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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.

_

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.

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).

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  • 4
    Why not just use a timer to come on 2 hours before the time of use? and off after, that would change the running time to approx 4 hours per day instead of 24...
    – Solar Mike
    Dec 21, 2023 at 8:26
  • How about just pointing a fan at yourself when you start working out? The fan will provide some immediate cooling, and as you work out and get hotter, the AC is bringing down the temperature in the room.
    – barbecue
    Dec 21, 2023 at 18:28
  • An overpowered A/C can cause condensation problems because it didn't have enough time to dehumidify. If you know which hour you plan to use the gym then consider getting a WiFi or bluetooth controlled unit so that you can adjust the climate 1-2 hours before use.
    – MonkeyZeus
    Dec 21, 2023 at 18:58
  • Using a minisplit for on-demand cooling might not save as much as you expect. Not only do you need to cool the air but also remove the thermal mass from the structure which requires far more BTUs than air cooling. Then there is the constant temperature changes which are terrible for the structure, walls, paint, humidity, mold, etc. For a garage, 18K might maintain temperature if ran constantly. An 18K unit will take several hours or more to cool down from 90 degrees. Factors like insulation, windows, sun orientation require many more factors than you are considering. Dec 21, 2023 at 19:40
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    The arguments regarding only needing to cool the air are not valid. The walls, floor, ceiling, etc are heaters. You must calculate how many BTUs per hour those structures radiate. Then you must also factor heat transfer from outside to inside. In summary, an 18K unit is small for a typical garage and will probably severely underperform for you use case. Dec 21, 2023 at 19:56

6 Answers 6

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  • 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.

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    The biggest trick in low-thermal-mass surface coverings is not re-creating the Station Nightclub fire situation. Search carefully for low thermal mass and good fire performance, combined.
    – Ecnerwal
    Dec 21, 2023 at 14:02
  • @Ecnerwal +1 ; exposed polystyrene is a huge fire hazard. Good materials would be cork, fire rated carpet, non foam based soundproofing panels (hemp, rockwool, etc) for example. Or just waste the energy and heat/cool the drywall, but put linoleum or carpet on the floor if it's a slab. If it's wood then it's okay. Lino is much better for a garage though.
    – bobflux
    Dec 21, 2023 at 14:49
  • Thanks, this answer was helpful to understand what was wrong with my calculation and intuitions. Observing that the inside temp rises slowly on a hot day, I figured that I could ignore the effects of the environment over the time scale of minutes. I thought of the walls as a shield which blocks a high % of the incoming energy. I should think of the walls as a reservoir, not just a shield. If the AC blasts the room full of cool air, the walls could quickly return the air to a higher temp. On the bright side, I can worry less about briefly opening the door and letting cold air out.
    – zed
    Dec 21, 2023 at 21:24
  • @zed Exactly! Well if your walls are well insulated they do act as a shield too, in the summer that's not always ideal as it prevents the inside for cooling down during the night unless window are open.
    – bobflux
    Dec 21, 2023 at 21:40
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    Conclusion: garage is a poor place to have a gym
    – gerrit
    Dec 21, 2023 at 22:10
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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.

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    And yes, I know you mentioned a "drying AC" but that AC will dry the air in the room, not the air on the other side of the drywall.
    – Edwin Buck
    Dec 22, 2023 at 4:18
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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.

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  • Thanks for the thoughtful answer. One thing I wanted to clarify. My motivation to get a larger unit is not to save energy by reducing how long the unit takes to reach the set point. It’s to get more convenient on-demand usage of the gym. My schedule is unpredictable, and what I’m optimizing for is a quick response time between when I decide to work out and when the gym is at temp. Energy saving is part of my overall goal (not keeping the gym air conditioned 24/7), but I am not super concerned about the difference in energy use between the large and small unit.
    – zed
    Dec 21, 2023 at 6:48
  • I will definitely look more into dehumidification. I’m curious if I get the small 9k unit, the math says it would get to temp in 6 minutes, is that enough time to dehumidify? Maybe I should be looking for the units that dehumidify fastest instead of caring so much about BTU.
    – zed
    Dec 21, 2023 at 6:55
  • Dry mode is actually a bit of an energy hog, as it runs the AC at full power (most minisplits are variable-speed compressors that are more efficient at less than maximum load) and the fan slower. I ran a test run on my building with mini-splits in dry mode and in cool mode and the power consumption was considerably more in dry mode.
    – Ecnerwal
    Dec 21, 2023 at 13:59
  • Thanks for making the edit. I've deleted my comment, as you've effectively covered the issue. Unfortunately, I can't help with the downvote, as it's not mine and don't know what the objection is. I haven't upvoted, because I'm unsure about all parts of your answer. In general, human comfort in a space will be complex and depend on multiple factors (at least temperature, humidity, and air flow over the human(s)). For example, just a fan blowing air might result in the human feeling adequately cool through sweat evaporation, even though running the fan increases the heat-energy in the room.
    – Makyen
    Dec 21, 2023 at 19:26
  • I don't love my answer either. I was focusing on the core of 'will this work?' and sticking with 'yes but maybe it isn't smart' and expanding that. The nuance of human comfort could be a hundred questions here. I did fold some of my comments in to the answer as well, to expand it.
    – KMJ
    Dec 21, 2023 at 22:09
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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.

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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.

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  • My garage doesn’t have windows. If cut a hole for a window unit, it would be hanging outside in an inconvenient location. I’ve considered cutting hose holes for a portable AC, but ultimately a mini split seems like a more reasonable choice if I’m going to put that much trouble into installing it.
    – zed
    Dec 21, 2023 at 23:11
  • A dehumidifier would heat up the air, right? I wonder if an AC+dehumidifier would be more or less efficient than an AC on dry mode.
    – zed
    Dec 21, 2023 at 23:30
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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.

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  • What is the inside temperature after 30 to 60 minutes? What is the outside temperature and time of day? BTUs? More details are needed. Dec 22, 2023 at 2:24
  • Does your AC have an IR remote? If so there are ways to rig a control that way.
    – zed
    Dec 22, 2023 at 4:49
  • I don't track the temperature change to the minute, but in the summer the room is well into the mid eighties in the afternoon if it is sunny, especially if someone has left the drapes open by mistake. The room will get down into the 60's within an hour. The AC unit is 12000 BTU. We rarely open the sliders, so the room is pretty tight and it doesn't get as humid as our garage. I'll have to check if it has a remote.
    – bill333
    Dec 23, 2023 at 15:13

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