A/C gurus...how long does it take to completely cycle/replace the air in a room?

Factual Questions
1

Just curious. Not looking for any health or living advice as I’m perfectly happy with my choices.

How long does it take to cycle/replace the air in a ~400sf apartment with windows closed? Curious because it takes ~12-16 hours to remove the smell of regular cooking, longer if I burn something or cook something smoky like bacon. And sometimes I probably just become nose blind to the odors.

I keep the A/C on whenever I’m home (the lowest setting, fan only) and keep my windows closed due to traffic noise and air pollution (several Korean BBQ restaurants within several square blocks). The windows are always closed, even when I’m not home (the curtains are set up to block all light).

I keep my A/C vent closed for efficiency, except for a couple of months in winter, when the outside temp is in the mid-60’s/low 70’s (I’m in Hawaii). Even then, the most I’ll do is open the vent and just use the A/C fan.

2

That’s a really open question highly dependent on the A/C unit. But it is doing nothing to remove odors anyway.

Dennis

3

I’m guessing you’re talking about a window/wall unit, not a central AC. As mixdenny said, it’s going to depend on the actual unit and more specifically, the fan in the unit. But, as also noted by mixdenny, it doesn’t remove smell. The only way it would do that is if you added a good quality/HEPA filter to it.

If you want to get rid of smells, you either need to add a HEPA filter to the front of it, open the vent, open a window or get an air cleaner with a HEPA filter.
A standalone air cleaner with a good quality filter is probably going to be your best bet to deal with cooking smells if you don’t want to open any windows.

4

It’s a 12,000 BTU wall unit.

My apologies for the confusion.

I’m not concerned about removing odors, just used it as an example of how (I assumed, seemingly incorrectly), the unit is cycling/removing the air in the apt, especially since the vent if closed.

My basic understanding (which may be incorrect) is that with the vent closed, a larger percentage of the air in the apt is cycled and cooled vs having the vent open, which mixes a higher percentage of outside (fresh) air into the mix.

I’m picturing a room full of smoke particles (the odor not being an issue). I’m assuming it would take significantly longer for the smoke to completely remove all the smoke particles with the vent closed than opened.

5

I’m not an HVAC engineer, but I’ve dabbled in simulating building airflow with computational fluid dynamics (CFD).

US residential HVAC standards include ANSI/ASHRAE 62.2-2013: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. Ventilation frequency is typically quantified in air changes per hour (ACPH). The exact number depends on which room (kitchen vs walk-in closet) and the number of people typically expected to be in that room.

It’s been a long time since I’ve looked at that spec, but I believe it calls out something like 6-8 air changes per hour for a living room. The catch is that this number is found simply by dividing the room volume by the fan flow rate:

ACPH = (60 * flow per min) / room volume

So there’s no guarantee that all of the air in the room is scavenged with each air change—in fact, a large fraction is quasi-stagnant. So “eight air changes per hour” doesn’t mean all the air gets changed eight times per hour.

Realistically, maybe 50% of the air gets changed 16 times per hour. Or it could be that 25% of the air gets changed 32 times per hour. It all depends on the floor plan, inlet/outlet location and how well sealed the dwelling is.

That’s a big reason why cooking smells tend to linger well beyond the first nominal air change. The only places that get fully scavenged with each air change are wind tunnels.

To answer the OP’s question, a 400 square-foot apartment might contain 4500 cubic feet of air. (I’m assuming ten-foot ceilings with some extra padding).

A typical 10,000 BTU window AC unit might blow air at 280 cubic feet per minute (CFM) or 16,800 CF/hour. Dividing 16,800 CFH by the 4500 ft[sup]3[/sup] apartment volume, we get about 3.75 air changes per hour.

I believe Joey P has a lot of practical experience with this sort of thing, so I yield to his expertise. But this covers some of the theory.

6

Wow! Thank you! Exactly what I was looking for.

I’m surprised at the number of times the air is exchanged. I thought it would take several hours at least to have a near complete exchange.

7

You could make it more efficient by adding a fan in an opposite corner.

Most cars with A?C have a ‘recycle’ option, which stops the unit from drawing air in and makes it recycle the air already in the car. This means that it cools down much faster since it is cooling already cooled air instead of starting from ambient.

