Furnace Duct Modification for Improved Air Flow and Circulation

[jd1923]

There must be a half dozen mods in this category, and I studied all I found in searching this forum. It's a shame the flexible ducting OTT uses, when upon original installation it would be so easy to plumb rigid ducting. Mine was torn everywhere in the area where you need to reach in and change the water valves.

I closed the rear duct at the furnace, using 5x6" sheet metal that I got inside of the furnace housing and used 3M 3350 tape to hold it airtight inside and out. I figured why not blow air at twice the CFM in the forward direction? I replaced the first 6 ft of ducting with semi-rigid dryer vent ducting and added a 4" steel junction to connect it. Placed foil insulation above it since it is now very close to the fiberglass under the bed. I kept it straight and tall as possible for better airflow vs. the downhill and tangled mess of the original installation. Wrapped the furnace hold-down with some old automotive heater hose to keep it cushioned and quiet.

I wish I could have reached all the way to the junction but would have to cut the floor of our kitchen cabinets to do so (if I ever have to replace the fridge, I will cut below and replace everything past the junction). I removed the underbed airduct completely, which is now a dummy vent, a return vent that I will place an air filter behind.

The airflow at the kitchen duct is now 2-3 times the old and at the bath it has certainly doubled. I also strung up the bath vanity ducting, so it has better straight airflow, not cramped by laying down at the bottom. While in there, I foam-wrapped the PEX which had been banging on the drainpipe very loudly when in use.

I will add an air filter to the return duct next (to catch dust and dog hair) and my last task will be to add the bathroom vent that @Rivernerd showed in another post (thank you!) This should really help when the bathroom door is closed as well as to vent/circle air around the basement and keep the water pipes above freezing in the rear basement. And of course, thank you @Snackchaser for your "Bathroom Heat - a more direct approach!" post that got me started in the right direction!

[jd1923]

Finished up the bathroom vanity. I had a supply of 1/4" u-clips but the fiberglass is quite thick at the opening and decided that would be too much work, the placement must be extremely accurate. 

I merely used thin panhead wood screws with no taper. Measured with a digital caliper to drill just the right size holes. Added a SS washer so the screw head would not drag on the plastic. Thoroughly cleaned off all the old caulk and sealant. This product has just the right size opening and adequate length: uxcell U Channel Edge Trim, 6.5ft Length Rubber Guard Seal Strip Edge Protector Fit for 1.5-2.5mm Edge, (13/64" W x 5/16" H) White - Amazon.com

To-dos left are the bathroom to under dinette vent and the return air filter housing. Waiting for parts...

[jd1923]

Would a household system have return air duct(s) without a filter? My next thought was to add one, to keep the camp/road dust and dog hair out of the basement furnace area. I purchased this part: 10" X 6 Steel Return Air Filter Grille for 1" Filter - Removable Face/Door - HVAC Duct Cover - Flat Stamped Face - White [Outer Dimensions: 12.5 X 7.75] - Amazon.com

This is an amazing addition for just $30 plus annual filter replacement. And think about the fact that with a filter in this location is would first impede airflow here, pulling more air from the basement AND it would result in less noise (dB) when sleeping! 

I was hoping, yes hoping, the idealist I am 🤣, for it to be a plug-n-play install! The OEM cover has a 10x6" opening, but not exactly go figure. OTT Service, if you are reading this, do your future owners a solid by either installing such filter as standard equipment, or at least cut a true 10x6" hole (measure twice, cut once).

Turns out it was actually 10" wide, but closer to 5 3/4" tall. Thank goodness for a high-end Dewalt cordless jigsaw, so smooth. Cut the opening to the proper size and cut the other imperfections. Measuring, taping and cutting was just another 20 minutes labor, NP! 

[jd1923]

BTW, I added some white rubber tape, I had on hand, for a good seal on the return air duct, just to be anal (see first pic above).

So, today I'm working @Rivernerd's suggestion next in adding vents in the closet and around the basement area. I'm NOT adding one that was suggested at the base of the dinette, since I believe that one would keep the basement return airflow from circling around and back to the furnace.

I have not sealed the outlet covers yet. Wish I had circles of 3M tape but will likely go to da Depot for some kind of sealant in a tube, no hurry, suggestions?

