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We recently bought a new (to us, built in '52) house that has A/C and a furnace. Great. The air distribution isn't... ideal - much colder in the bedroom than other parts of the house. For the time being we've tried shutting registers everywhere except the bedroom to pull warmth in; jury's still out on whether or not that's working.

Regardless, we want to eventually finish the basement - and with the way the ducting looks, we'll have very low ceilings in some places. We'd like to fix that - and we'll need to modify it SOME way to condition the finished basement, too. Trying to get opinions on good DIY options: I'm more than willing to learn and run my own ducting.

As a side-note: the electric in this house is grandfathered, but far from code. We're bringing it up: are there elements of the ducting/HVAC that don't look to code that we should be aware of? The returns seem to be just a flat sheet taped to the bottom of two joists to turn the gap into a duct; I think that's no longer allowed?

Specific questions:

If my research is correct, this looks like a "reducing trunk" layout; except there's no 2-foot run beyond the end of the trunk to convert air velocity into static pressure. Ironically, the two ducts that branch from the end of the trunk go to the bedroom and office, both of which are the least warm rooms in the house.

Is a reducing trunk the best option for this space? It's probably a 30 foot run or so from the furnace to where the last branches split. Without any experience but a few google searches, in my head I've thought of some sort of extended - shallow but wide - plenum that could stay near the joists (to avoid reducing ceiling height) with branches coming off the top in between joists.

I guess I'm at the "I don't even know what I don't know" stage, but I'd love to get insight and pointers; I want to do as much as possible myself both to save money and to learn; it's safe to assume my time is mostly worthless. ;)

When we do the basement finishing, should we just try to keep the existing ducting and add new around it? I've been debating a motorized damper at every room with dedicated temperature monitors per room (effectively a zone-per-room approach) to balance things better.

Furnace/Plenum Final branches

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  • Depending on any work being done, first thing a 52 house means poor insulation/air sealing/drafty, might be number one problem for the bedroom. In the second picture with the odd 4x4 on posts, on the duct you can see a wing nut with a piece of metal sticking out. That is a handle for a damper/valve inside of the duct. Some tuning of the ducts can be done with them. That 4x4 might need it's own question(never mind).
    – crip659
    Apr 25, 2023 at 18:13
  • @crip659 Oh, there's a question about that 4x4, haha. I didn't catch the wingnut damper adjustment, thanks! I'll see if I can open it up more (if it's not already). Definitely going to be putting in better insulation when we can, but our current order of operations is: Kitchen, finish basement (with good insulation) while living upstairs, then redo upstairs walls/etc (with new insulation) while living in basement.
    – user112697
    Apr 25, 2023 at 18:23
  • That damper shown should be in full open position, inline with the duct. Warmer rooms should have their dampers closed a bit and the bedroom make sure the dampers are full open. Probably a couple of weeks to get it right.
    – crip659
    Apr 25, 2023 at 20:17
  • Do you have a routing sketch?
    – Huesmann
    Apr 26, 2023 at 14:27

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A couple of possible approaches. These may not be right for you, but hopefully the ideas help you:

  • The large main trunk going down the middle of the ceiling could take an indirect route along the far wall. Then you build a closet or shelving beneath it. Design the space for a "low part" near the wall.
  • If the air handler is near end of life you might consider replacing with a high velocity system (Spacepak or Unico). These use smaller ducts and drive the air at higher pressure and speed. A complete replacement will be expensive, but if you want to maximize a basement reno it's something to at least size up.

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