Can I use an overpowered mini split to rapidly cool a space?

Question

I have a garage/gym that I’d like to condition with a minisplit. Since I only use the gym 1 hour a day, I have the idea of cooling it on-demand instead of keeping it at temperature all the time. At the extreme, this could mean lowering the temperature from 90 to 64 degrees. Or in the winter, raising it from 50 to 62. I’d like this cooling to occur quickly so I can use the gym on-demand. Looking at the single-zone mini splits Mitsubishi sells, they range all the way from 9k to 42k BTU.

My questions:

1. Will this work? Is there some limit where a larger machine can’t cool down the air any faster? Some pitfall I am not thinking of?
2. How to compute the temperature drop time for a unit with x BTUs? Most calculations I find are about sizing an AC unit to maintain a temperature. I didn’t see anything about estimating how long it will take to reach a temperature. I want to make sure I understand the performance I’m going to get when I decide which machine to buy.

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Attempt to answer question #1 about pitfalls

When I looked up information about oversized AC units, I saw two issues mentioned:

• Short-cycling could lower the life of the unit. I am guessing this would be less important when the unit is only used an hour a day.
• Less dehumidification would occur with a shorter cooling cycle, so the air could be damp. The units have a dry mode, which I could use to mitigate this issue.

I know about the cost, physical size, and beefier circuit required by larger machines.

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Attempt to answer question #2 about computing temperature drop time

This is the formula I came to:

Edit: Don’t use this formula! It will make you buy an undersized unit!

( (Specific heat of air isobaric) * (density of air) * (volume of room) * (temperature drop) ) / (BTU/h of air conditioner) = time to reach temperature

Dropping values into Wolfram Alpha [1], I got:

( (1.006 kJ/(kg*K)) * (1.204 kg/m3) * (12 ft * 8 ft * 20 ft) * (26 degrees f) ) / (18000 btu/h) = 3 minutes

This formula ignores factors like solar heat gain or insulation, but my intuition says this may not affect the result much, because the temperature in that room does not rise meaningfully over 3 minutes.

If this is correct, the 18k BTU unit would be do a great job. However, I am not confident that I’m doing this calculation correctly. The result almost seems too good to be true - from my admittedly foggy memories with window/portable AC units, they did not cool down a hot bedroom room in a minute like this would seem to imply.

[1] Link. I clicked for Wolfram Alpha to interpret temperature as a temperature difference instead of an absolute temperature. The results seemed wrong if I left it as absolute temperature - it would give me a longer duration that was not sensitive to the temperature drop (56 minutes for 26 degree drop vs. 53 minutes for 1 degree drop).

Answer 1

• Time to cool the room

Heat capacity of air is negligible compared to the heat capacity of walls.

You say your room is 12 ft * 20 ft * 8 ft or 3.6mx6mx2.40m which means 67m² of walls and ceilings. Assuming 13mm drywall at 9.7kg/m², that's 656kg of plaster, with a heat capacity of 1kJ/°C/kg that's 656kJ/°C. So a heat pump with 3kW output will cool the drywall by 0.27°C/minute. That's without acccounting for the slab.

Of course, heat exchange between walls/ceiling/floor and air is neither instantaneous nor perfect, but you get the idea.

All the energy spent bringing the walls/ceiling/floor to temperature is lost when you leave the room.

High thermal mass is great in some circumstances, but not in your case. For rooms that are heated/cooled occasionally, you want low thermal mass, and especially low thermal effusivity. This means materials that have low thermal mass (quick to heat or cool) and low thermal conductivity.

If the garage has bare cinderblock it's even worse than drywall, because it has higher thermal mass and higher thermal conductivity.

In other words, if you put some carpet on the floor, maybe some of these kitsch and very flammable polystyrene tiles on the ceiling... it will have a lot more effect than a bigger aircon. It doesn't matter if the material is thin: the most important is that it acts as an insulator between air and walls/ceiling/floor. Carpet is fine, so are gym foam mats, curtains, cork, acoustic soundproofing panels, whatever. The most difficult part is finding something that isn't too ugly.

