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funfool 10-05-2012 04:15 PM

another duct sizing ?
Lets hope I do not sound to stupid on my first post :laughing:
I have a small 900 sq ft mobile home. It was completely gutted, new 3/4" cdx flooring, dual pane windows, electrical and plumbing.
I framed in a adequate sized furnace closet, proper exhaust venting, I now need to size the duct work.

It is a 75k btu furnace, I am limited for supply lines at 8" because is a trailer and has axles in the way. I plan to have two 8" trunks coming off the down draft plenum.
One will feed the bathroom and wye to a 7" to feed the bedroom.
The other side will be 2, 8" for living room and wye to 7" for kitchen.
5 drops total.

My problem is trying to size the return air duct.
I had a local shop make up the plenum's for me, they are 12"x22"x24" tall.
I want to run 10" elbow from top of plenum to a adjoining wall for the cold air return.
But am concerned if a single 10" return will be enough. I also wanted to install a grill which will hold a filter.
What size do I really need?

beenthere 10-05-2012 04:31 PM

You don't have enough supply for the furnace. And you would need 2-10" returns. And the return grille sizes would be 16X16.

funfool 10-05-2012 04:49 PM

Thanks for reply, still in rough framing and installing boots.
I have some construction restraints, but do have some flexibility.
Could you name a size for me where only one return would be needed?
Am thinking maybe build a chase out of plywood, will be in ideal location taking air from hallway where t-stat is located.
Also takes up cabinet space over the washer and dryer.
Heating is more critical then cabinets, but trying to figure out correct return air sizing has proved to be a challenge..

I can install 2, 10" ducts, would you have a guess on what size for 1 duct?

beenthere 10-05-2012 05:01 PM

14" for only one return. Which would put you at a 20X24 return grille.

funfool 10-05-2012 05:20 PM

Thank's again
20"x24" with 14" sounds much more reasonable.
I went to 3 hardware stores today, home depot, a local mom and pop store and a well known commercial supplier of hvac duct work.
Nobody could answer this question, I came home empty handed.

Glad I found this forum, I actually am a carpenter, been doing remodel work for over 20 years, hope to return the favour to others with questions I can help with.

joecaption 10-05-2012 06:32 PM

PS CDX is the last thing I would have used for subflooring in a mobil home.
Full of voids in the core and flaws in the top layer, plus all the moisture from under the home is going to cause it to delaminate and fungus to form.
Been there done that.
I only use Advantec subflooring with constrution adhesive on the joist.
Now your going to have to add a layer of 1/2" Subfloor rated plywood to use as an underlayment.

beenthere 10-05-2012 06:33 PM

As you probably already saw. We have several forums where a carpenter can aid people.

thehvacguy 10-05-2012 08:42 PM

You should do a downflow and run the ductwork under the house, and cut the return into the side of the furnace closet, that is if it is somewhat centrally located...

funfool 10-06-2012 10:08 AM

It is centrally located. down draft and will put the return out the side of closet.
Ideal place would be bring the return right out the front into the hallway, of course the b-vent is in the way and would not work.

I built a niche for my desk, is a old mahogany desk I like.
The desk determined the size of the furnace closet, meaning it is a few inches wider and deeper, Once sheet rocked, desk will look like a built in.
So options are to bring it out the side into the living room, that will be a feature wall and have a oil painting planed to hang there.
I could bring it out the back, will be above my desk, could be distracting.

On the other side is the bathroom, along that wall will be the washer and dryer with a bank of cabinets above them.
Am thinking ideally would be to build a chase that would go into the bathroom and put the grill back in the hallway.
Would lose some cabinet space, but they just collect junk anyways and ok with that.

About the cdx flooring. The original mobile home was in great shape. Everyone thought the single older woman was crazy to hire a carpenter to gut it out and rebuild.
The insulation and moisture barrier, original flooring is all in good shape, The new cdx was just added to stiffen the floor up.
She finally ran out of money after new windows, doors, plumbing, electrical, new bath, water heater. She moved out of state to live with her daughter.
I would not have put that kind of money into the place. The mobile home park could not rent it as is, and offered it to me for free, I sent her $500.
Sorry for off topic.

beenthere 10-06-2012 11:22 AM

That was nice of you.

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