What size mini split for garage?

[ps2cho]

Anyone able to give me advice on size of mini split for garage?

-Phoenix AZ so 110F summers
-Attached Garage is standard 2 car, about 20 x 15
-Northeast facing
-insulated walls, will be insulating the garage door this week.

Everything online is saying either 18k or 24k unit.

Downside to going 24k is both more up front expense and risk it’s oversized and then it’s harder for variable speed motor to run efficiently.

Any thoughts or advice?

 

[Jackfre]

I’ve found it better to slightly undersized mini-splits. Yes, the bigger models can modulate down to about what an average day is going to be, but better to let a unit run across its range than have it loaf along in the lower ranges. Kinda like driving your truck in granny all the time. I’d put in a 15 which is probably still to big and if you are hot some days look at adding insulation To bring the load back to the unit. Bigger ain’t better. It’s just bigger.

 

[bonneyman]

Any neighbors or friends locally have a similar situation? Nothing beats personal recommends and actual experience.
But I'd agree with the above suggestion.

Size the unit for normal operating conditions in your area. The weatherman usually post something like the 30-year average for your locale. Rather have the unit run decent on cycles to maintain temps rather than be oversized and excessively cycle. That'd play havoc with temps and unit longevity.

 

[WisJim]

It also depends on the quality of the insulation. And is the roof/ceiling also insulated? That can be more important than the walls.

 

[PoorUB]

20x15 two car garage? What two Smart cars?

Are you certain of the dimensions?

If you dimensions are correct, less than 12,000 should cool it easily, assuming typical construction and insulation.

Here in North Dakota, in 95F days I could cool three times that size with a 12,000 BTU window unit.

 

[ericm]

24k for a 20'x15' garage would be extreme overkill.

Loadcalc.net has Manual J and S calculators. Look at the Manual J one carefully and fill in every field that's applicable.

 

[ps2cho]

20x15 two car garage? What two Smart cars?

Are you certain of the dimensions?

If you dimensions are correct, less than 12,000 should cool it easily, assuming typical construction and insulation.

Here in North Dakota, in 95F days I could cool three times that size with a 12,000 BTU window unit.

Maybe I misremembered the width - there’s only about an extra 1-2ft of either side of the cars.

Sounds like the 18k unit would be the right balance of not too oversized but not too small to take forever to cool down.

Ceiling is insulated yes! Felt about 6-8” of insulation when I cut into it to look at wire feeding

 

[ericm]

A quick Manual J for 20x15 with 10' walls and one 120sq ft garage door, R11 walls and R24 ceiling, says 4.2k for heating and 3.8 for cooling. This is for Ft Lauderdale, your location and everything else may vary. Manual S on that says 6k for "emergency".

Maybe your insulation is not R11 and R25, or it's super leaky, or there's something major in the calculation that I missed, but unless I'm very wrong, 18k is way too big.

 

[Dig Doug]

your garage is probably more likely 20x20.
15 feet deep is to short & 15 ft wide is to narrow - couldn’t get out of the car if 2 cars are inside. A 2 door jeep is 14ft long and 6ft 2 inch wide

I have 2 friends in St George utah that have over size 3 car garages one has a 2 ton the other has a 18k. They both work great and over kill!
they are very active and do a lot in the garage

another buddy in Bullhead city has a boat deep 3 car w/ a 1 ton and it’s cool inside

a few factors to consider -

how often does the door open
wife going in out all day taking kids to school & sports or shopping

do you plan to work in the garage, tinker around, or car maintenance etc



if your just going to take some heat out of it a 12k would be fine.

Setting the temp at 75 /78 even 80 is comfortable when it’s 110 or 115 outside

 

[ps2cho]

your garage is probably more likely 20x20.
15 feet deep is to short & 15 ft wide is to narrow - couldn’t get out of the car if 2 cars are inside. A 2 door jeep is 14ft long and 6ft 2 inch wide

I have 2 friends in St George utah that have over size 3 car garages one has a 2 ton the other has a 18k. They both work great and over kill!
they are very active and do a lot in the garage

another buddy in Bullhead city has a boat deep 3 car w/ a 1 ton and it’s cool inside

a few factors to consider -

how often does the door open
wife going in out all day taking kids to school & sports or shopping

do you plan to work in the garage, tinker around, or car maintenance etc



if your just going to take some heat out of it a 12k would be fine.

Setting the temp at 75 /78 even 80 is comfortable when it’s 110 or 115 outside

Thanks -
This is the hard part as I found some Phoenix subreddit posts where they all are using 1.5-2ton units and nobody is saying a 1 ton unit…maybe their HVAC guys are just collecting a bigger check then!!

