Panned ducts
Hello,
I was looking to have a LEED certified home and was moving along nicely until unfortunately due to a lack of proper communication my HVAC company put in panned ducts for the returns (all in the conditioned space). The project has grinder to a halt and I am now faced with deciding to scrap the LEED thing and keep the panned ducts or put in a central ducted system as there is not enough room for fully ducted returns from all rooms. My house has a basement and two additional floors and my HVAC company is able to put centralized ducted returns on all levels with enough cfms to handle the output of the system and then plans on putting in transfer grilles for the rooms, but my builder and HVAC people think it will not be as good as the current system (and it will cost me more money). Talking with other people in my area, panned return ducts seem ubiquitous, however everything I read on line seams to state that panned return ducts are the worst thing ever. I don’t want to do something stupid just to be able to get LEED certification and aesthetics wise it would look way better as is currently set up and I wouldn’t have to worry about an ugly centralized grille and noise problems with the transfer grilles, but in the end I just want to do the best thing for the home. I would appreciate anyones opinions on what to do.
Andy
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Replies
Panned joist bays are not allowed here, haven't been for years.
I wouldn't want them myself. Centralized returns rarely work but in compact square designs.
Minisplits almost always win whether ductless or ducted.
Ill keep thoughts on LEED to myself.
I just went through something similar although I wasn't looking for a LEED certification. Panned ducts sound like unfortunately what the industry does. I was shocked to show up at my job site and see panned ducts. A question I asked about duct sealing and aeroseal digressed into some comments about panned ducts that might be worth readying in the thread here :
https://www.greenbuildingadvisor.com/question/acceptable-duct-leakage-and-aeroseal
In the end alternatives were just too difficult to work with given how much work had been done. I chose to stick with the panned ducts but had the installers mastic everywhere possible.
I got somewhat acceptable duct leakage numbers as can be read in the thread here:
https://www.greenbuildingadvisor.com/question/hvac-duct-leakage-test-results
If you can get good leakage numbers in your duct test does it really matter if they are panned ducts or not with respect to your LEED certification?
Steve
Panned return ducts are nearly ubiquitous in Ontario. We have full basements here and all our ductwork is in the conditioned space. And that conditioned space is usually relatively clean and free of mold, moisture, and other nastys associated with many crawlspaces with panned returns. So duct leakage isn't nearly as big of a problem. You end up with some return air from places you didn't plan to have it from but it works fine. You can end up depressurizing your rim joist area, so make sure it's sealed up tight (which you are/should be doing anyway). It is NOT best practice, but it does a suitable job on the cheap if you have a clean conditioned basement.
Other problems include condensation on wood framing members, not a problem if it's in conditioned space. Panned returns are difficult to clean and often have a lot of construction debris in them. Depressurization of top plates, seal those up too to prevent pulling air from the attic. Other subs have a tendency to run wiring and plumbing right through your return, adding to the leakage.
I'm confused though, if there is room for panned returns, how could there not be room for ductwork? It would go in the same space.
Central returns with properly sized transfer grilles work fine, although the reduction in acoustical separation is a problem for some. The key is to size them right, I've attached a pdf that will help you size them if you go that route.
Hey Yupster, they ran supply ducts in a couple of the areas where the return ducts are I believe
Is the basement conditioned, insulated space? Does LEED prohibit panned ducts?
I don't see much of a problem with panned returns as long as they meet the airflow requirements and leakage requirements. There are lots of ways to screw up panned returns, but done properly there isn't anything wrong with them if they test properly.
Is the air handler installed? Have the system static pressures verified with the blower at the highest setting that will be used.
Thanks Matt. Basement is conditioned. I am told LEED prohibits panned ducts. They plan on doing blower door test when done, but do not check the duct system prior to that is my understanding.
R403.3.5 Building cavities (Mandatory) Building framing cavities shall not be used as ducts or plenums.
I would think you would want (and are paying for) an installation that at least meets minimum code requirements.
Still legal in our building code, and others. He doesn't mention where he is located, so we can't tell him whether or not it meets building code.
>" He doesn't mention where he is located..."
Grand Rapids MI, would be my guess:
https://www.greenbuildingadvisor.com/question/help-with-hvac-for-green-home-in-grand-rapids-mi
See section M1601.1.1. section 7:
https://up.codes/viewer/michigan/mi-residential-code-2015/chapter/16/duct-systems#M1601.1.1
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7. Stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following conditions:
7. 1. These cavities or spaces shall not be used as a plenum for supply air.
7. 2. These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
7. 3. Stud wall cavities shall not convey air from more than one floor level.
7. 4. Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fireblocking in accordance with Section R602.8.
7. 5. Stud wall cavities in the outside walls of building envelope assemblies shall not be utilized as air plenums.
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So unless locally dissallowed in Grand Rapids, in Michigan panned returns are apparently legal, if terms 7.1---7.5 are all met.
What code are you referring to Jon? IRC 2015 R403.3 is frost protection and 403.3.3.5 doesn't exist. The 2015 Mechanical Code R403.3 is mechanical ventilation - outdoor air and local exhaust airflow rates.
I'd like to find that code so I can show my builder.
Thanks,
Steve
R403.3.5 from the 2015 IECC. That is the code in effect in my area. Obviously you need to check the actual version in effect in your area.
https://codes.iccsafe.org/content/IECC2015/chapter-4-re-residential-energy-efficiency?site_type=public
Andy,
A bit of a side note, but unless you see some financial benefit from pursuing LEED certification, I wouldn't bother. Use its requirements as a check-list of things you might want to include, but apart from that, I don't see the point.
Thanks Malcolm. I thought it was a convenient way to make certain things that I felt passion about were done, but it is a bit of a pain. I may not have to worry about it much longer though haha.
Thanks for everyones comments. Dana is right I am in Grand Rapids, MI and I am told it is code and almost always done here.
Andy,
I researched a bunch of residential leed stuff a few years ago for a build. But I don't know if by heart anymore. I suggest double checking the leed requirement is, as in look it up for yourself. From what I remember, Most of it is performance based instead of prescriptive. By that I mean I have a feeling the leed points would come from a minimum amount of duct leakage and placemnt in conditioned spaces.. and not so much the shape of the duct.
Because of you think about it, a panned return duct is just a 14-1/2" x 11-1/4" square duct (or whatever size Your joist bay is).
If I was in your shoes, I'd get them to mastic all the seems, and if they were planning on using the backside of the drywall as the fourth side of the duct, then place tin sheets (with mastic) over the joist bay to essentially created a closed and sealed duct.
And because they messed up, assuming they'll use a brush for the mastic, I'd make them go over it twice to be sure it isn't too thin and crack later on.
Your HVAC system should be an independent system within your building envelope system. the duct pressure test should happen as soon as all the ductwork is in and well before drywall is done so you can address any duct leaks. I can only assume they want to do the duct leakage test with the blower door because it's probably the same contractor doing both tests. If you can confirm that they doing a pre drywall blower door/duct test, then that's perfect.
I'm sure some HVAC people on here can provide more info than I can. I'm just some guy on the internet.