HVAC flex ducting rerouting

masraum · 06-27-2021, 01:48 PM

Our home has a hip roof with 3 dormers. The upstairs is essentially a T-shaped space with the bottom and one of the sides of the T separated into two bedrooms

The HVAC in our home is completely plumbed with flex hose.

Upstairs, we have a 1/2 bath that is best suited to hobbits. We want to move the bathroom to the other side of the top of the T where there's more space. Currently, some of the HVAC ducting runs through that space so to create a bathroom there, we'll need to reroute the ducting. I think I've thought of a way to do that, but since I don't know jack about hvac ducting, I thought I'd check with the brain trust and see if there are any major flaws with my plan.

In the diagram below, on the left side, there are 3 ducts (red lines "point" to them). One runs through the corner of a room where a portion of the corner was enclosed with drywall at a 45º angle. It then pops out into the attic area and runs over to the vent in the ceiling of the downstairs bathroom. The middle most vent is in the opposite corner of the room. There's a wooden box (wart) built in the corner of the room that houses that vent and the feed for it. The existing ducting pops down from above the upstairs rooms into corner attic areas, and then splits off in a Y to feed the 2 downstairs vents.
My thought is that I add another 45º angle bit of drywall and re-route the existing ducting down through the corner of the room similar to the other side. This would require a Y or T immediately above that middle vent to break out the ducting to go to the bottom most vent. It seems to me like the best option would be to have the duct feed a "wye" and then one end of the wye go to the middle vent and then more flex hose go to the bottom vent. Adding the drywall to that corner at a 45º would make the two corners match except for the wart. Then the theory is that I'd built something like a window seat that would run all of the way across the wall that would essentially hide the wart by integrating it into a feature. I should be able to make the wart shorter because right now, it's a little higher than the bottom of the windows.

Do you gents that are familiar with HVAC ducting have any opinions on whether this would be acceptible or for some reason, a really bad idea?

a not completely accurate or scale representation of the upstairs as it is now.

Showing the difference in how we want to reroute the one duct to the 2 vents.

I think this might be the best bet if I can get it plumbed directly to the top of one vent.

In the background in the corner is the wart (covered in stained and finished beadboard paneling).

This is the 45º corner that the one duct is currently run through.

908/930 · 908/930's Garage

I don't do duct work for a living but I have done quite a lot of it, If you want to branch off do not use a T fitting, will add a lot of restriction, the WYE will flow better. Are you changing the actual length of the run by anything or just altering how it gets there? If overall length is close it should be fine, if you are adding length use hard pipe as much as you can it should flow better then flex, and try to keep the diameter of the pipe at least as large as it is now, larger diameter helps keep the flow, when you restrict it the air will just find the path of lest resistance, unlike what some people believe.

masraum · 06-27-2021, 02:36 PM

Quote:

Originally Posted by 908/930

I don't do duct work for a living but I have done quite a lot of it, If you want to branch off do not use a T fitting, will add a lot of restriction, the WYE will flow better. Are you changing the actual length of the run by anything or just altering how it gets there? If overall length is close it should be fine, if you are adding length use hard pipe as much as you can it should flow better then flex, and try to keep the diameter of the pipe at least as large as it is now, larger diameter helps keep the flow, when you restrict it the air will just find the path of lest resistance, unlike what some people believe.

I don't think this will be much of a change of length if at all. It might be a tiny bit longer. But I'll see what I can do about using non-flex stuff. I can probably manage that for some of it.

masraum · 06-28-2021, 08:40 AM

OK, so the big issue is change of length (change in diameter would probably also be an issue as well, but I don't anticipate that).

Anything else? Recommendations of youtube videos to watch that are decent or any recommendations for how to join/seal ducts to hard lines, etc...?

908/930 · 908/930's Garage

How much longer do you expect the run to be? Does the heating cooling adequate in those areas now? If not, now is the time to tweak it. I do not use flex lines but would think you can just hose clamp them on to the fittings. Once inside the building envelope slight leak is not a big deal, different when in uninsulated area. With pipe to fittings just use a couple of duct screws to secure, you can also use sealant if you really want it air tight.

look 171 · 06-28-2021, 09:35 AM

Get one damper on the of the branch in case it flow more air then the other. This way, the flow can be controlled as long as you can get at it after the dust settles.

908/930 · 908/930's Garage

Something else to check into, I do not know if your steel fitting will need to be insulated to keep them from getting condensation on them? Up north we worry more about heating, well it is about 100f outside today.

masraum · 06-28-2021, 10:25 AM

Quote:

Originally Posted by 908/930

How much longer do you expect the run to be? Does the heating cooling adequate in those areas now? If not, now is the time to tweak it. I do not use flex lines but would think you can just hose clamp them on to the fittings. Once inside the building envelope slight leak is not a big deal, different when in uninsulated area. With pipe to fittings just use a couple of duct screws to secure, you can also use sealant if you really want it air tight.

I suspect the run will be if not the same, then within 2-3'. Upstairs gets warmer during the day, but I think that's more about inadequate insulation (thread coming soon) and possibly HVAC design (need additional return?) than HVAC capacity. Miraculously enough, the entire HVAC system was installed in 1991, and other than a slow leak, still cools REALLY well. We are looking into replacing the system, probably within 9-18 months. The attic has 2 whirlybirds, but no soffit vents, so that's another issue that I plan to rectify.

masraum · 06-28-2021, 10:27 AM

Quote:

Originally Posted by look 171

Get one damper on the of the branch in case it flow more air then the other. This way, the flow can be controlled as long as you can get at it after the dust settles.

Good recommendation, thanks.

Quote:

Originally Posted by 908/930

Something else to check into, I do not know if your steel fitting will need to be insulated to keep them from getting condensation on them? Up north we worry more about heating, well it is about 100f outside today.

I want/plan to greatly increase insulation in the attic. I'm going to start a new thread on that.