 

[ericm]

Oops I thought you were in Florida for some reason. I don't know what a standard 2 car garage is but for sure it's not 15x20. 20 deep is pretty shallow and 24 wide is about the minimum for a useful garage. Yours may be smaller. Measurements would help.

Assuming a 24x24x10 and one 16x8 garage door, R11 walls, R19 ceiling, somewhat leaky construction and door, I get a Manual S for Phoenix of 9k btus cooling, 11k heating and emergency 16k of heating. That's with the default target heating and cooling temps for inside a house of 68/75. For a shop I'm ok not heating it that much in winter or cooling it that much in the summer.

HVAC installers often oversize units.

 

[IRQVET]

20x15 two car garage? What two Smart cars?

Are you certain of the dimensions?

lol

I had the same though when I first read this . . .

But man what I’ve learned over time and geography, west coast two car garages are around 20x26x20. East coast (modern) two garage is lucky if they're 20x22x20. Hindsight- no room for a average work bench snugged along the side walls of the garage.

 

[shade]

I'm in Arizona.
I have 2 24k units.
So many people on this board will say not needed...
Sorry but go away.
My garage is 24x36 and very well insulated but in the summer, those units run full out.
I do have computer equipment so 48k of cooling is needed.
In your case there's no reason NOT to go 24k.
It'll ramp down just fine if cooling isn't needed and it's not hard on the unit.

 

[WildBill]

Any neighbors or friends locally have a similar situation? Nothing beats personal recommends and actual experience.
But I'd agree with the above suggestion.

Size the unit for normal operating conditions in your area. The weatherman usually post something like the 30-year average for your locale. Rather have the unit run decent on cycles to maintain temps rather than be oversized and excessively cycle. That'd play havoc with temps and unit longevity.

Thats not really a thing with mini-splits like it was with non-inverter/non-variable compressor old school setups. They will just happily run at idle until you open a door and let in that 110F hell wind people in Phoenix seem to enjoy, then going bigger you will have the cooling capacity to drop the temp back down much faster. Maybe something really overkill like a 36k would not drop low enough to run intelligently, but the difference between 18k and 24k would show up in recovery time, not normal temp maintaining. At least that's been my experience installing mini-splits in shops and garages. If it was the same size house without garage door size openings and hot cars or if OP never opens his garage door maybe 12k would be fine, but personally for a garage or shop I always go up a size or two. You can look up the minimum output on mini-splits to help decide. There is also generally not a huge difference in price between a 18k and a 24k with the same efficiency and other ratings, it was less than $250 difference between the 18k and 24k I looked at for my shop, and I am really glad I went with the 24k every time I have a door or two open for awhile in the summer.

 

[ps2cho]

I'm in Arizona.
I have 2 24k units.
So many people on this board will say not needed...
Sorry but go away.
My garage is 24x36 and very well insulated but in the summer, those units run full out.
I do have computer equipment so 48k of cooling is needed.
In your case there's no reason NOT to go 24k.
It'll ramp down just fine if cooling isn't needed and it's not hard on the unit.

I just remeasured and I am 22 x 20
Would I get away with 18k?
Appreciate you steering me away from 12k that is abundant here in this thread..

 

[PoorUB]

I just remeasured and I am 22 x 20
Would I get away with 18k?
Appreciate you steering me away from 12k that is abundant here in this thread..

A 18k should easily cool it, I would bet a 12k would easily do it.

Granted, I am in North Dakota and rarely soee 100F but we see mid 90s. I cool 900 sqft with 18k upu have about half of that, so if everything was equal, 9k would do it. 12k would be fine, 18k at the absolute largest. 24k is nuts!

 

[bonneyman]

Thats not really a thing with mini-splits like it was with non-inverter/non-variable compressor old school setups. They will just happily run at idle until you open a door and let in that 110F hell wind people in Phoenix seem to enjoy, then going bigger you will have the cooling capacity to drop the temp back down much faster. Maybe something really overkill like a 36k would not drop low enough to run intelligently, but the difference between 18k and 24k would show up in recovery time, not normal temp maintaining. At least that's been my experience installing mini-splits in shops and garages. If it was the same size house without garage door size openings and hot cars or if OP never opens his garage door maybe 12k would be fine, but personally for a garage or shop I always go up a size or two. You can look up the minimum output on mini-splits to help decide. There is also generally not a huge difference in price between a 18k and a 24k with the same efficiency and other ratings, it was less than $250 difference between the 18k and 24k I looked at for my shop, and I am really glad I went with the 24k every time I have a door or two open for awhile in the summer.

