Mr. Cool sizing

[kaiser715]

I am thinking about a Mr. Cool DIY in one section of my shop. Overall, shop is 42x60. My welding area is 20'x24', one 12hx10w insulated garage door, and one 3-0 man door. Ceiling is 16-6 on the low side, and 20-8 on the high side.

All 4 walls are insulated (2 exterior, 2 partition walls from rest of shop.

So...only 480 sq feet...but with the tall ceiling, lots of volume (almost 9000cuft).

What size unit should I be looking at?

 

[DHCrocks]

i'm no expert on AC by a long shot, but from my experience up size the unit. I did an online manual J calc for my 400sf garage and it said 12k would be sufficient. From what I've read most guys recommend about 12-15k for a double garage. but I know that this is way too small for my application. I had a 12k portable unit and it didn't make a dent in cooling my garage. my garage door which is insulated faces the afternoon sun and the heat coming off the door is ridiculous. ambient outside air temp can be about 85 and the garage would heat up to high 90s more then 10 degrees higher then the outside air. in addition I have a stand up freezer and water heater in the garage. the contractor recommended a 24k unit and I explained the situation and how hot it gets and he said that a 30k would do the job. I had a 30k Mitsubishi Mr.Slim installed and couldn't be happier. the mini split can throttle down to 10k so over sizing is not an issue. I can get the humidity from mid 60s outside down to 40 inside. I have the unit set to maintain 79degrees constantly and when I'll go in the shop to do some work I'll drop it to 74 which is comfortable to me. if i'm getting sweaty I'll go down to 72.

My only recommendation for you is to go big get at least 1.5x what is recommended for the space. Mini splits are variable so oversizing is not a problem and humidity can be easily controlled. Even with my 30k unit after pulling in my car from a long drive and the heat coming off the engine it can take about an hour to get the temp down to a comfortable level. I don't know how much heat comes off a V8 but it must be incredible as it will instantly heat my garage I can see the temp rising in a few minutes.

in summary don't worry about oversizing a mini split, it can drop down to below 50% of its capacity so that's not a problem. You will appreciate the faster cooldown times with the larger system.

 

[soob]

Cubic feet is the wrong way to size air conditioners or heaters. The real measure is exterior (temperature) surface area. Like how a radiator cools better than a solid tube.

But, anyway, I agree -- oversize that sucker if the space is only going to be used intermittently. Traditional a/c sizing is based on the assumption that it's going to be cooled 24/7.

 

[PoorOwner]

i'm no expert on AC by a long shot, but from my experience up size the unit. I did an online manual J calc for my 400sf garage and it said 12k would be sufficient. From what I've read most guys recommend about 12-15k for a double garage. but I know that this is way too small for my application. I had a 12k portable unit and it didn't make a dent in cooling my garage. my garage door which is insulated faces the afternoon sun and the heat coming off the door is ridiculous. ambient outside air temp can be about 85 and the garage would heat up to high 90s more then 10 degrees higher then the outside air. in addition I have a stand up freezer and water heater in the garage. the contractor recommended a 24k unit and I explained the situation and how hot it gets and he said that a 30k would do the job. I had a 30k Mitsubishi Mr.Slim installed and couldn't be happier. the mini split can throttle down to 10k so over sizing is not an issue. I can get the humidity from mid 60s outside down to 40 inside. I have the unit set to maintain 79degrees constantly and when I'll go in the shop to do some work I'll drop it to 74 which is comfortable to me. if i'm getting sweaty I'll go down to 72.

My only recommendation for you is to go big get at least 1.5x what is recommended for the space. Mini splits are variable so oversizing is not a problem and humidity can be easily controlled. Even with my 30k unit after pulling in my car from a long drive and the heat coming off the engine it can take about an hour to get the temp down to a comfortable level. I don't know how much heat comes off a V8 but it must be incredible as it will instantly heat my garage I can see the temp rising in a few minutes.

in summary don't worry about oversizing a mini split, it can drop down to below 50% of its capacity so that's not a problem. You will appreciate the faster cooldown times with the larger system.

