Adjusting HVAC vent covers (most efficient position)

The adjustable part of my HVAC vent covers are horizontal flaps, which are controlled by a small lever. Horizontal meaning when considering them from side-to-side, but their position from front-to-back can range from vertical (in the closed position) to horizontal (in the open position). They look like this.

The most closed position would be with them fully vertical. One would assume on first thought that the most “open” position, meaning the position which allows for the most airflow, would be the opposite, with the flaps fully horizonal, and that having them angled mid-way between the two positions would be somewhere in between.

But the way they were installed is that when they’re partially open, the angle of the flaps is upwards back-to-front. And the ducts are coming up from the basement. So I’m thinking that the least obstructed position relative to the flow of air is actually the middle position, and that having them fully horizontal is actually less efficient in terms of airflow.

In my case, my issue is in being able to partially close them on the colder floor of the house. And I’m thinking angling them is not going to do it - the opposite, in fact. If I’m correct, then the only thing which might work is to reinstall them upside down, such that partially closing them goes against the flow of air.

Am I off-base here?

I can’t say for certain, but with these types of louvers there does seem to be a rather large zone starting at fully open where all you’re really doing is redirecting the airflow rather than restricting it. Then at some point you actually start limiting the airflow pretty significantly. It’s very much a non-linear relationship, so 50% closed may have a small effect on airflow while 60% closed chokes it off pretty fast.

As far as whether angling the louvers to be more parallel with the incoming airflow actually improves it, I don’t know if that’s actually the case here. For that to happen the fully open louvers would need to create more turbulence than angled ones, and without looking at each and every one I don’t know if it’s possible to determine that. You may also not be able to tell the difference between straight-out 100% flow and angled 90% flow.

Regardless, to achieve what you want you’d be better off closing those vents to the point where you can definitely tell the airflow is reduced. Even better would be to adjust dampers in the ducts, if you have them, because these sorts of vents can whistle and woosh a lot when they get closed down much, especially if you have a strong furnace blower.

I had an undergraduate course in a/c cooling. We are supposed to control airflow by calculating the percentage of floor area (related to volume) to the area of the whole house to ensure equal heating or cooling of each room. A larger space needs more air flow than a smaller space. I don’t recall how to measure the flow out of a vent.

I’d like to help you find your way into the rabbit hole:

Go nuts :slight_smile:

And stay cool !

That might be too much. The question is whether closing them an intermediate amount reduces airflow as well. That was my first thought, but on further contemplation it occured to me that the angle of the incoming airflow needs to be considered.

Couple of big problems with the dampers are 1) they were mostly sealed in when we finished the basement, and I’m not inclined to open up the ceiling down there, and 2) even before we did that it wasn’t practical. The problem is that the relative temperature of different parts of the house fluctuates too much (e.g. when there’s major cooking/baking being done, the kitchen and adjacent areas become a lot hotter) and it would involve constantly changing the dampers back and forth.

Actually, what’s ironic is that in my case it works the other way around. When the oven/stove is not on, the first floor is generally a bit cooler than the second floor (where the themostat is). The hotter it gets outside, the more that difference is magnified, because the a/c runs more. So with the daytime temps in the mid-to-high 90s these days, the first floor is getting a bit on the cold side.

You could still look at air balancing with that CFM anemometer gizmo, or something like it.

But cold air falls, and it’s not at all uncommon for a first floor to be colder than a second floor in cooling season, and vice versa in heating season.

Do you think there’s more than that going on ?

It’s worth noting that it isn’t always harmless to use registers to reduce airflow. HVAC systems often suffer degraded performance, excess loads on the motor, or increased wear if there’s too much ‘resistance’ (static pressure) from actions like these.

I don’t know, but I suspect that it may have to do with the a/c unit being in the basement, such that the air coming to the first floor is colder than that which reaches the second floor.

Curious question: if the issue is that “cold air falls”, then why would it be “vice versa in heating season”?

It’s a good question, but the answer is multifactorial:

One thing that some people notice is … if they use a fireplace (or, as you mention, the oven) … particularly in a room where the main thermostat is located … other rooms elsewhere in the house – notably the upstairs – will get really cold as the t’stat is ‘tricked’ into thinking it’s warmer than it ‘really’ is, so the heat won’t come on as frequently.

This is also possible. Some people have luck with this sort of gizmo where this is the issue:

I’ve never experienced a situation where the upstairs is colder, and I doubt it’s ever “always colder.” That’s such a weird absolutist take.

In older homes that were built before air conditioning, the furnace was sized and the ducts arranged to provide more heating to the first floor to account for warm air rising. That would ideally lead to mostly even temperatures, with more/bigger supplies on the first floor and most if not all the returns on the first floor. A fair amount of the heat from the first floor wafts up the stairs, and the returns try to capture the coldest air which would be at the floor. Easy.

Problem is that sucks for air conditioning. Since cold air is denser the furnace has a harder time pushing it up to the second floor, doubly so because most of it wants to pour out the more generous first floor registers. It’s working against the dense sinking nature of cold air, whereas in the heating season it’s working with the natural buoyancy of hot air. So you close some of those first floor registers off, but the furnace can only push so hard against those fewer and longer duct runs to the second floor. Also, with few or no returns upstairs, and the supply registers likely in the floor, you get a bubble of hot air up near the second floor ceiling that the system has no way to siphon off. The valuable cold air that does get upstairs pours down the steps and slides along the floor to the returns. It’s much harder to balance.