HVAC install – cardboard returns, duct air tightness
In our current build we have a fully ducted system on 3 levels with an ASHP and ERV. At the rough-in stage I pointed out the cardboard returns as I was a bit horrified. Our PM asked the installer and said it was no problem, they always do that. The large gaps I could SEE in the returns? Again, no problem because it just pulls air from throughout the house through the gaps. Back then I was a bit more trusting than I am now. I honestly think there might be a few spots upstairs where they “used drywall as the duct”.
Now we’re in the final stages of interior finishing and I’m questioning the work. As I was cleaning the filthy floor vents upstairs and low wall returns that did not get covered, I noticed the cardboard ducts are tricky to clean and seem pretty flimsy. Then I read an Energy Vanguard post about cardboard ducts. I also noticed the round rigid ducts have a bunch of screws exposed, making them hard to clean as one cannot simply insert something to wipe them out.
Experts, enlighten me: is there any problem with foil faced cardboard return ducts? Do returns need to be airtight? What is the impact of leaky returns on stack effect? Is it normal to have screws protruding into ducts?
I’m not sure if I should say something about the installation quality to my GC, I thought all the ductwork was supposed to be metal and I don’t want to offend anyone… but I just don’t see how this is supposed to be an airtight installation when I did not even see mastic being used. It bothers me that the narrative is that I am the problem when I’m simply holding people accountable but because of my age and gender it is perceived as “micro”managing to simply point out the best practices that we expect. Like, is using mastic for sealing not industry standard? I feel like learning the little amount I have learned and watching all the YouTube builders has really ruined me because 99.99% of contractors are nowhere near that level of attention to detail and management capability so everything is just a huge disappointment in the discrepancy between expectations and reality.
I don’t have a pic yet of the giant cardboard return on the main floor, but here’s a fun one of a bedroom return full of crap.
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Replies
Do you have a quote from the HVAC contractor? Does it specify metal/flex ductwork?
I don’t have the quote but I have a previous version before we changed the equipment a bit and it does not specify the type of ducting. I know we’re paying the HVAC contractor like $70k CAD though.
The engineer’s HVAC design is 3 pages of the layout for each level and then 1 page of specifications. The specs list all equipment and relevant details. Attached below is the ducting section. People don’t read, people don’t communicate. I honestly cannot believe I failed to take photos of all ductwork. That was back when I still thought they basically knew what they were doing. Now that we’re in the finishing stages I feel like I’m coming out of a dense fog and of course if I knew what I know now about ductwork back when they did the ductwork - I absolutely never would have let this stuff fly.
There shouldn't be trash in a duct, return or otherwise, ever. That's not good workmanship and should be addressed. The fingers of that branch run should be bent over tight to the inside of the rectangular duct too. Someone rushed that it looks like.
I have never worked with cardboard ducting. I doubt I will ever see it on a commercial project (where I usually work), since we have a lot more stringent fire codes to deal with. I would not want to use it on my own home. That said, air returns are commonly run using the framing and drywall to form the "duct", and while not ideal, it does work. I would want the walls to be detailed airtight in those areas though (assuming interior walls here, I'd never want a duct in an exterior wall), to avoid pulling low pressure in the rest of the wall and sucking in dust and dirt at every penetration.
Tape is used instead of mastic at times. If that's the case, you want the METAL FOIL tape, NOT the more common cloth "duct tape" stuff. Mastic is better though, but tape can work too when done properly using quality materials. The common problem with mastic is that it's installed in a too-thin layer. Mastic is supposed to be installed "nickel thick", with a layer about the thickness of a nickel. I see it installed all the time looking like thick paint, where you can see screw heads clearly through it -- that's TOO THIN.
Your GC should be keeping the contractors on schedule and the project built to plan. If you see any deviations from the plan, bring those up and ask why. Sometimes there is a legit reason for a change, but even then, not all changes are acceptable (you CANNOT change an engineered system, for example, without the engineer's approval).
Don't worry what they think of you checking. They work for you, on your house. You're the owner, and you pay the bills. Make sure the work is done the way YOU want. The only thing to watch out for is don't constantly try to change things to something other than the plan (it's OK to try to make things go the way the plan says), and don't second guess every last detail -- try to stick with what matters. Three or four screws on a duct transition isn't a big deal. Air sealing the transition is a bigger issue, especially where called out on the plans or required by code.
Bill
The sheet metal screws sticking out into the duct is normal; three screws required per connection up to a certain size diameter I believe. But there is a chance the screw length is longer than what was required and could be sticking out further than necessary.
Assuming the leaky ductwork is totally inside the conditioned space duct leakage is no problem as nothing is lost outdoors.
If like me you are one that tends to leave most doors open return ducts don’t really matter much so long as when they connect to the equipment the opening is large enough not to restrict the air flow and the filter is next to the equipment. In my house the contractor accidently did an experiment by forgetting to cap the joist bays being used as returns so almost all the return air was coming from the basement. It was over a year before I happen to notice. The comfort level remained unchanged when the error was corrected after more than a year.
If your supply ducts are leaking and the ductwork was computer designed to deliver X number of cubic feet to each room the guy doing the adjustments is going to have a bad day. This is almost never done in residential work. Most residential jobs get one of the same size registers in each room located under the farthest window and the big room gets 2 registers. Let’s just say not a lot of effort get spent on most designs.
I like old school steel ductwork but it all but a lost art in residential work. When you give people the bid for steel and another for flex ducts the low bid tends to win so often, they stop writing the steel unless someone asks for it.
Screws inside the ductwork are very common. Dryer vents are the only place they are prohibited I can think of.
Walta
Interesting. What do you make of Allison Bailes’ old article about cardboard being inferior and leading to issues with stack effect? https://www.energyvanguard.com/blog/do-cardboard-ducts-belong-in-a-leed-certified-home/
The scenario seems similar with a cold climate, 3 levels in the floor plan so a very tall building, big cardboard returns in basement ceiling and overall not airtight returns including upstairs on the top floor.
Allison Bailes generally knows what he's talking about, but I don't think you'll have stack effect issues from carboard duct IF it's all within the building envelope. If that cardboard duct extends OUTSIDE of the building envelope, then I agree it's likely to lead to trouble over time (critters could chew into it, etc.).
I personally am not a fan of using the space framed out by 2x4s and drywall as air returns, but it's done all the time and it works. I do detail those areas airtight when I see them, so that the "return space" is at least sealed off from the rest of the wall, especially at top plates underneath attics.
Bill
The way I see it the basement is well connected to the rest of the house and almost no pressure differential can be created by operating the blower relative to the outdoors.
Walta
very normal, don't worry about the drywall used as returns and same with the foil cardboard as long as the installer cared a bit and cut around i joists and used tape at end of runs. Metal duct for supply is a must but i worked in hvac for 8 years and did not see a metal return duct in anything but commercial rooftop ducted units.