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We are adding new heat supply runs to our basement ceiling. We have had 3 HVAC contractors look at it and all 3 want to use 5" round air diffusers instead of the normal metal square registers. Here is an example.

We do not have a/c and are only supplying heat. They want to place these within a foot or so of the exterior wall by the window.

None of the contractors gave a good reason as to why to use these instead of square registers. They just state that's what we use now and it doesn't really matter. My understanding is they are great for A/C but cant find any info on heating.

Question: I would like to know if we should use these round diffusers to provide heat in a basement, will the heat just sit up in the ceiling?

If we use these, how far from a window should they be installed to be efficient.

We are putting cold air returns near the floor. Also I am up in Canada so have to deal with cold winters.

Thanks for the help

  • What makes "square" (I'm guessing 2x10 rectangular) "normal"? Since the late 1990s we've been seeing 6" round ducts with round grilles in the ceiling here in Minnesota. We have basically the same climate. 2x10 rectangular are still common in floor supplies. – isherwood Apr 5 at 12:59
  • Existing drywall ceiling? It's possible they are planning on running flexible ducting as much as possible and that adds a lot of air restriction so it's not always the best solution just the easiest. If they ran normal metal ducts, they would need to remove the drywall ceiling, air seal all the seams, and then re-drywall, tape, sand, and paint or match the existing ceiling finish. – Dotes Apr 5 at 13:04
  • And to be accurate, many grilles simply referred to as "registers" are also diffusers. They often have slats directing air in multiple directions, thus spreading or diffusing the heat. Even those with just one direction serve to tumble and mix the air. – isherwood Apr 5 at 15:03
  • @Dotes The ceiling is open right now, but will be finished with drywall. Everything will be rigid duct work. – OverStacked Apr 5 at 17:44
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I'm in Toronto, and I'm a HVAC tech. I do use round diffusers from time to time but I've never noticed them being the "go to" by any tradesmen. There must be an undisclosed reason for it. Possibly a local supplier has been pushing them out at a low price. They (round diffusers) work well enough in most cases. Basements should be supplied low wall anyway but that often results in boxes and nobody likes boxes.

Also rectangular registers have an effect called throw. Depending on the orientation of boot (A boot, end boot or universal boot) the outgoing air will have momentum and upon hitting the vaines of a rectangular register deflect but continue slightly in the starting direction.

rectangular register with an end boot

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continental industries

Round diffusers have different effects illustrated by the two following images. Round diffusers

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researchgate.net

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Slideshare

Honestly the dynamics involved are chaotic and almost impossible to predict accurately. I studied this topic deeply years ago. It's something I still consider when designing a system but ultimately customer want to eliminate boxes first. If given the choice, not so mechanically inclined customers will always go the "no box route" and customers who take great interest in the design of the system may be okay with a box.

Back to the OPs question round diffusers may or may not work as well as a rectangular register. For cooling they are definitely better, for heating, in my experience at least, they don't work as well. But in a basement where half of the exterior is underground heat loss is reduced compared to the floors above when it's exceptionally cold outside.

In short there is no clear answer but at least there're taking low wall returns.

  • I think diffuser is meant to mean the round diffuser type, while the rectangular type would be considered a rectangular register? I think he's answering the question, the issue is just how the specific terms are understood. – PhilippNagel Apr 5 at 13:12
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    Yes colloquially round diffusers are just referred to as diffusers and the square ones are called registers. At least in my neighbourhood. I edited for clarity. – Joe Fala Apr 5 at 13:53
  • Thank you @JoeFala for taking the time to provide so much information. Its understandable that having the registers near the floor would provide the best heating option. We did consider this as our basement is framing is exposed right now. But the cost of that plus the box outs etc turned us off of the solution. In any new build I would make sure this was part of the solution. One consideration for the round diffusers is comfort level so that air is not blasting down at the people below. We have a few small bedrooms and the bed will be near the location of the heat supply. – OverStacked Apr 5 at 17:56
  • As far as placement of a round diffuser near the windows would you recommend within a foot of the window or to give it a bit of space maybe a couple feet away from the window? – OverStacked Apr 5 at 17:56
  • Close to the windows is good, within a couple of feet. You get what's called skin effect and it sets up a convection currant. The skin effect is a result of the cool window cooling the hot air out of the diffuser and causing it to drop. – Joe Fala Apr 5 at 18:03
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When I finished my basement 20 years ago I installed those same ceiling diffusers with shut off dampers so I could close off the supply air if I choose. They were a poor choice for me. years ago, I replaced them with multi-vane supply registers so the air could be directed towards the floor for better comfort. I have found that basements can be very hard to heat depending upon the homes construction and the fact that heated air rises and can easily run up the stairs to the upper floor. Make sure that you install enough supply registers and return grills. My "rule of thumb" is to have as many supplies and returns as you have on the floor above since the heated square footage is probably the same. I have 7 supplies and 4 returns in my basement and I could use more. If you can install the supplies on the main trunk you will get more supply air and can reduce the number required.

  • I am not sure to whom you are referring to with your comments about diffusers and registers. In the HVAC trade, registers are usually mounted on a verticle surface such as walls and the like and diffusers are the round type that are usually mounted on the ceiling. the shape and the construction dictates the name – d.george Apr 5 at 13:52

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