Hvac Being moved to conditioned space
Gavin12
| Posted in General Questions on
Hi all!
So during our new construction, we opted to have units and ductwork in the attic. We quickly realized it was the wrong choice and ripped everything out.
We then decided to follow guidance and install ducts and units within the thermal boundary. Basically we drywalled the entire home and painted and are now opening up corners of room (previously framed for this work) and running trunks and ducts on the interior finished side.
If you look at the image attached- we are wrapping ducts with insualtion in either case, but can the trunk lines be pressed up against the drywall, or does there need ‘breathing room’
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“Can the trunk lines be pressed up against the drywall, or does there need ‘breathing room”
The ductwork can be tight to the drywall on the exterior wall. Note you want the drywall behind the ducts to be taped and muded not pretty but air tight.
“We are wrapping ducts with insulation in either case”
There is almost no advantage to insulating the ducts inside the conditioned space. Whatever energy escapes the ducts still goes into the conditioned space so there is zero loss.
Will the ductwork be old school steel or low-cost flexible ducts?
If steel consider going square and making the bulkhead smaller.
The drawing shows 2x4 framing around the ducts. In my opinion that is over kill for strength and adds needless bulk. A 2x2 in the corner with 1x4 connecting it to the walls is more than enough.
“We quickly realized it was the wrong choice and ripped everything out.”
I have to ask do you live in the south?
Did you get much push back?
Did you have to give up a closet to make room for the equipment?
Great choice
Walta
Excellent. Thank you.
We ended up painting everything anyways so exterior walls are totally sealed. Our ductwork is rigid metal, opted out of flex as it's difficult difficult clean on the interior.
I was wondering about the 2x4s and appreciate your comments that we can use smaller. That's helpful as the bulkhead will be getting close to the window trim on those exterior walls.
We live in NJ. We didn't get much pushback, more of a why from first hvac crew who wouldn't redo it. The 2nd crew was well known for working with mold avoidance ideologies and helped us tremendously
"There is almost no advantage to insulating the ducts inside the conditioned space. Whatever energy escapes the ducts still goes into the conditioned space so there is zero loss."
There is benefit to insulating ductwork even within a conditioned space. Namely delivering heat where it's intended. I.e. if you're heating air to 110F in your air handler but the exit temperature in your furthest bedroom is 85F then you're likely going to have balance and comfort issues. Best to deliver the air as close to the air handler supply temperature as possible.
I also wonder about condensation on the ducts during the cooling season. Supply temperatures get pretty close to the dew point depending on conditions.
Article that discusses:
https://www.achrnews.com/articles/135246-ducts-in-a-conditioned-space-to-insulate-or-not
Condensation is usually only an issue near the A coil in the furnace, but it depends on conditions. You can sometimes mitigate the issue by running higher air velocity, which results in a lower delta D for the same deliverable BTUs.
There isn't really a downside to insulating the ducts aside from cost and the need to allow a bit more space for an insulated duct run. Note that if you go with square/rectangular ducts, you can use the commercial rigid fiberglass insulating board made for this purpose, which should be available at mechanical supply houses. On commercial projects, this stuff is usually secured to the duct by nails through the insulating board that are tack welded to the duct, but for smaller ducts you should be fine just taping the corners (there is a tape made for this purpose). You can get more cross sectional duct area in your square columns by using a square duct.
Bill
What about just the bubble wrap on the interior?
That's probably fine for condensation control, since you don't need much R value to deal with that. Bubble insulation is likely to get you around R1-R2 or so. All you need to do to control condensation is to keep the first condensing surface up above the dew point temperature, and that's not hard to do when the air temperatures of "inside" and "outside" the duct aren't very different.
Bill
“I.e. if you're heating air to 110F in your air handler but the exit temperature in your furthest bedroom is 85F”
I have not done the math but my guess is if this duct was 8 inch round, the 110° air was moving at 800 feet per minute in the duct surrounded by 68° still air the length of the duct would exceed a ¼ mile before the exiting air was below 85°.
Ducting will move a bit when it expands so you don't want it rubbing on things. Typically ducting is hung with straps or from a leg formed by bending the ends of the joiner cleat. Either allows for that bit of movement so you'll have noise free ducts. They can still be close to the drywall just just not touching.
The bulkhead is typically build out of a combination of 2x2 and 2x4 on flat. It only holds up drywall, so you don't need all that much structure there. For tight spaces I've build bulkheads out of combination of hat channel and 1x2 framing angles. This gets it down to about 3/4" thick.
I'm in a cold climate with hot and muggy summers. Here ducting is never insulated and it is not an issue. About the only time I've seen duct sweating is when it was run through a vented crawl or after mudding a floor of drywall. Doesn't hurt to insulate, but I would not loose sleep over it.
Typical trunk and branch setup is not bad for sound transfer but if you want a bit better, a length of flex for bedroom feeds doesn't hurt. For flex straight is best for airflow but for best sound a lazy S band with slightly upsized flex does wonders. You can also get in-line duct silencers if you want to avoid any flex. These are not that expensive and are also very effective.
If you are doing a remote blower for your range hood, I would definitely recommend a duct silencer between it and your hood.
Good on you for insisting on ducting inside conditioned space. This is pretty much the norm here, blows me away to hear about ducts in attic it in new builds south of the border.
Do you have any details on that framing you mentioned? I want to explain it to my carpenter. In a way that I can understand? Maybe like a drawing or something! Thank you
Find attached a picture of the bulkhead with hat channel.
For wood, it would look something like post #9 here:
https://www.diychatroom.com/threads/framing-around-ductwork.7568/
Doesn't have to be anything beefy, it is simply holding drywall in place.