Suburban Furnace Question

[jodeb720]

I have a Suburban Furnace Model SF-30F.
Since I've owned my 5er, it's never really put out a lot of hot air from the floor vents.

A few years ago, I pulled off the return grill and found the majority of the air is going into a plenum located under the front - bottom of the unit, under the floor and then the plenum runs under the entire bottom of the 5er -with a 4" connection under the stairs feeding the upper area. At that point in time, I turned on the fan, and felt the volume but it didn't add up to what was coming out of the vents.

Since my ordeal last fall (2019) my wife and I have had to rebuild the upper area and we're doing a full renovation.

Today, I pulled the furnace front off, when I fired up the heaterand it filled the entire coach (200sq ft) of space in about 5 minutes from 60 to 70 degrees. If I use the ducts that would normally take about 15 minutes.

here's what I'm thinking.
1. where the current heated air is pushed under the unit into the plenum, block it with a piece of sheet metal.
2. Put a 4" residential output pipe out of the unit, and then connect it under the stairs, connecting it to a "T" and having it going out under the floor - and then connecting to the two other vents.
3. This would bypass the plenum (which I'm sure is leaking under the Rig in the floor) and I'd get much more heated air.

in the installation manual - it stipulates a return minimum of 56 Sq Inches -

On the ducts it's a bit confusing. For Side Ducts it's 36 sq inches and 3 ducts. For the bottom/top it's 56 sq inches.

Since I am considering closing off the bottom duct (and I'm assuming that's 56 sq inches because it was done by Keystone) I would need to use 3 - 4" ducts (1 -4" duct is 12.5 sq inches) to equal the 36 inches required by the manufacturer.

My question is this. Can I aggregate of the two ducts or do I need to run three separate ducts? I'm thinking of aggregating the 2 of the three ducts into a single 6" duct (6" duct has 28-1/4" sq inches).

Thanks in advance!

Josh

[jodeb720]

Not to beat a horse to death but.....


I’m at the RV and it turnout the plenum that runs from the heater (passenger to drivers) isn’t closed on the driver send - and I can’t feel the passengers side without removing the chloroplast.

So as Time2roll suggested I’ve been crawling around looking for a path to put 4” ducts. It’s going to be ugly but it’s a better option than messing with the plenum under the coach and hoping it works.

Now I wait until I get the upstairs finished and I can remove the stove and get to area to run the duct.

Thanks

[jodeb720]

@Time2roll - I think you've hit the nail on the head. I'm trying to reuse the existing vents but that may not be the best solution. I'm going under the rig today to look to see what's below the existing vents and access to the vents.

I'll need to run at least one 4" vent to the rear floor vent so it may be easier to run a single 6" plenum and T off it for each vent. I'll know more later today. Love the idea though.

The install manual stipulates 56 Sq. Inches of return - and the current "grill" is 216. I'm thinking I can do two things - one is reduce the return to something like 60" and two is to baffle the flow of the air so the sound of the fan doesn't have a direct path into the rest of the rig.

Again, thanks all for the ideas.

josh

[time2roll]

My floor plenum connects to a floor duct that runs full length fore and aft under the floor. The plenum that connected to the long duct was collapsed and severely restricting airflow. Also where it connected it actually extended almost half way into the long duct giving more restriction. I propped the collapsed portion open and drilled holes in the restriction. Some improvement but still cycled on the limit switch due to low air flow and overheating the fire box.

The ultimate solution was to leave the existing ducting in the floor and add two 4" ducts. These are connected to the side knock-outs and routed direct to the living space through the cabinet face. One on each side of the furnace.

I now get tons of heat and the burner runs continuous until the thermostat is satisfied.

Not sure this story helps your situation. In the end I recommend just leaving the existing and add more ducts. The heat moves around just fine even if the main heat is now provided mostly just in the central living area. I would overshoot the minimum duct area specification.

[jodeb720]

first let me thank all of you for the responses and letting me know i'm on the right track.

@KTMRFS - when we first got the rig, the flexible duct (as you described) was punctured by an overly aggressive wife who wanted her chairs to stack in a certain way. I did exactly what you suggested - right pipe with insulation around it. Alas, no difference - but you and I were/are thinking along the same line.

@Stircrazy - i have a 2" output pipe that blows under the rig to protect the tanks. I went so far as to cap it off to see if it made a difference - it didn't. Again, you and I were thinking the same thing.

@Opnspaces - I can't access the plenum. It's sandwiched between the linoleum (on top) and a piece of wood underneath the plenum. based on the leakage my AC has - I'll bet the thin aluminum tape they used to seal the sections together are torn and leaking, badly. Unfortunately, I can't access the plenum (based on some youtube videos of how the coaches are built today.

I think, and that was my question regarding the sq. inches of ducting - that my best result is to open three 4" ducts into a single plenum - either 6 or 8" which is much more than required (56 sq inches), and split it off into 3 4" ducts. One for the upper area - two for the lower.

But, I think I'm going to have to go under the coach to feed the two vents that exist - cut through the floor plenum and bypass it completely.

The good news they are standard 4x10 vents so the big box stores all stock the standard duct work. I can probably find insulated pipes as well which will retain more of the heat (when I need it).

One thought is that I can use those Dryer vents (4" round to a square body and then back to a 4" round) and if I use a couple of those, I can connect those into a larger duct - but since they are facing the inside of the rig I don't have enough room for 2 4" round ducts.

Again, if my math is correct ,as long as the total number of sq inches equal the manual (36 sq inches for 3 4" ducts) then I should be able to imagineer this together and still be within manufacturer specs.

Does anyone see anything wrong with my math?

Thanks so much!

josh

[opnspaces]

It sounds like you have a lot of heated air leakage running under the floor in that long plenum. Your fan test where the amount of airflow from the fan is much greater than the amount of airflow from the registers also suggests you have leakage from the plenum.

The weird thing about air (or water) leakage, is that what may seem like an inconsequential amount of leak can add up to a surprisingly significant leak.

Are you able to see the plenum from under the coach? If so I would go to Home Depot and get a few rolls of foil backed HVAC duct tape. This is not the 1 dollar or even 5 dollar a roll plastic duct tape. This is 20 dollar a roll HVAC tape Link

This tape is very sticky and holds it's shape when folded or bent and will seal off the leaks. Tape off all the edges of the plenum especially where it goes to the floor so that the heated the air goes to the registers and you will probably get much better performance.

[StirCrazy]

just watch how good you seal it off, some have leakage and.or a open duct to the basment by design to keep tanks and such from freezing. Just check to see if this is a possibility before you redo how it works.

Steve

[garyemunson]

A problem I found on our Winnebago was the registers were actually made to have adjustable shutoff louvers that were not there. The registers did have the side/end plates that would have held the louvers and these plates extended down into the ducting interfering with the flow of air through the duct and out the register. In our case the plates were just stuck to the register grille with 3M double faced tape so I was able to remove them. Greatly increased the air flow out the registers.

[ktmrfs]

when we got our outback, I also felt that I wasn't getting good airflow, nor was it as hot as expected. I easily fixed both problems.

Airflow: the ducting was the thin flexible pipe with spiral wires. this type of ducting has very high airflow resistance due to the convolutions in the piping. Replacing it with 4" ridgid ducting like in houses drastically increased
Next, I wrapped the new duct with the foil backed insulation. Now the airflow is great, heat is much better.