Greetings on this Memorial Day,
I need to add return duct-work to an addition (approximately 14' x 20') that is on a slab. The original house (a ranch) has a basement, so the heating and air conditioning duct-work goes up from the basement to the attic crawl-space and then down to the addition. It's not ideal, I know. The addition does not heat/cool very well, so I think (hope) that adding a return vent will help.
It will be a challenge and I've never done duck-work before. The most limiting factor is a wall cavity that is a scant 3 1/4" x 12 1/4". Here's a picture of that cavity Restrictive wall cavity to house return air duct. Currently, there is a return vent in our hallway that occupies that wall cavity and the cavity next to it (there is another pair of cavities on the other side of the hallway, but that is a bearing wall). A guy in a building supply store recommended that I use an oval duct for the cavity that I want to use.
Beginning at the room addition, this is the route I'm planning to take:
> Large (14" x 14") grill in the sidewall of a closet.
> Up 8', through a normal 2 x 4, wall to the attic crawlspace.
> West 4' (to accommodate truss bracing) in the attic crawlspace.
> North 19' in the attic. I'd like to use large, round, duct to minimize resistance.
> East 5' in the attic.
> Down 9' in the restrictive cavity to the existing return.
Here are my questions:
1. How do I attach the grill to the first duct that goes up to the attic crawl-space? I'm thinking that the first duct (going up) will also be oval, but could probably be rectangular.
2. How do I connect the "up" duct (which will likely be oval), to the 4' West run (which will connect to a round duct)? I want to avoid flex-duct.
3. Depending on the answer to #2, how do I connect the West run to the North run?
4. Similar to question #2, how do I connect the East run to the "down", oval duct?
5. How do I seal this "snake"? I've seen that mastic is the best bet, but the duct-work will be sitting on insulation, so it would be a challenge to get to the under belly. And, keep in mind that it will likely be very hot up there.
6. How do I insulate the "snake"? I understand that I need R8 or better in the unconditioned crawlspace. So, with some of the same concerns as with sealing the ducts, how do I get that done in the limited space?
7. Where the duct pokes through the ceiling, what should I use to seal the gaps?
I realize that I've asked a lot of questions here, so thanks in advance for any guidance you can provide.This message has been edited. Last edited by: SturdyNail,
As you have a lot of twists and turns, the simple answer is that you will need to get creative with modifying whatever stock duct pieces you can find. A note on that: You'll find a lot more duct piece choices at a mechanical supply store rather than a big box store.
The key to the whole thing is to keep your different shaped duct pieces at about the same volume size. To start with, I would find two side by side wall cavities to use as the return from the attic to the unit. That will give you about 80 square inches of size to work with. As it's a return, it's a little more forgiving so it can be oversized without hurting anything.
To keep it simple, I would use mostly 14" flex as the main return (just under 80 SQ in). Make custom boots to get from the two wall cavities to that attic and from the attic space back tho the wall cavities to the 14' round flex. As you could guess, you'll need larger than a 14" x 14" return grill as you need something that spans both cavity spaces. 20 x 20 should do it.
The hardest part is making 2 metal boxes at the top of the wall plate to convert the two cavities into 14" round. Plus the takeoff at the bottom end of the wall cavity to get to the unit.
Thanks for your ideas Jaybee.
I would like to use two wall cavities. The problem with the cavity to the right, is that there is a light switch taking up some of the space as shown in this picture Cavity to the right has a light switch
Even if I could squeeze in a duct, it may not be code to have the duct nearly touching the switch box.
I "might" be able to move the switch to the other side of the hallway if there's enough length on the Romex. I'd have to drop it down the cavity, so there would be no staples holding it to the wall stud (again, I don't know if that's code).
