ducting through floor trusses
I have a question that I can’t seem to find an answer for…
I am helping my sister-in-law build their house and they are using floor trusses (14 inch depth, open web trusses at 19.2 oc). There is a nice rectangular ope area to run the HVAC ductwork through in the middle of each truss. My question is this: How do you get the ducting in there? Once the walls are sheathed, I can’t figure out how to get long pieces of duct into the open areas… Would it be easier to slide the ductwork in before sheathing the outside? We just got the trusses on, the bracing and strongbacks in and the subflooring is installed. it would be easy to slide ducts into the openings now, but if we wait it seems like we would be cutting short pieces for the main trunk lines and doing lots of seams.
As you can probably tell, I’ve never dealt with floor trusses before. I have experience with i-joists and 2x floors, but have never worked with these before. Any help would be appreciated.
Darren
Replies
I think you have some choices. Put in before the wall sheathing. Leave a piece of sheathing off or loose (screw it in place so you can pop it off to put in the duct. Short pieces probably not a good idea. You could just put in now before your sheath the wall. It depends on how your wall gets sheathed. Some don't sheath the joist band until later. If water/weather is a bit of an issue, you could drape it w/ poly to drain off water until you are weather tight and then pull it off. For example cut a long poly strip 2-3 ft wide and wrap the duct ... now a good time to use duct tape ... to hold the poly in place. Slip the duct in and you're set.
Flex Duct
If you use flex duct you won't have any concerns.
Except
all the usual concerns one has with flex duct. Bad idea usually.
Thanks guys, I have been olooking into flexible ducting, but I think it would be better to just slide some sections of ducting in before sheathing. Especially since we are not too far along yet. And it is -10 degrees here this morning in Montana and about all we will get done for the next few days is feeding cows and shoveling the new snow off the subfloor! Gotta love fall in Montana!
I appreciate your responses, it really helped me know that I am on the right track...
Ducts
I wouldn't slide any duct work in until after you have it dried in. Just screw a piece of sheathing board on the trusses at each end of all the chase runs. It is easy to pop them off and slide the duct in after you have everything buttoned up. I even painted mine yellow so the HVAC didn't have to find them. I also cut a flap in the hose wrap that could be folded up away from the openings, tastapled and taped back when they finished. The duct should not be be exposed to water for a long time.. It is galvanized but will start to rust at cut ends, and sometimes on the bends.
When you get to the point of installing the duct runs put a couple of them together with the drive slides. duct sealing mastic, and foil tape. Cut a piece of 1/4" luan 8' long and a little less than the chase width. Slide it in first and then slide the duct on top of it. That takes a lot of the headache out of bumping 5' sections of duct across the bottom cord of the trusses and dinging it all up. You can pull the luan out after you get your major runs in.
What Clewless said.
I had my floor trusses designed with a chase in them for my main supply and return trunk lines. They are 24" deep and 19.2" o.c., and even then it was going to be tight.
Your 14" deep truss is not going to allow but an 11x12" duct run. Clewless can give you the supply CFM you can expect for that size. I don't have a duct-a-lator handy.
Best advice is get a good HVAC company to run your load calculations, determine your unit size requirements, and plan your runs for you. I seen more systems messed up from poor distribution systems than improper sizing of equipment.
BTW flex duct is way easier to screw up than rigid duct. It should only be used for tail end runs to supply/return registers. The shorter, the better. A poor flex duct job can rob you of from 30 to 50% of you total CFM. In case you haven't guessed, i don't like flex duct!
I feel the same way about flex duct, especially the more I read about it. I haven't measured the chases left in the trusses for HVAC, but I would guess they are about 24 inches wide. And there are two sets of them, one chase running down each side of the house. And the furnace room is pretty close to the center of the house, which will help.
I concur with getting a good HVAC company involved. But since it is not my house, all I can do is politely make suggestions. I hope they will take my advice, since my little suggestions have saved them about $400 so far, and we aren't that far along yet! Well, I guess I have probably cost them about $400 in 'suggestions' to make sure that they are getting the correct materials (yes, it HAS to be a treated bottom plate in the basement!) and enough of them. Running out of lumber, screws or adhesive is really a pain when you are 45 miles from the nearest store!
You can
work 5 ft pieces of 6" round duct in to the trusses but its not easy. Between floors, I have 14" trusses on 24" centers. I had no option to run a trunkline down the center rectangular opening before the "ends" were closed up. I ran a 20 x 12 between two trusses and tapped in 6" rounds to run to each room. Worked out well, but took a lot of work and materials.
What need to be fed from this space is important to work out before hand. 12x8 duct is good for about 450 cfm, 12x10 - 600. Crossing DWV could be a significant concern.
ductwork in floor trusses
we worked with this system a lot
Install ductwork as Clewless suggests,
Do not use flexduct
It's like: put in all your plumbing lines before pouring concrete floor