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
Best Answer
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
continental industries
Round diffusers have different effects illustrated by the two following images. Round diffusers
researchgate.net
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.