There is no cavity to the left. There's a door.
can't legally have power and "stud duct" in the same cavity. not a good idea, either. something about sparks and lint means no house and tired firemen. so you're down to one wall cavity. really, for a branch duct, that ought to be sufficient. don't downsize the overhead duct until you get to the return vents in the rooms.
one complete turn in a duct (4 90s) is all you can have as a rule, believe this cuts it to half the airflow. I'm not a tin-knocker, just another DIYer, but if Jaybee doesn't have the numbers on hand, they are on the internet.This message has been edited. Last edited by: swschrad,
sig: if this is a new economy, how come they still want my old-fashioned money?
I see no NEC problem with duct work and a switch box occupying the same wall cavity. As long as the duct work will fit in to wall and not interfere with the switch box [or Romex] presently in place. See it time to time in remodel work and never had an inspector make a comment about what is present.
Now this is residential work we are talking about. But I routinely see Romex running though fiberglass ductwork as the Romex is plenum rated.
Popeye only reached for the Spinach can as a last resort...
Thanks 'swschrad' and 'CommonwealthSparky' for your input. There appears to be some disagreement with respect to using a return duct and light switch in the same cavity.
thats not your only problem
12.6x3.6 is 24 +- sg inchs. you ned a minimum 8"duct
thats 8x8=64 sq inch your wall caviety is undersized. you need to think about a chase for the duct work. or your room will not cool
the return air duct will be of no use
to make a tap into a round duct you need 1 of 2 things. 1st is a tee fitting 2nd is a square box with sq to round spin ins installed in the rd duct then you can tap into it with a a spin in duct tap
True. But as long as you do not mount a switch box in duct work you are fine to place both in the same wall cavity. What is paramount here is duct work placed in the wall and not using the stud cavity as duct work. This is where it may get a bit cloudy.
Have I ever seen someone place a switch box in duct work? You would not believe me if I told you the answer to that. To bad it was back before I had my smart phone. Even then you might think it was a photo shopped image. Did the switch work? You bet it did. Did the opening in the return suck air into it? You bet it did. You could not hear it but you did feel it for sure.This message has been edited. Last edited by: CommonwealthSparky,
Popeye only reached for the Spinach can as a last resort...
This message has been edited. Last edited by: CommonwealthSparky,
Popeye only reached for the Spinach can as a last resort...
Frodo, thanks for your input. You said
Do you really mean "no use" or "not as effective as it could/should be?"
CommonwealthSparky, once again, thank you for your input. You give me some hope that this job is, at least, possible.
If you don't mind, I'd like to add another question about this project. As I said in the original post, there are existing returns on either side of the hallway of my home. They are basically holes in the wall where the flooring is cut out to dump air into a return below. Here's a picture:
There is nothing above the hole either, so, currently, the return can try to suck air out of that "chimney" that goes up to the unconditioned attic crawlspace. That is a bad setup that was pointed out to me when I had a home energy assessment. I'm going to take advantage of that "chimney" though by passing my new return duct-work for the addition down the "chimney". My question is, should my new duct come down low enough to block one of the existing holes in the wall (restricting existing returns) or should it end just above the hole (possibly lessening the ability of the HVAC to draw air from the new duct-work)?
Thanks in advance for your input.
not as effective as it should be. cold/warm spots
I was all set to buy the duct-work I thought I needed today. The guy at the store had everything I wanted, but he talked me out of it. He said the cool return air is not going to travel up. I'd be wasting my time and money.
Oh well. It seemed like it was worth trying. I guess there is no sense in opening up the walls and crawling around in the attic crawlspace if it's not going to help the situation.
If this advice came from a guy working at a big box store, I would just forget whatever he said. Sad but true that most Lowe's / HD employees really don't know very much about building or what their products can really do.
No reason why a sealed return system should not work - no matter the direction. Yes, the more distance and the more turns in the ducts will reduce the efficiency of the system, But anyone who claims that the cool return air will not travel up just doesn't understand how these things work.
The key to your return air system is to size the duct to balance out the supply air and to try to limit the distance and turns as much as possible.
