Designing Framing for Ducts
i’m working a build with a basement, and trying to avoid cc-spf in favor of cellulose.
the plan has some cathedral-ceiling (parallel-truss) which i’m thinking to dense-pack, and some flat-ceiling (energy-heel) which i’m thinking to loose-fill,
these would be vented-assemblies, so outside envelope.
was thinking 2×4 service-cavity on interior of exterior-walls and ceilings.
interior walls would presumably be 2×4.
given those parameters, seems like this only accounts for ducts inside the envelope that would fit in a 2×4 service-cavity.
this could be fine with a hydronic setup (if that is the hvac strategy),
but what about hrv duct-work, and what if hydronic isn’t on the table for some reason (eg price-point).
i’ve seen mention of a “reverse-bulkhead” built into the truss design, is this (in combination with some select in-wall channels), the way, or are there others, and generally can someone talk to conventional wisdom on this topic?
it seems like you need to know insulation and hvac strategy in advance to get framing right…
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"build with a basement" Well that is your answer.
The main trunk runs in the basement under the floor joists, supply ducts branch off from the top of the trunk and run between floor joists to floor registers and up interior walls for high mounted ones. Upsizing some of your interior walls to 2x6 makes life a lot easier and barely noticeable. With careful design, you can eliminate almost all bulkheads.
If you don't want the trunk in the basement under the floor joists, you can order floor trusses with a duct chase.
"it seems like you need to know insulation and hvac strategy in advance to get framing right…"
That is correct! The earlier ductwork is considered in the design, the easier it is to accommodate. You can get 3" and 3.5" oval duct that will fit in 2x4 walls.
thanks akos and michael.
so possible that erv/heating/cooling ducts can effectively operate with only floor and wall registers?
i guess, if i have a 2x4 service-cavity in the ceiling, i could get some of that slim profile duct work michael mentions up there anyway.
i forgot to mention that this would have two floors above the basement, (think ~1000sf per floor), so with that in mind, a couple of follow ups:
(1) if i wanted to consider a cold-climate/air-to-air setup, do you think it's possible to keep those mechanicals in the basement and still effectively service a three-story structure with 3" ducts?
(2) same question for erv's, are there units that can effectively service a three-story structure with 3" ducts?
(3) any special considerations for range-hood make-up-air?
(4) bonus question: if you guys were considering an hvac system for such a structure, what would your go-to setup currently look like?
(1) No problem at all. Ducting needs to be designed and sized for the air handler you pick. Most manufacturers offer a high static pressure air handler that has the same capacity blower as a standard furnace. The key to good cooling comfort is a largish return near the ceiling on the 2nd floor. This takes space to run but don't skimp on it. The 3x14 / 3x10 ducting is not the easiest to work with and can be restrictive if you need to jog it. Usually most ducting is done with round pipe (hard or flex) from the main supply trunk, thus my recommendation of a 2x6 chase where needed.
(2) 99% of HRV/ERVs on a new build are plumbed using simplified ducting, that is straight to the air handler return. Home run systems are unheard of and you'll find a hard time for anybody to install it. If you want better, my preferred setup is a hybrid ducted setup where the fresh air is supplied to the air handler return but the stale air pickups are ducted to the bathrooms / kitchen area / basement or crawl. This saves a bunch of exhaust fans associated wall caps which pays for the extra bit of ducting.
(3) Pick a range hood that doesn't need makeup air. For most codes this is under 400CFM. The key too good range hood performance is not the blower size but how it is constructed. You want it to be wider than your range, baffles on either side and with a large capture volume. A good setup will get you much better performance than one of those modern low profile 1200CFM so called "professional" one.
(4) There is no easy answer here. If you are paying for the HVAC work a single zone in the basement is your best. Maybe add in a zone damper for the trunk feeding the 2nd floor for a bit of extra local control.
A couple of other pointers. Assume the basement will be finished, if not now eventually. This inevitably happens so make your life easier. The utility room should be against an outside wall, corner is not bad if the ducting can work. Main trunk should run along an outside wall not in the middle of the space, trusses with a duct chase are even better.
Good spot to run the main supply and return trunk up to the 2nd floor is behind built in storage or closet. Back of a pantry or fridge are good spots. The 2nd floor supply trunk can lot of times be run in a bulkhead right above the kitchen cabinets.
If you have big windows, you need supply vents under them. Ceiling vents can work but you need high throw registers which are hard to get right and add a fair bit of restriction.
High efficiency construction has much more even temperature so it is pretty forgiving to supply vent locations. I've had no issues with heating/ cooling bedrooms with a high vent on the inside wall. The only time I've installed a ceiling vent is when the HVAC was in a dropped ceiling, almost always wall/floor vents.