I think it depends on the heat gain of your particular garage facing and insulation of course. I have had a garage with door that faces west and there was alot of heat on the garage door, it acts like a 16x8 feet radiant heater. originally it was a wood garage door and not have much insulated value. If you don't go near the door it doesn't feel so bad, but it was always the AC fighting against the heat there with a 12000 BTU mini. It seems to hit some kind of stale condition for a while where it could not pull down more, until around 6PM.. usually that's time to finish up it would feel more comfortable. It also depends how much sun and heat is beating on the outdoor condenser.

If I set the mini to 68 the problem seems like before it reaches that the compressor starts to slow down to reach the set point so it won't over cool, but that is probably not what I want, I want 68 ASAP. I guess maybe that's what they have a turbo mode to override this logic.

Next garage faces north that it does alot better, most of the day the garage door is not in the sun. The side wall of the garage gets some morning sun. alot of times it is cooler than inside the house in the hottest days. None of the garage is insulated but they have drywall.. common practice in CA


I have heard, you don't really want to oversize too much, although the minis can throttle down, although the system slows down on a big unit and the moisture is on the evaporator and the fan blows the moisture back into the space. Ideally you want to keep the temperature controlled 24/7 and then every item in the garage is already cold enough so it does not have to work as hard to pull down. So still, if that's the way to use it if you don't go oversize it will be the most comfortable. Like if I had the mini on all day w(regular sized BTU) and by 6pm finally it is cool and dry. If I had turned it on a day before it would probably worked better.

 

[Razorsedge]

I ended up with a 36k btu in my 25x30x12 with 2 10wx10.5h insulated doors. I wouldn't want anything smaller. I don't run it 24x7, only when I plan on being out there. It works well. I originally planned on a 24k Mr Cool, but I'm glad I went bigger.

 

[eddieK]

i'm no expert on AC by a long shot, but from my experience up size the unit. I did an online manual J calc for my 400sf garage and it said 12k would be sufficient. From what I've read most guys recommend about 12-15k for a double garage. but I know that this is way too small for my application. I had a 12k portable unit and it didn't make a dent in cooling my garage. my garage door which is insulated faces the afternoon sun and the heat coming off the door is ridiculous. ambient outside air temp can be about 85 and the garage would heat up to high 90s more then 10 degrees higher then the outside air. in addition I have a stand up freezer and water heater in the garage. the contractor recommended a 24k unit and I explained the situation and how hot it gets and he said that a 30k would do the job. I had a 30k Mitsubishi Mr.Slim installed and couldn't be happier. the mini split can throttle down to 10k so over sizing is not an issue. I can get the humidity from mid 60s outside down to 40 inside. I have the unit set to maintain 79degrees constantly and when I'll go in the shop to do some work I'll drop it to 74 which is comfortable to me. if i'm getting sweaty I'll go down to 72.

My only recommendation for you is to go big get at least 1.5x what is recommended for the space. Mini splits are variable so oversizing is not a problem and humidity can be easily controlled. Even with my 30k unit after pulling in my car from a long drive and the heat coming off the engine it can take about an hour to get the temp down to a comfortable level. I don't know how much heat comes off a V8 but it must be incredible as it will instantly heat my garage I can see the temp rising in a few minutes.

in summary don't worry about oversizing a mini split, it can drop down to below 50% of its capacity so that's not a problem. You will appreciate the faster cooldown times with the larger system.

Over sizing A/C means less run time...and less humidity removal and more ice ups of evaps...

I am an A/C guy. Also remember that with any Mini split it is a disposable purchase...you can get some parts some of the time with the bigger companies...but with the smaller ones it is obsolete in short order and basically disposable. Take care of it.

 

[eddieK]

Cubic feet is the wrong way to size air conditioners or heaters. The real measure is exterior (temperature) surface area. Like how a radiator cools better than a solid tube.

But, anyway, I agree -- oversize that sucker if the space is only going to be used intermittently. Traditional a/c sizing is based on the assumption that it's going to be cooled 24/7.