To a point I agree with the fellow that you spoke with at the Big Box store. Your return airflow through the plan cold air return will have many bends, and turns and changes in the size of ducts which will have a total length of 45 feet or more. Over this distance you will have a reduced air flow without adding additional fans within some of ducts to assist with the flow of air which is reduced because of the changes in the size of the ducts as well as the various turns that interfere with the airflow. I believe your plan for the air ducts will have a negligible value in air return and will be of little value for the money and effort you will spend to install them.
Is it possible to reduce the overall length of the duct by trying to shorten them where the addition attaches to your home?
You stated that the addition is a slab foundation and thus it forces your to consider the air return route through the ceiling of the addition. However since it is an addition to your home it must be attached to your home somewhere where a wall of the addition butts up to the original foundation of your home where you may have either a crawl space or basement below. Using that wall that butts to the home foundation is it possible to find wall space where by you can run the cold air return just above the slab foundation through the wall that butts up to the addition to your original home and then through the crawl space or basement to your HVAC system. Using this method may allow you to use larger ducts and/or the spaces between the floor joists with sheet metal and allow a greater airflow so that your get the comfort of both heat and A/C as you desire and save you money as well in this project in the long term.
These are just some thoughts to consider.
Thanks Jaybee and Simply_Me,
The guy I spoke with is reputed to know what he's talking about. It's a, let's say, "medium sized" box store that actually runs clinics on installing your own HVAC systems etc...
OTOH, a guy from the HVAC company that maintains our system didn't "poo poo" the idea when he came out around 6 months ago to assess our system. He recommended, as you guys have, using the biggest duct-work and grill that I could get away with.
I've racked my brain trying to think of other configurations. OTOOH, the guy who did an energy assessment of the house did tell me he thought the work wouldn't be worth the effort when I told him what I had in mind.
It's difficult to describe, but the way the addition butts against the house makes running the duct-work from the "butt end" into the basement darn near impossible. That's why the supply runs were routed through the attic (through a transit space that allowed them to run 10" flex). The slab is about 8 inches lower than the original house and the "butt end" is parallel to the joists in the basement (so I'd need to cut through joists or concrete to get duck-work down there).
My window of opportunity for doing this job is quickly slipping away, because I put in for a couple of days off from work to do the job--thinking I'd have all the materials purchased on the weekend and be ready to go on Monday.
So, I think, "What would Mike Holmes do?" Probably; tear down the addition, bust up the concrete slab, build a proper basement under the new addition, put in a new HVAC system, and then seal the whole thing in several inches of blue spray foam
Then I come back to earth
Thanks for for considered thoughts and ideas. I do appreciate them.
Well actually, Mike would do whatever his construction chief and crew said to do.
Going with the givens that there is no path from underneath and there is space available in the attic then there are only two choices:
1. Find enough available area in wall cavities to carry the return load.
2. Build in some form of chase to carry return ductwork from the attic space to basement.
Choice #2 can actually be fairly easy. The space available in the back or side of a regular closet is large enough to hide ductwork. Or, a mininum use closet, like a linen closet, can be made shallower with the duct encased at the back. It's even possible to use a corner of a room without too much impact. In each case, you can install the ductwork without anything in the way, then build a chase around it, drywall and paint and you are done.
I'm disappointed to say I'm going to have to shelve this project for a while. I'm just not going to have the time to get it done during the time I took off from work. My son will be back in a couple of days (he's occupying the room) so I won't have the necessary access. I would have to totally empty his closet to put in the grill and the upward duct-work.
I "think" I would have had enough carrying capacity if my downward duct-work occupied the side-by-side cavities. I was thinking of using two 6" ovals. That would have required me to move the wall switch to the other side of the hall.
My wife and I had a new concern with the plan. It's likely that there will be noise transfer through the return duct-work. Since my son tends to work evenings and stays up into the wee hours of the morning, I don't want his noise and he doesn't want ours. Maybe I'll need to wait until he moves out
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