• Size of the unit and increased humidity due to oversizing

Modern heat pumps have variable speed and output power thanks to inverter technology, so oversizing is only a cost problem. Oversizing used to result in on/off cycling which is undesirable, but inverters will usually run without cycling from 25% power to 100% power (although some can go lower than others). In fact, with inverters, you have several options. In silent mode, COP will be about the same over the power range because the fans slow down at low power. In noisy mode, the fans spin faster, so COP will be better at low power due to lower deltaT.

If your aircon cycles, when it's off, it's not dehumidifying outside air coming in through the air system. But you're going to get an inverter so it won't cycle, and it's not a whole house system, so it's not handling the outside air intake. So this problem does not apply.

If you run it for just the time it takes to work out, it's not going to cycle anyway because it won't have finished bringing the walls to temperature when you end your workout...

So oversizing is not really an issue.

• Consider the use case

For working out, low humidity and airflow are a lot more important than temperature. It's possible to go full power on a bicycle outside in 30°C air temperature or more because the speed means there's enough dry wind to cool you very well. It's only during slow climbs that heat becomes a real problem, because there's not so much air cooling anymore!

So to work out, in the summer, indoors without airflow you do need quite chilly temperatures like your 64°F (18°C). But if the aircon dries the air and there is good airflow, either from the aircon itself or a separate fan, then you should feel comfortable at higher temperatures like 25°C, which is much easier and cheaper to obtain with a heat pump.

In the winter you'll have a humidity problem. If you do a good workout you'll sweat a lot, that will evaporate, and the room will turn into a cold damp sauna. As air humidity reaches 100% I bet you're going to turn off the heating and open the windows! All this water will also condense on the walls which are still cold due to intermittent occupation/heating, and you'll have mold everywhere. So in the winter, you'll use the heat pump to get a comfortable temperature, but you'll also probably need either a dehumidifier, or let in a substantial amount of dry outside air.

Portable aircon units suck as aircons, but if you put one in the room without connection to the outside, it makes a great dehumidifer. Also much cheaper than a dehumidifier, if you get it used.

Answer 2

My brother was in the trade, and the idea of over cooling a room is one that's not new. It has a lot of problems that come with the idea.

The main problem is humidity. By cooling something too quickly, it will be cold enough to create condensation on the surface that touches the warmer areas. If using drywall, that means water condensing on the "other" side of the drywall. In a room setup, that other side is the interior of the wall, where the water will wet your insulation.

While a little water once or twice doesn't matter, a little water with some frequency will eventually build up mold and rot within the wall. In severe cases you can eventually destroy your building, or at least make it very expensive to fix.

There is a reason that ACs are rated for a house / room. It's because they cannot cool the room down faster than a slow rate which prevents this kind of condensation.

Answer 3

This is veering fairly close to opinion, which we don't really trade in. Because of that I'm not going to make a super long answer.

Oversizing units is generally a bad idea because it does, as you mention, result in poor dehumidification. This results in poor comfort. Your plan to get around that by using the drying mode of a unit somewhat gets you around this issue. I still think it's a poor choice, and you're better off correctly or slightly oversizing the unit.

Why, you may ask?

Two reasons:

1. Comfort. Many (most?) people would prefer to be in a space with well controlled humidity and higher temperatures, vs one with poorly controlled humidity and lower temperatures. A correctly sized unit that you start on a schedule or via app an hour before you want the space to be cool and ready for use will do a much better job of making the room comfortable, and as a bonus you won't have to spend the energy to run it all the time in dehumidification mode.

2. I think your energy savings will be less than you might think. Running the smaller unit for longer should use a very similar amount of energy, as it's removing about the same amount of heat.

Don't compare to single-hose portable units. They suck. Literally. They remove the conditioned air they just created via their heat exhaust hose. A 12k BTU single-hose portable cools a lot like an 9k BTU window or a 6k BTU mini split, from a human comfort perspective. Unless you're sure you have used a dual-hose unit before, you've probably used a single-hose unit.

So in a strict sense, yes, you can totally throw a 20k BTU mini split in a small room and successfully use it to cool the space. So the answer to the question you asked is yes. You can use an oversized mini split to rapidly cool a space.

I still suggest you don't oversize. Properly size the unit. You'll enjoy the outcome more, and the economy difference is small.