Those fancier mud in registers and floor vents with inserts look good but are pretty restrictive. They can definitely be used but upsize them.
these are helpful hints, akos, ty!
yes, my plan was to finish the basement during initial construction:
- overall
- 70' x 50' foundation foot-print (excluding garage portion)
- targeting pretty-good-house performance
- erv
- cz-6
- walkout-basement:
- ~1000' sf
- mostly open area
- one separate room
- bathroom
- mechanical-room at center of the long dimension along the outside wall
- first-floor:
- ~2000' sf first floor
- two-story great-room
- mbr-suite with bath
- small office
- 1/2 bath
- second-story
- ~1000' sf
- 3 bed-rooms
- 2-baths
- top-part of great room
what might an air-to-air system look like in terms of mechanicals and duct-work?
- are there single units that could handle that entire space, or would it require multiple?
- what might the duct-work look like in terms main-branch and sub-branch sizes to account for in framing?
i would love to have the blue-prints account for duct-work instead of winging it in the field.
i'm trying to reconcile the following considerations:
- hvac installers want prints to provide a design
- good hvac installers in the area are so busy i'm having trouble getting callbacks
- not sure if all hvac installers will install from a third-party design (i read one story about a guy who dropped 5k on a design, but was then unable to find an installer to implement)
- hvac installers seem to specialize in types of equipment, so they don't seem like good unbiased sources for a holistic recommendation
"i would love to have the blue-prints account for duct-work instead of winging it in the field"
This is easy to ballpark. A main supply trunk will be about 20x8, this needs to run from your mechanical room all the way across the house.
Return about the same size to a central return on the main floor and say a 14x8 to the 2nd floor.
Takeoffs for each room is between a 6" to 8" flex (one size smaller for hard pipe). Each room needs at least one supply duct, larger rooms 2, living space probably 4 to 6.
Best location for supply vents is bellow windows or along outside walls. In a high efficiency construction with good windows, location is more flexible.
Single HVAC air handler is no problem for a 2 story house.
You can sit down and sketch out how these will all connect with minimal bends and jogs. Floor trusses with a plenum chase make your ducting life a lot easier.
Once you have a rough layout, you can update the house drawings to include the changes you need to make it work (ie duct chase from the basement to the 2nd floor), bumping up interior wall thickness to make room for ducting and bulkheads.
This will also give you a reasonable linear feet of ducting your HVAC can use for budgeting purposes.
Once your house design is finished and equipment is selected, you can get a pro to design the actual duct plan which hopefully be smaller than the space you set aside for it.
thanks akos,
my current house has a unit in the basement which handles the first floor via ducts running under joists in basement to registers in floor and returns low on walls. the second floor is handled by a unit in a small insulated mech-room in the attic with ducts running across attic joists to registers and returns in the ceilings.
in this build, on second-floor i was planning on dense-pack-cellulose over vaulted-ceilings, and blown-in cellulose over flat ceilings with ducts remaining somehow in envelope.
i can easily visualize duct layout for the first floor based on ducts running either under the joists in the basement (or through if i spring for floor-trusses) , but for the second floor, i guess either floor-trusses between first and second floors or roof-trusses with plenums would be required?
i feel like roof-chases with plenums may by more difficult to insulate with blown-in, have you ever seen people do floor-trusses between first/second floors and run duct work at that level, or have u seen mostly the plenum strategy (or other)?
I know bulkheads can be a swear word in some areas of the country but around me they are pretty much the norm.
The usual way to feed the 2nd floor is to have a bulkhead in the ceiling of the main floor (above kitchen cabinets is a good spot or above built ins in the living) for a the trunk feeding the 2nd floor. Supply ducts can now run from this trunk along the 2nd floor joist to each room.
A separate zone for the 2nd floor is better for comfort but comes at cost. If your budget allows and you can find a place for an air handler within the conditioned space it can work. Some of the multi position air handlers can run in a downflow configuration so if you use plenum truss for the 2nd floor (or bulkheads bellow) it is a pretty simple install. Since you don't have a lot of rooms, you can also go with a full home run with flex ducts which means you can use standard trusses or I-joist for the floor.
how crazy would i be to consider a hydronics based distribution system for this?
downsides i've heard so far are:
(a) they are expensive (but i don't know how much more expensive yet)
(b) use for cooling incurs risk of condensation damage (but i just read that code where i am building requires r-3 on water lines anyway)
(c) erv will require ducts anyway (not sure what erv-only ducts sizes would be, but u also said something about installers not doing home-runs, would that be required if an existing air distribution system wasn't present to tap into as u described)
Yes and no. If you have an extra $25k to $30k to spend, by all means look at hydronic.
A typical dedicated fresh air feed is 3". You probably want to make that 4" to keep noise down when the unit is on boost. To also heat and cool a bedroom using the same duct, you would need to bump that up to 5", so the cost saved is pretty much zero.
buildzilla,
One big advantage of doing the duct and other services layouts now is to know what will actually be where. There is a good chance many of service cavities in the walls and ceilings will have either nothing, or one or two electrical outlets, meaning it may not be worth installing them everywhere.
true malcolm, i was planning on using the inner wall of the double-stud for electrical, but i was probably gonna insulate unused space before rocking that up.
sounds like i will need specific spots for 8" supply and return cuz 3.5" ain't gonna cut it for those :)