I had purchased one 4" vent suggested on another thread (installed above the closet), and then bought 3 of these, installed without the center black insert: https://www.amazon.com/dp/B077HJC5T9?psc=1&ref=product_details

I'm thinking I'm done, unless I install an inverter charger in the streetside basement which may require more airflow! Almost done here, wadda ya think? 

Edited June 25, 2024 by jd1923
Added pics...

[Jason Foster]

There are numerous other "holes" in the basement for air infiltration, but the worst offender is the one right where the dog lays down. Thanks for the idea.

 

[jd1923]

This is the most straightforward, simple mod you can do to improve the operation of the Oliver (Suburban or Atwood) furnace system. Please let me know if you have any questions. After 20+ nights camping in the SW winter, we had no idea the wet bath was heated, until I read it here! Absolutely NO heated airflow into the bath until now, Thanks for reading! 😂 

[Jason Foster]

49 minutes ago, jd1923 said:

This is the most straightforward, simple mod you can do to improve the operation of the Oliver (Suburban or Atwood) furnace system. Please let me know if you have any questions. After 20+ nights camping in the SW winter, we had no idea the wet bath was heated, until I read it here! Absolutely NO heated airflow into the bath until now, Thanks for reading! 😂 

I have the Truma heater and even though the ducting seems better, it still flows about 80% to the bedroom area and the bath is cold.  I will have to pull some things apart when it gets cooler outside.  It got up to 100 today with 70 humidity.  I could always run the ac I suppose.

 

[jd1923]

12 hours ago, Jason Foster said:

I have the Truma heater and even though the ducting seems better, it still flows about 80% to the bedroom area and the bath is cold.  I will have to pull some things apart when it gets cooler outside.  It got up to 100 today with 70 humidity.  I could always run the ac I suppose.

That's the main reason to delete the bedroom duct, given yours has the similar 3 duct arrangement. Now the main airflow comes out through the kitchen duct, aimed at the floor away from the beds, and much greater airflow into the bathroom. We had that duct "where the dog lays down" closed as much as possible, but the other ducts truly get 2x airflow when the first duct is bypassed.

We've been hot too. Not East Texas hot & humid of course, but my A/C has been running afternoons so I can get this work done while I have time off work. I stay plugged into shore power during summers since our Oliver is parked outside in the Arizona sun. Set at 84 so it does not get up to high 90s inside. At elevation, we cool off at night, so it generally runs on-n-off about 6 hours a day.

[jd1923]

I had one more step to finalize this mod. Love it when I can cross something off the to-do list! Don't we all. 🤣

I deleted the duct under the bed and installed a return duct filter, so the deleted air duct needed the finishing touch. What is important is that no return air comes through this opening. The other consideration is the added return duct will throttle the return air, pull it a little more slowly, creating a stronger air pull through the basement. This is key to the basic design and will certainly help to protect the entry water plumbing (City Water and Fresh Tank inlets) in the rear of the Oliver (objective #2).

Bought this item on Amazon: Amazon.com : NVAAV 4 Inch Boat Round Inspection Deck Plate Hatch with Detachable Cover and Pre-drilled Holes, Water Tight for Kayak Marine Boat Yacht Outdoor Installations : Sports & Outdoors

Not the best quality, basically a cheap aftermarket part. I would not use it as an external deck plate, as the rubber seal was no good, so I went without it. The duct is a 4" opening and the deck plate for the water heater shut-off is 5", so my first thought to use the same part would not work.

It was a snug fit, so I filed the opening just a little. Replaced the screws included in the package with a size smaller that I had on hand. Looks good in the end. I enjoy the look of our water valve switches, the return duct and deck plates, all now complete!

Edited July 10, 2024 by jd1923
typo

[MAX Burner]

@jd1923: Very clean mod, brother!  Nicely done - BRAVO ZULU!

