Curious about impact of high wall vents
_Stephen_
| Posted in Mechanicals on
I went on a walkthrough of my house with my builder today, and noted something odd:
All the HVAC vents are high up on the walls not on the floor, and so are the returns. The builder claims that this causes the better air mixing in the room, so it’ll be more comfortable. I thought it was a bit odd to have a vent and a return next to each other, high up on the walls.
Is there any truth to this claim? Could having high wall vents and returns cause issues? it’s kinda nice that you don’t have to worry about having a couch cover a vent or return for sure…
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Replies
It depends. You really shouldn't have a supply and return right next to each other, because the conditioned air tends to short-circuit without mixing into the room air. How close is too close depends on the style of registers, air velocities, and other stuff.
What climate zone are you in? if your location is more driven by air conditioning than heating, supplies and returns on the ceiling works pretty well. If you are in a heating climate, not so much.
I agree with Peter's answer.
This is climate zone 6. The house is so well insulated that the modeled peak heating load is actually less than the peak AC load at design conditions. There's a fair bit of south facing glass, with a SHGC( solar heat gain coefficient) of .34, probably about 40 sq.ft in this room.
I think air velocity would be pretty high, as the ductwork itself is 3.5" * 5" oval ducts. Not going to get a lot of air flow through there unless it's reasonably high speed air flow.
Minisplits have their "return" right next to the "vent" and they don't seem to short circuit too badly, do they? At least, not under normal circumstances?
With proper face velocity in a reasonable (ie, away from the return) direction, there isn't a problem. Velocity in the duct doesn't matter.
If the house is truly that well insulated, you probably won't have a problem either way. With higher AC loads, the high registers will provide better mixing in cooling mode. At worst, you might notice slightly colder floors in winter. Wear socks.
One advantage of placing HVAC supplies near the ceiling is the coanda effect--the air sort of skips across the ceiling, distributing it across the room. Another is that it gets the air flow above the range where you feel it moving--probably more noticeable for those who wear dresses or skirts in the winter, and less effective for tall people and short ceilings. For well-insulated and air-sealed homes, there should be little temperature stratification anyway.
According the the HVAC engineer that designed it, the high wall vents will distribute the air in a cone shape down from the ceiling towards the floor, and promote better mixing across the room.
The house is pretty well insulated. In zone 6, peak heating load is 18k BTU, but realistically, is probably closer to 15k at design temperatures, for a 2800 sq ft house. Mitsubishi hyperheat ducted system will be the supply of heat. It'll probably never run at anything other that it's lowest modulation, but that's OK. The 18k minimum output of the system is a lot closer to the design load of 15k BTU than the 80k BTU furnaces that most houses seem to use.