Recently purchased a home built in the 1960s. We have baseboard diffusers in 3 bedrooms that are getting carpet installed this week.

The diffusers are old and big and I was looking to replace them. They are 24 ½" long and the actual vent opening is only half that.

Is there a reason the diffuser would be so much bigger? I'm having trouble finding one of the same size and would like to downsize them if it's not detrimental to the cooling and heating.

Carpet is coming in a couple days so I need to figure out if I can rip them out and they can lay carpet assuming we'll have the smaller diffusers.

Here's images of what I'm working with:

Diffuser cover removed

Diffuser cover in place

3 Answers 3


The vent opening is unrestricted (open area.) The diffuser has some sort of grille/vanes/slats that partially block the open area. So a diffuser that is larger than the vent (overall dimensions) is going to be the right size for the vent (effective vented area) while one the same size as the vent will be a restriction. Whether the restriction will matter in your system is hard to know, but it will be a restriction.

  • It sounds like what you are getting at is that the cold air will somewhat "fill" the interior diffuser and also flow outside the side that is not directly over the vent? This is what you mean by the "effective vented area"? Effectively its as if the covered portion is also providing venting?
    – bgura
    Commented Oct 18, 2021 at 16:10
  • 2
    No. The grill blocks part of the opening. So you need more opening with grill partly blocking it to equal a duct with nothing blocking it to let the warmed or cooled air supplied by the vent out the grilled opening.
    – Ecnerwal
    Commented Oct 18, 2021 at 17:05
  • 2
    Except for virtually every floor vent ever. They're not oversized.
    – isherwood
    Commented Oct 18, 2021 at 18:27
  • @Ecnerwal based your follow up, sounds like if the 12inch opening is receiving the full capacity a 12 inch opening can physically handle, I would also need to sure I have an unimpeded 12 inch opening to not inhibit any flow. Assuming this vent is putting out the full capacity a 12 inch supports. The vent would need to be longer to allow the full capacity to flow. Applied to just Ecnerwals explanation, Isherwood's comment could also be justified in that modern vents have smaller grates and the vent may not be pushing the full capacity it could support.
    – bgura
    Commented Oct 18, 2021 at 21:01

It probably wouldn't make much, if any, difference in airflow if you went with a shorter diffuser. A few thoughts on why they're twice the size necessary.

  1. This could be the size the original installer had on-hand and didn't want to make a trip to the supplier.
  2. Based on wall lengths on each side of the current register, it may have meant not needing to use more than one board for your baseboards and have to butt (or overlap) them baseboards together.
  3. Could have just been an architectural decision for appearance.

If it were me, I'd just re-use the existing ones, as you'll have trouble matching the baseboards if you use a shorter diffuser, and wouldn't look "right" with the baseboard patched in, especially with the likely multiple layers of paint that are already on the baseboards. If you end up needing to find replacement ones of the same size, you may need to go to an actual HVAC supplier (assuming you're checking at big box stores for what they have available), as an HVAC supplier has access to a lot more variety than a big box store. Or perhaps, depending on where you live, an architectural salvage company.

  1. Metal products weren't as cheap as water in the mid 20th century like they are now--mining and manufacturing were a more costly venture. Fewer sizes and variations were available, so a builder may have purchased a number of the same thing and used them for various duct opening sizes.

  2. Some additional distribution and diffusion of airflow occurs with a larger vent grille. This can help soften flow to eliminate draftiness, and can reduce noise from air turbulence.

In my 1955 home one 6-foot louver covered two openings in the floor under a picture window, probably to help keep frost off the window during extreme cold. I replaced it with two smaller, more modern vents (without 37 coats of paint on them) with no ill effects. Obviously I had to refit the baseboard trim to match.


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