[johnwen]

John,

I resurrected this to ask is it possible to snake a new stretch of ductwork from furnace to bathroom?  Here is a picture I took (when you could still take pictures) of the ductwork before the inner hull was installed and it looks as if it might be possible.  Wendy is pretty sensitive to dust/allergens/pollen etc. and that flex hose is a dust trapper for sure.  I put a filter on the return hatch and that stopped the persistent sail switch clogging.  As you stated "Would a household system have return air duct(s) without a filter?"  Google says there are no RV furnaces made like household furnaces so everything in the belly, including what comes in through the scuppers, gets cycled into the cabin.  So I think I'd like to at least replace all the ductwork, since I can't clean it, and experiment with filters on the exhaust vents to keep things from infiltrating into the cabin.  I'll be searching for a different type duct design.  I suppose too much airflow restriction entering and exiting the furnace may cause problems.  Just trying to see if others have tackled this dirty air issue.  I thought I was through with the furnace this season but 34 degree temps this morning ( on 2 August?) brought it back to life :) ... we're at Warren Bridge Campground, US Hwy 191, Wyoming headed to Baker's Hole CG in West Yellowstone :)

Thanks,

John

 

[johnwen]

So, I already have an access under the sink and after seeing things revealed a little better I think I'd better learn to live with this problem.  I suppose I could replace ductwork up to the "T" at the vent under the drawers and put an open restrictor on the refrigerator side (and not even use the ductwork to the bathroom) to allow airflow over the freshwater tank and circulate throughout the basement .  I'm not getting warm air into the bathroom anyway, don't really need it.  

This last picture is of the duct going under the refrigerator.

Edited 13 hours ago by johnwen

[jd1923]

4 minutes ago, johnwen said:

I suppose I could replace ductwork up to the "T" at the vent under the drawers and put an open restrictor on the refrigerator side (and not even use the ductwork to the bathroom) to allow airflow over the freshwater tank and circulate throughout the basement .

This may be your best option. I deleted the vent under the curbside bed (didn't like hot air in our faces when sleeping) and replaced the ducting up to the T with rigid yet flexible dryer vent ducting (illustrated in this thread above).

I cannot imagine why OTT uses the soft flex ducting we have in our hulls. Our original ducting had holes everywhere along the bottom and a lot of dust inside. During manufacturing, it would be so easy to put solid aluminum or PVC ducting that could be cleaned inside like household ducting. Not easy, or near impossible as a retrofit!

We do not have openings under our kitchen drawers as in your hull. Cutting into that area to open it up like yours would be easy enough, but getting under the fridge, under the cabin entrance, by the closet and all the way to the bathroom is another story!

There may be a way to get under the floor and run ducting to the bathroom via the streetside basement, but you'd have to deal with it being in the way every time you need to be in there and would likely produce weak airflow in all its length. 

Yes, you could delete the bathroom run (leave the bathroom door open on cold nights). You could add another duct as far forward as you could reach and have all new ducting to that point. I would do that if/when we have to delete the bathroom since I had deleted the one under the bed. You need two 4" openings at least. The 10x6" filter added to the return duct certainly helps re dust. In this case you would have all new clean ducting and greater airflow in deleting the bathroom run which likely has leaks in it along the way.

Glad to hear it's getting cold again up north! We had to use too much A/C when we were up in WY during June!

[johnwen]

Thanks, John

I appreciate your help and insight, as always!  Hope to see ya'll in Quartzsite next year :)  Safe travels...

9 hours ago, jd1923 said:

Our original ducting had holes everywhere along the bottom and a lot of dust inside

Ditto on above...

John

[Snackchaser]

12 hours ago, johnwen said:

I'm not getting warm air into the bathroom anyway, don't really need it.  

Hey John,

It might be worth checking out my post  Bathroom Heat - a more direct approach!  It's an easy and significant improvement to bathroom heating.  It won't help with your dust issue, but it includes replacement of some of that problematic flexible duct.  It would be near impossible to replace that long duct going to the bathroom, but I think with a filter installed the dust problem should go away.

Since I've done that mod, I've added more semi rigid duct from the furnace to the kitchen.  It's tucked up out of the way into the space above the return air register as seen in the photo, and it eliminated some restrictions and puncture hazards.  I also cut a vent hole in the wall on the left side of the toilet through to under the dinette seat as others have recommended.  It helped a lot!  Afterall, bathroom heat is a nice luxury, and it helps with the wet bath humidity too!

Cheers,  Geoff