enter image description hereWe are going through a home remodel. I want to replace our baseboard diffusers because I feel they are too big for the vent. The size of them are 25inches. The dimensions of the vent are 12 Length x 2.5 width. I’m unsure what size I need. I’ve seen people say that you want one two times the size of the opening then I’ve seen people who use the exact size or a bit bigger.

  • 1
    The 25" diffusers must have one or two more dimensions you've failed to give. If the system works as built, changing the system may make it work less well, due to altering the disribution with additional resistance (the grille over a vent cover has a significant impact on the actual area of the vent cover, so it should always be larger than the vent behind it, unless the vent behind it is oversized.)
    – Ecnerwal
    Commented Feb 22 at 14:06
  • What other dimensions do I need to give? Commented Feb 24 at 13:31

2 Answers 2


The answer lies in two things:

  • The actual area of the duct opening
  • The need for diffusion around the room

Years ago I removed cumbersome, oversized diffusers spanning two duct openings from my 1950s home and replaced them with two modern, compact diffusers which were only slightly larger than the openings. I never saw problematic effects even in our severe climate.

As long as you don't reduce flow enough to leave the room uncomfortable and air is mixed adequately, have at it.


Diffusers do two things. They diffuse. They restrict flow.

Less diffusing might cause some parts of the room to be warmer or cooler, it might cause drafts to be felt near the register, it might be more easily obstructed by furniture. Or none of those things might happen, or you might not care. If the register is away from people and furniture, the air should mix all by itself.

More or less flow restriction will change the balance of heating and cooling between rooms. Again, you might not care. It happens all the time when you open and close doors. If you replace all the diffusers make sure your new ones don't all provide less air flow. That could be bad for the equipment or for energy usage.

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