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Building Science

Grilles, Registers, and Diffusers Part 3: Supply Air Outlets

Properly sized and located supply outlets will maximize airflow and contribute to an HVAC system's longevity

A register high on an interior wall throws air across the ceiling, promoting mixing while keeping fast-moving air out of the occupied zone. Source: DOE Building America.

Grilles, registers, and diffusers (GRD) are key components of forced-air systems, impacting comfort, efficiency, and noise levels. In part one of this series, I explained the differences between grilles, registers, and diffusers. I also discussed the placement, selection, and sizing of return grilles. In part two, I talked about return-air pathways, which allow air supplied to bedrooms to flow unimpeded to return grilles in the main body of the house. In the final article in this series, I discuss registers and diffusers. These fittings serve as outlets for the ducts supplying heated and cooled air to the living space.

As construction practices have evolved, so has thinking about where these supply outlets should be placed. Modern buildings have lower air leakage rates, higher insulation levels, and better glazing than those built a few decades ago. These envelope improvements reduce the total amount of heating and cooling needed. They also allow more flexibility in placing registers and diffusers; we no longer need to rely on supply air to correct comfort problems created by hot, poorly insulated ceilings or cold, single-pane windows.

Less total airflow and more location options allow for compact duct designs. Shorter, smaller duct runs mean lower costs, fewer space requirements, and less heat loss or gain through the ductwork. In this article, I’ll outline key considerations for register and diffuser placement and selection, then look at how high-performance building envelopes can affect these design choices.

Registers and diffusers

Registers are square or rectangular fittings with movable dampers to adjust airflow volume. Lightweight stamped-steel registers are used for wall and ceiling applications; heavier-duty cast or welded registers are used in floors. Diffusers cover supply openings in ceilings. They have angled vanes that discharge air in a spreading pattern. Round and four-way diffusers blow air in all directions, while…

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  1. Expert Member

    Thanks Jon. The design flow is very helpful. It's logical, but the temptation is to move from whatever air handler you have towards the terminations, making the necessary decisions along the way.

  2. AC200 | | #2

    This is really interesting. However, I'm finding it hard to implement the theory. I'm in rough in stage at the moment for HVAC. During design I had a discussion with the HVAC engineer and stated my priorities were performance and efficiency as number ones, with value as secondary (opposite what a production builder might do)

    He tweaked the design based on my requests, enlarging the return, moving it to high wall and adding a supply to a bedroom with a large westerly window. Critical returns were made of metal ducting instead of panned joist.

    My HVAC contractor, then added more returns and supplies based on their experience. Some of the new returns are close to the furnace blower and I'm concerned that it will reduce air flow from the distant returns.

    There was no specifications for registers and diffusers, only that I know Aria vents are bad even though Designers love them. So now, I'm on the hunt for vent covers that offer good airflow with good aesthetics, not that easy.

    I need to do another walk through with my contractor to discuss all of these issues before rough in is completed and drywall.

    It seems to me that this area is often overlooked and we end up with uncomfortable homes. The most glaring example are the tall and narrow 3 storey town homes with budget HVAC that result in freezer temperatures in lower level and broiler conditions on the top level.

    1. Chris_in_NC | | #3

      Have you looked at bar linear grilles? They're really nice looking (to my eye), and can be found in frameless flush mount and drywall bead versions if you want a super clean modern look.

      1. AC200 | | #4

        I've only seen them online in pictures. It would be a leap of faith to order them. They look nice in pictures and have open area specs which is a bonus.

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