Yeah I have to admit operating in Phoenix is always full load! Missed that in the OP's post.
My SIL lives up there and the summers are brutal.

 

[BrandonV]

18k or 24k, for sure.

As someone else mentioned, the moment you park a hot car or open the garage door during the summer here, whatever unit you have will go into overdrive trying to recover.

 

[Scully1]

I lived in Chandler and had an 18k in my 20x20 marginally insulated south facing garage. During July & August, I’d kick it on Friday night and it would be 78* in the morning. Mid day it would creep up to 82* and then back down to 78* when the sun set.
Running it every weekend for my 3 day weekend in those months would add $20-25 to my SRP bill.
Sounds like your garage is better insulated than mine was. You should be good with a 18k unit, will just have to plan ahead during the hot months to knock down the heat soak in the garage.

 

[Notgrownup]

Eastern NC here so hot in the summer . 24x24 well insulated. 18k btu 18 SEER runs 24/7 leave it on 81 in the summer to keep the humidity down and 81 feels good when you walk in from 105. Even 95 degrees.

 

[ps2cho]

I lived in Chandler and had an 18k in my 20x20 marginally insulated south facing garage. During July & August, I’d kick it on Friday night and it would be 78* in the morning. Mid day it would creep up to 82* and then back down to 78* when the sun set.
Running it every weekend for my 3 day weekend in those months would add $20-25 to my SRP bill.
Sounds like your garage is better insulated than mine was. You should be good with a 18k unit, will just have to plan ahead during the hot months to knock down the heat soak in the garage.

Thanks that’s some actual hard numbers to work with. Sounds like 18k will do the job but 24k would cool quicker. Thanks all!

 

[dlwilson]

I'm in south Florida with a 960 sq ft wooden building, with foam on the underside of the roof. I have a 36k mini split with two air handlers. It's probably too big, but the insulation keeps the temp lower all the time, and when I turn on the AC the temp drops quickly to a comfortable temp.

 

[Modagger]

Anyone able to give me advice on size of mini split for garage?

-Phoenix AZ so 110F summers
-Attached Garage is standard 2 car, about 20 x 15
-Northeast facing
-insulated walls, will be insulating the garage door this week.

Everything online is saying either 18k or 24k unit.

Downside to going 24k is both more up front expense and risk it’s oversized and then it’s harder for variable speed motor to run efficiently.

Any thoughts or advice?

After 24 years with a wood stove in my 20x20 wood/metal/car shop, I cried uncle one freezing January morning while waiting for the stove to get the shop up to working temp. Had an 18000 btu Daikin, (it’s my understanding that they invented the mini split), installed.

I have it set to come on at 6:00 am at 60 degrees and when I go out to the shop, the first thing I say when I step inside is, “ahh yes, good decision”.

I probably won’t use the a/c much because I enjoy having the doors open when the weather turns nice here in New England. It’s a decision that I haven’t once second guessed.

 

[maplewood]

In Phoenix with a small 3 car garage with very little insulation, 24k unit works great. Only have it on when needed, but if left on it'll eventually cool the slab and things will stay nice.

IMO you can't get advice for AC from non-Phoenix locations. Our home HVAC is "oversized" compared to online recommendations and Manual J calcs, but works perfectly. If we'd have gone smaller it'd be struggling over the summer, at best.

 

[u3b3rg33k]

it's really hard to oversize single-headed minisplits. they can easily run at 25% rated capacity, so 24k can turn-down to 6k.
Also you'll find that many 18k units are just 24k units, software limited to 18k.

any time a minisplit is running under rated capacity, it's efficiency goes up, because the condenser effectively becomes oversized.

 

[ps2cho]

Decided on the Pioneer 24k Mini split

Pioneer Low-Ambient 24,000 BTU 2-Ton 20 SEER Wi-Fi Ductless Mini Split Inverter and Air Conditioner with Heat Pump 208/230-Volt WYT024GLFI22RL - The Home Depot

Keep cold air, birds and rodents from entering your home by using this Pioneer LowAmbient WiFi Ductless Mini Split Inverter and Air Conditioner with Heat Pump.

www.homedepot.com

It went on flash sale for $1350 2 days ago and its already ended, glad I snagged it!
Plus 30% off for tax credit $945 total!

Concrete pad laid down - working on the electrical and fishing the wire needed...