpret this. Will see when it arrives tomorrow and may need to get the snips out.

What am I missing???

[grantrekker]

Regarding response 6) above, Isee that I am in error as you mentioned the cover, not the collar. Well, both will be here tomorrow so I may have some reduction to do on the collar

[RussOnTheRoad]

Originally Posted by grantrekker

Thnak you RussOnTheRoad for your most informative and timely reply. I will reply to your comments:

1) there was a large retangular knockout on the top of the unit and is in the same place that your picture shows the indented area or what I assumed to be a knock out as it was still hanging down into the plenum by a shred of a tab

2) I will measure the now large opening on the top and order the cover you suggested, That is a much more elegant covering than my tin & tape on-the-road solution

3) I do need to find the model number on this unit. Am afraid it is on the top and if so would entail destruction of the cabinet to some degree to view.

4) Airflow requirement as you state "...for ducting are in the installation instructions and are provided in terms of square inches and number of ducts required to be used.... As best as I can determine, the only outlet from the plenum that was used was what I assume to be downward and into the basement area. Further it seems to be a pretty straight shot for the ducting without any severe bends. However, if the manual describes the airflow requirements in terms of output from the plenum, this would be simply one large downward knockout into the main lateral vent line with the four cabin registers and whatever holding tank exits exist. Wouldn't covering/shutting 2 of the factory cabin registers then restrict the airflow at the end distribution point to the same effect as shutting it down at the plenum (except considering the downflow directed to the holding tank areas)? I have effectively closed down 32s.i. of flow so reasoned that opening up one 4" hole withing 1-1/2" of the plenum would be replacing the outflow by only about 12.6s.i. So I might be shy by quite a bit for a one to one replacement of the closed vents if that is to be considered.
5) Regarding placement of the new 4" outlet, it is in a "wall", can be directed with the register cover and will be about 30" from the air return at the end of the bar area. Will have to check the manual when I find the model etc, and confirm this works.
6) Thanks to your research and part numbers, I have ordered the parts on Amazon and I read the collar depth as 1.1" although am not sure as it gives product dimensions as 4 x 1.1 x 5 inches and I'm not sure how to interpret this. Will see when it arrives tomorrow and may need to get the snips out.

What am I missing???

I have wondered about the rectangular piece on top of the furnace as to whether or not it's intended to be used as a vent. The Suburban rep I spoke to did not mention that as being a vent nor the other way around. He did say the unit has 11 vents counting 7 on the sides and the one on bottom as 4. He may have missed the vent on top if that's what it is. The installation manual does seem to indicate it is a vent if I read it right.

Re: your #2: The knockout cover I referenced was for the 4" round vents on the furnace, not the rectangular one on top or bottom.

Re: #3: The model number for my furnace was found under the front cover where the high temp limit sensor is located. You could call Northwood and ask or possibly query other owners at nroa2003.com. Either would be easier than pulling the unit or cutting it into pieces.

Re: # 4: It seems you may be missing the fact that there is one 90° bend in the ducting at the furnace where either the top or bottom rectangular vent is located if the hot air is redirected from the vertical plane to the horizontal which it is, and another 90° bend where the ducting turns upward through the floor where the in-floor registers are, if you have those. If you have 4 in-floor registers you have at least 5 90° bends assuming your furnace vents through the bottom. Suburban told me that using just the bottom vent met the bare minimum requirements for the SF-35FQ and using more was better. Also that 90° bends in the ducting work against that minimum requirement. Do you think then that Northwood has satisfied the minimum venting requirements where it may use just the bottom vent for that furnace and incorporate 90° bends in the ducting? Also, the manual specifies vents into sealed areas such as holding tank areas do not count and in my unit that vent comes off the ducting anyway and not the furnace itself. It does not add venting capacity. Please refer to the installation instructions yourself so I don't have to do it for you repeatedly. Closing dampers on registers acts against the minimum requirements as to venting. So, where you have the minimum requirements being met, or less than those, installing damped registers is, IMHO, irresponsible.

[grantrekker]

Thanks RussOnTheRoad, your help has been most valuable.