Traditional A./C is sized by the common load factor and heat gain of the space in question and it is CUBIC because that is the total space to remove heat from. Nothing to do with 24/7 run times.

 

[soob]

When you're conditioning a space, you're not removing heat that appears magically out of the aether -- it's heat gain, like you said, which depends not on how much air you have in the building but on how much surface area of walls and ceilings you have. It's elementary geometry that a taller building has a better ratio of interior cubic space to walls, and thus has less heat gain compared to a building with a bigger footprint but lower walls.

Like take a 10x10x8 building, that's 800 cubic feet volume, it has 420 sq ft exposed to the outside through which temperatures exchange. 1.9 ratio.

But an 10x10x12 building has 1200 cubic feet volume -- 50% more, but only 580 sq ft exposed, only 38% more. 2.07 ratio -- a better ratio and thus a slower heat gain per cubic foot.

This kind of thing matters a lot when you're sizing heaters for greenhouses... maybe not as much for a/c since there's no trip point (freezing) where everyone dies.

But, at any rate, your sizing tables are based on the assumption that the space is going to be conditioned 24/7. They are calculated so the thing won't cycle too much over the whole time it's used. The result is that an a/c sized according to the household charts will take way too long to bring a shop that's 90 degrees to a comfortable temperature.

 

[eddieK]

When you're conditioning a space, you're not removing heat that appears magically out of the aether -- it's heat gain, like you said, which depends not on how much air you have in the building but on how much surface area of walls and ceilings you have. It's elementary geometry that a taller building has a better ratio of interior cubic space to walls, and thus has less heat gain compared to a building with a bigger footprint but lower walls.

Like take a 10x10x8 building, that's 800 cubic feet volume, it has 420 sq ft exposed to the outside through which temperatures exchange. 1.9 ratio.

But an 10x10x12 building has 1200 cubic feet volume -- 50% more, but only 580 sq ft exposed, only 38% more. 2.07 ratio -- a better ratio and thus a slower heat gain per cubic foot.

This kind of thing matters a lot when you're sizing heaters for greenhouses... maybe not as much for a/c since there's no trip point (freezing) where everyone dies.

But, at any rate, your sizing tables are based on the assumption that the space is going to be conditioned 24/7. They are calculated so the thing won't cycle too much over the whole time it's used. The result is that an a/c sized according to the household charts will take way too long to bring a shop that's 90 degrees to a comfortable temperature.

Conventional systems have air movement through the space. If you have a large cubic space and you have returns located high and supplies low...or vice versa you are removing heat from the entire space...and if you are not doing that the stratified air above the conditioned area helps lower the load. There are all of the other factors as well, insulation, fenestration, orientation etc...

Over sizing in heating or cooling creates several unsatisfactory issues, I've seen it often in the 40 years I've been doing this. Short cycling, icing, high humidity, vast temp changes during cycles...etc etc

 

[Fixin'Stuff]

Conventional systems have air movement through the space. If you have a large cubic space and you have returns located high and supplies low...or vice versa you are removing heat from the entire space...and if you are not doing that the stratified air above the conditioned area helps lower the load. There are all of the other factors as well, insulation, fenestration, orientation etc...

Over sizing in heating or cooling creates several unsatisfactory issues, I've seen it often in the 40 years I've been doing this. Short cycling, icing, high humidity, vast temp changes during cycles...etc etc

You're thinking of the "it's either ON or it's OFF" operation of a standard system. You can oversize a minisplit and still be comfy, thanks to its ability to throttle back the compressor via the inverter. The compressor will ramp up to full speed to bring a room down to temp (moving it's max BTU/HR out of the space), then slow down to a much lower BTU/HR speed to maintain the temp (and provide dehumidification) instead of just dropping out a contactor and powering down until the next call for cooling.

Having an oversized minisplit can be VERY helpful for those that don't want to cool a garage 24/7. The initial BTU load in a hot garage can be insanely high on a really hot day because everything in the garage has become heat soaked and will spew that stored heat into the air just as fast as the a/c is trying to cool the same air.