If you really want totally 'on demand' use I would still size properly, then set it back when you're not in the space, instead of focusing on how quickly you can chill the space when starting from nothing. I.e. set the temperature to cool to, say, 80F instead of what you prefer. Adjust the setting when you want to use the space. That would keep it barely idling over with minimal energy usage when you're not in there, do a generally good job of keeping things dehumidified, and not cool when it is not needed. It would also offer pretty good comfort.

Dehumidification speed will be very similar between similarly physically sized units, assuming similar conditions. Any time you aim to quickly get to temp, you won't do a good job of dehumidifying, regardless of the size of the unit. Proper dehumidification takes the unit running for a while. The water vapor only falls out of the air so fast. Bigger units will possibly dehumidify quicker due to having more coil area, but they suffer in ability to properly control latent heat and to work well in the early and late part of the cooling season due to being oversized.

Answer 4

I'm not sure bobflux's answer is correct. We're not interested in cooling to thermal equilibrium: we want to cool the air in the garage to 18 C, and then maintain that temperature by balancing the rate at which heat enters the room from the walls with the rate of cooling input. What matters is that you feel the room is 18 C, not that you cool the garage in totality to 18 C. In that sense, this happens all the time with domestic air conditioners, like a NYC apartment with a window unit, since as soon as you turn the unit off the temperature rises again. I think this is a roundabout way of saying, just size the unit for the space and go workout.

Edit: To add more details, you would calculate the rate of heat transfer from AC to air, and room to air, then you would solve for the fixed rate of cooling from the AC that leads to a stable temperature at 18 C. In reality, the AC probably has a control system that varies the amount of cooling based upon distance from the set point. It will surely output at max BTU when you first turn it on, then as it approaches 18 C it will dial its BTU back to the point where the cooling it outputs is balanced by the room. So, in my opinion, the entire problem is taken care of by the AC, and under or oversizing is moot if you only intend to use it transiently. Sizing an AC is important for continuous operation.

Answer 5

Have you considered installing a standalone dehumidifier in the space? If it's a garage there's presumably a drain, so you won't have to keep emptying it by hand. It'll only run when it needs to, so it's not a constant energy drain. If you use weatherstripping etc to keep exterior air infiltration down, even better. Dry warm air feels much cooler than humid warm air, because sweat evaporates more rapidly. Add a fan pointed where you'd be exercising, and you may not need the AC at all. And if you install it, it'll work better.

If I were doing it, I'd also use a two hose unit and put it just behind the aforementioned fan. Then you're getting the cool air where you actually need it and whether or not it cools the rest of the room isn't really relevant. As an example, I have a ~400 sf rental unit that's all brick and plaster and fully exposed to sunlight on 3 sides. We were installing new flooring during a period where the heat index was well over 100 degrees. After one day of nearly dying while using a window unit to cool the place, I ordered a Wynter something or other portable AC (with two hoses), and it became quite comfortable within a minute or so of turning it on.

Answer 6

I have almost the exact situation you describe with the same 'can't set schedule' workout issue, although my space is a bit smaller (20x8 foot room with 7 foot ceilings). From what I've gleaned from the other answers, it's a terrible space in terms of quick cooling (it was an outdoor patio we enclosed on the open side with glass patio doors, so it has stone on 2 sides, glass on two sides, and slab on floor and ceiling.). Since the windows are on the west side with no shade, even with low E glass and thermal drapes, it can get really hot in the afternoons, even in winter (>85 degrees in summer and 80 in winter). I also like to work out at around 64 in summer and 62 in winter.

What works pretty well for me is:

1. constant low level dehumification, even in winter, since the room is at ground level. For the winter, I run two of those really small, 1 pint dehumidifiers that draw almost no energy. In the summer, I use a larger dehumidifier to keep humidity ~50% or less.
2. A portable AC unit. It's only a cheap 1 hose unit, because I could not find a 2 hose unit anywhere when I installed it.
3. A box fan.

30-60 min or so before working out (depending on how hot it is), I turn on the AC and the fan, which sits right behind the AC unit. By the time I start working out it is close to the right temperature, and with the colder air blowing toward me it 'feels' cool and dry enough to work out. This was relatively cheap and easy to set up, and while not perfect, works pretty well. There is never any condensation on the windows.

Unfortunately, on these digital control AC units, you can't use a timer to turn them on beforehand.