Grading an Attempt to Get Ducts Inside Conditioned Space
There's a ton of new construction and remodeling going on in Atlanta. On a lot near where I live, a developer tore down the old house and is building two new ones. This weekend, I took the opportunity to check out their progress on the first one and saw the mechanical system roughed in on the first floor. They did some good things in their attempt to keep the air handler and ducts inside conditioned space. They also did some not-so-good things. Let's grade their work.
Exterior walls - Grade: A-
As you can see, before they installed the equipment, they insulated the walls and covered them with a layer of interior sheathing, Thermo-Ply in this case. They also sealed the edges with spray foam. I gave them an A- instead of an A because there's one place where the T-Ply is coming apart. I assume they'll fix that.
Furnace - Grade: A
They used a high efficiency, sealed combustion furnace. For combustion equipment inside conditioned space, that's definitely the way to go. And yes, they installed both pipes, intake and exhaust.
Placement - Grade: C
The mechanical closet is in the back of the house. While it's nice to have it out of the way and leave the middle of the house for espresso makers and powder rooms, the result was long duct runs to the front of the house. Worse, the two vents on the left side of the house had to go around the stairs (see 2nd to last photo below). From the air handler to those vents, the air has to make its way through ten 90° turns and four 45° turns. Finding a way to get the air handler into the middle of the space would have avoided long runs.
Installation quality of ducts leaving air handler - Grade: F
The duct system here is radial rather than trunk and branch. It's all flex duct. Because of the constraints, the flex doesn't meet the Air Diffusion Council's standards for flex duct installation. Notice the mess of ducts coming up out of the mechanical closet. Notice the turns and the squishing. With a trunk and branch duct system, they would have installed only a few ducts, ideally hardpipe rather than flex, coming up out of the closet.
Keeping the ducts inside conditioned space - Grade: C
Oops! It looks like that attic above the mechanical closet is meant to be insulated at the drywall ceiling, not the roofline. That means those ducts will travel a bit through unconditioned attic space. A couple of the ducts also go out into the attic over the porch, as you can see below. Also, because of all the ducts in that little attic space, they're going to have a heck of a time insulating it properly.
Straight runs - Grade: A
Where they could, the installers ran the flex duct to the vents in nice, straight runs. It looks like they pulled it tight, too. Although it's the inner liner that needs to be pulled tight, if they got the outer liner pulled tight, it's likely the inner liner is fairly tight.
Turning the air - Grade: D
They're not the worst turns I've seen, but they did make every turn in the flex duct using the flex itself. That adds much more resistance than installing rigid metal elbows to turn the air. And there are a lot of turns, especially in the duct that has to go around the stairs (below).
Butt joints - Grade: C
Flex duct comes in 25' lengths. When you run 50' of flex, you've got to connect pieces together. They did use rigid metal connectors at the two joints I checked out. But I can't give them higher than a C here because if they had used metal elbows, they wouldn't have needed any butt joints. Even better, if they had gone with a trunk and branch system, they wouldn't have needed any butt joints.
The best way to get ducts inside conditioned space
So, in this home they got the air handler in conditioned space. They got the ducts mostly in conditioned space. And they had to make a lot of compromises. I don't blame the HVAC contractor here. No, the real problems in this home go back to the design phase. The architect who drew that mechanical closet should also have considered how the ducts would get to the different rooms on the first floor.
If you're going to put an air handler and ducts inside conditioned space for a first floor system like this, here are a few tips:
- During the design phase, bring in the HVAC designer and contractor.
- Also bring in someone who can spot weak points in the building enclosure so everything gets insulated and air-sealed properly.
- Try to locate the mechanical closet centrally to eliminate long duct runs.
- Consider open-web floor trusses with spaces designed for trunk lines.
- Use flex duct only for straight runs.
- Consider using a central return. If the system is sized and designed properly, it doesn't have to be noisy.
With a first floor system inside conditioned space, it's important to get it right upfront. This isn't like a system in an attic or crawl space. The ducts will be buried behind drywall when the house is finished. Good, integrated design is the way to go.
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