I am replacing the subfloor in my turn of the century house. After ripping up the old floor, I realized that the floor register boots are huge compared to the size of the supply ducts. For example, this a picture of a 6" round duct with a 10x12" register boot.

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Look at that thing! It is eating up so much floor space! I have four of these on my first floor that I'd love to make smaller. As I understand it, a 4x10 or 4x12 register boot is pretty standard for a 6" round duct. Is there any reason I can't make this change?

While I'm at it, there are three returns in the same space that are even larger. I'm talking 8x30", 6x30", and 24x30". There is no way those returns need to be so large, right? My assumption is they just used the old gravity fed registers when upgrading the HVAC system some years ago. But that is just a hunch.

So I have a couple of questions:

  1. Can I reduce my floor register boots for the 6" and 7" round supply lines without negative effect?
  2. Can I reduce my return sizes as well?

I'm well aware that I should have a Manual J/D performed to get exacting answers. I'm looking for safe rule of thumb at this point.

Thanks all!

  • Was there a three-way register there before? Maybe smaller sizes only come in 1-way and 2-way so they bumped it up to use a 3-way? Just a random guess.
    – JPhi1618
    Commented Mar 30, 2020 at 21:00
  • I would make some temp templates out of cardboard to limit the flow on all 4 of them, check the back pressure and flow before and after , that is large but we don’t know how your system is setup. If you don’t see a change in air flow I would say it sounds like a good plan. Remember short lengths of ductwork don’t restrict as much as longer.
    – Ed Beal
    Commented Mar 30, 2020 at 21:16
  • The register is usually always larger than the duct that feeds it. But keep in mind that the grills in the register significantly reduce the area for air flow, so that the effective area for air flow is much less than the size of the register. This parameter (can't remember what it's called) is used in calculations for pressure drop along the air distribution route.
    – SteveSh
    Commented Mar 31, 2020 at 12:41

1 Answer 1


Many of the very old houses had gravity style furnaces that utilized large registers and grills that offered very low resistance to the flow of air. Since there were no fans or blowers the delivery pipes and registers had to be very large. Depending on the size of your room/house update it may be a good idea to ask an HVAC company for a bid on a duct work and register replacement package. You do not want to restrict the air flow to the furnace today and have to do it "again" if the work is done wrong. my 2 cents

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