8

Air changes per hour on a window unit is only the volume of air that passes through the unit. It’s going from inside the room, through the unit, and back into the room. So while the volume of air passing through it may be enough for, say, four air changes per hour, that doesn’t mean it’s fresh air, it’s just recirculated air. It’s a calculation to use to base cooling capacity on, not fresh air volume. As others have said, the amount of fresh air brought in from outside is essentially zero, unless the vent is open, and then it’s some very small number greater than zero. In practice window a/c units have pretty lousy seals around them and even internally, so there’s going to be some air exchange between inside and outside, but it’s not much. That’s why the smells don’t dissipate.

9

Of course if you move one room volume, you haven’t actually moved all the air through the unit, because the air mixes. So even if you were actually exchanging air, and using nitrogen-15 to distinguish new air from old, you’d still see the labeled old air after many exchanges.

10

To answer what you asked:
Cooking smells is a VERY BAD measure for air movement.

Go to an Indian restaurant’s kitchen. One that has been CLOSED and abandoned for 5 years. You can still smell the spices. Sooo, does that mean the air in that kitchen has not cycled once in 5 years?
Nope.
Cooking smells reside mostly in aromatic oils, which settle on walls, floor, and ceiling.
To answer what you intended to ask: How much air does a wallmount 12000BTU aircon move?
It depends.
If your aircon is in cooling mode(not fan only) then it moves as much or as little air as is needed to achieve the requested temperature while staying within humidity limits. This can result in WILDLY differing airflow rates, depending on outside temp & humidity, and requested inside temp.

If the aircon is correctly sized and not straining, it should move about 1 cubic foot per square foot of floor, resulting in volume equal to whole apartment in under 10 minutes.

Assuming your aircon is really struggling, it will still be moving better than 150 cubic feet per minute or higher. This is equal to the volume of air in your 400 square foot apartment every 21 minutes!
You problem is not in air movement, but in the stickyness of normal cooking smells.

11

But, does it? Does ALL the air EVER get completely cycled? If only some % of the total cycles through in a given amount of time then isn’t there always some molecules remaining that never get cycled? Seriously.

12

Possibly, but, so what?

13

Some molecules never get cycled. If the air in the room is constantly being perfectly mixed, and the AC is processing air without replacing any through an indoor-outdoor vent, then the time to process a roomful of air is the room volume divided by the AC flow. During this time, the fraction of original air that has not gone through the AC will be 1/e, where e is a constant equal to about 2.718281828. That fraction is also 1/exp(1) because exp(1) = e.

You can wait for twenty exchanges, that is, the time to process a roomful, multiplied by 20. The fraction of air remaining, that never got processed, will be 1/exp(20) or 0.000000002061154. For a room the size of the one I’m in right now, that’d be about 20,000 air molecules.

14

FWIW, if you make it far enough along in college math to get to Differential Equations, you’ll spend an entire semester working out the answer to that question.
It’s nothing philosophical, there’s some pretty straight forward equations to work it out.
It doesn’t bother me in the least that at this point in my life, I wouldn’t even recognize them if you put them in front of me (and my degree is in math).

15

It’s been a while, but, didn’t we do that in just the first class?

16

I do remember it was pretty early on in the semester, but I don’t remember many specifics. I took that class 20ish years go. I remember thinking mixing equations were kinda fun to work with. Beyond that, it’s all just a blur. IIRC, that class was lots and lots and lots of arithmetic. Like single problems taking 2+ pages to work out. That class is when I started putting a line through by Zs. Getting a wrong answer because you turned a Z into a 2 got old in a hurry.

In any case, my point was that how much air in a room never gets cycled is a question for college sophomores, not the great minds of ancient Greece.

17

And when you go to Max AC the air quality in the car will begin to decrease. O2 levels will drop as the CO2 levels increase.

18

Ah different Qs. What I have forgotten.

19

No cars are sealed well enough for that to be the case.

20

Not modern cars. My Chief engineer and I were in his mini van traveling from one location to another. We had just purchased a meter that measures CO2 and CO levels in a building, checking for sick building syndrome. We were on the freeway in slow traffic. New meter you have to play with it. We were thinking how bad is the air in this traffic. The readings came close to the normal range, the green area. WE were kind of surprised. WE closed the windows and turned the AC to Max AC. The meter began to rise and in a few minutes it was in the yellow range for CO2. We turned off the Max AC and the meter began to return to the normal range. New cars are sealed tighter today.