Sizing window AC BTUs for a small shop

[goofiefoot]

For those following, I'm in the process of building a 10x20 Tiny Workshop in my backyard. I live on the south Texas coast, so heat and humidity are pretty much a constant. In designing my shop, I elected to have a window-style AC unit, rather than a mini-split for ease and cost of replacement - the salt air down here is hard on ACs and they don't last very long. I'd rather replace a $300-400 AC every 3-4 years than an $1800 system every 5-6 years.

The shop is 200 sqft with vaulted ceilings (about 11ft at the interior peak). I will be insulating the walls and roof area, and have installed radiant barrier OSB as the roof sheathing. There are two small windows and a windowed french door. The building is fairly well shaded by the oak trees in our backyard. Plugging this all into a typical BTU calculator, I'm showing the need for around 10k BTUs - which is easily obtainable in a 120V window AC.

The question - given the factors of possible "extreme duty", should I oversize the BTUs for my situation? Would running a 12-14k BTU unit do anything more for me than the 10k, or would those simply work less and negate the dehumidifying in this space? I do plan to keep the unit running to maintain temperature and humidity.

Any thoughts are helpful!

 

[Jim greengo]

Being a little undersized on a hot day just means it will run a little more,beats short cycling.

 

[Bert_]

I started reading and thought 5-6000btu would be about right. Oversized ac won't pull enough humidity out of the air.

I realize you are in Texas, but, I run an 18,000 on the main floor of my house which is about 1k sqft with lots of windows. It's doing fine with a high of 97* today.

 

[goofiefoot]

This is the calculator I used to get to the 10k BTU number. With the high ceiling vault, I believe the cubic feet makes more difference than square feet in my case:

 

[PoorUB]

I believe it is oversized too. If it makes a difference I have worked in HVAC for twenty years.

Before I put a central air unit in my garage I cold cool it with a 12,000 BTU unit, 935 square feet, 9 foot ceilings. i can't believe the smallest window AC you can but will not do it easily, 6,000 BTU for example. You don't want it over sized. You will end up with cold damp and uncomfortable.

Your high ceiling still makes it a small space!

My garage is about 8300 cubic feet, your space is 2200, or roughly 1/4th the size. You could probably cool it with 3,000-4,000 BTU, but you can't find units that small.

I wouldn't buy anything larger than 6,000 BTU.

 

[Bert_]

This is the calculator I used to get to the 10k BTU number. With the high ceiling vault, I believe the cubic feet makes more difference than square feet in my case:

I'm going to say it, that calculator is wrong.

I used another calculator that actually has you input information about wall and ceiling construction / insulation not to mention windows and doors. It came up with 3700 btu for cooling.

The volume of the room really doesn't matter, it all about the area of walls and ceiling

 

[greenskeeper]

Constant use or only when you are out there? If constant, go slightly larger, if only when out there, get the largest unit available

 

[PoorUB]

Bad advice from Groundskeeper. This is only a 200 square foot building. Buy the smallest one you can find!
6,000 BTU will be too much!

 

[PoorUB]

I used another calculator that actually has you input information about wall and ceiling construction / insulation not to mention windows and doors. It came up with 3700 btu for cooling.

I agree with that number.

 

[greenskeeper]

Bad advice from Groundskeeper. This is only a 200 square foot building. Buy the smallest one you can find!
6,000 BTU will be too much!

Depends. The OP never answered whether they are constantly cooling the space or only when occupied. If the latter, a larger “oversized” unit would get the space down to temperature when occupied while a smaller unit may never get down to temp.

 

[goofiefoot]

Thanks for the replies! I would not have guessed such a small unit would be the best solution. Bummer that my initial calculator was so far off.

As PoorUB mentioned, I will have a hard time finding a unit under 6k BTU, so I'll target somewhere around that size.

greenskeeper, I do plan to have the unit on at all times to keep it as dehumidified as possible - probably keep it around 78-80° when I'm not in there and drop it down to a comfortable level when I'm in there working.

I'm finishing up the siding now, and will be focusing on getting the electrical run soon. I will have the AC on its own circuit, but with such a small unit, I won't have to really beef anything up.

 

[American Locomotive]

Something to maybe look into is the LG 9,500 BTU inverter unit. You could get the quick pull-down, but the inverter drive should allow it to modulate capacity and control humidity well.

 

[goofiefoot]

Something to maybe look into is the LG 9,500 BTU inverter unit. You could get the quick pull-down, but the inverter drive should allow it to modulate capacity and control humidity well.

Interesting. I am unfamiliar with an inverter AC. Looking at the specs, the 9500 model has a larger dehumidifying capacity than similar size non-inverter units, and supposably it's also quieter. Would this type of system make any difference in the oversizing discussion here? I see that 9500 is the smallest BTU they offer in the inverter models.

 

[PoorUB]

Depends. The OP never answered whether they are constantly cooling the space or only when occupied. If the latter, a larger “oversized” unit would get the space down to temperature when occupied while a smaller unit may never get down to temp.

It doesn't matter if he wants to run it all the time or not. He needs 4,000 BTU or maybe a bit less and the smallest he can find will be in the neighborhood of 6,000 BTU. the 6,000 BTU unit is 1-1/2 to 2 times bigger than he needs. I used to cool my 925 square foot garage with 12,000 BTU.

Plus your previous post said to get the largest available unit. Well, he can find a 12,000 BTU easy enough, and bigger if you look hard enough. A 12,000 is 3-4 times the size he needs.

Would you put 5 tons in a 600 square foot home?

A 6,000 BTU will cool it down fast enough.

 

[Jagmandave]

5K BTU units are available all over the place - Walmart even has them....that should cool it just fine especially if you set the temp and just leave it on.

 

[Showkey]

There are 5000 btu at the Corpus HD for $160.
Way more choices at 6000 btu.
No small inverter units.

 

[Terry D]

Depends. The OP never answered whether they are constantly cooling the space or only when occupied. If the latter, a larger “oversized” unit would get the space down to temperature when occupied while a smaller unit may never get down to temp.

It might get it down to temp, but it will not remove the humidity. It will not feel comfortable. A unit has to run for longer period to remove humidity

 

[greenskeeper]

It might get it down to temp, but it will not remove the humidity. It will not feel comfortable. A unit has to run for longer period to remove humidity

Hence the reason I asked if it was going to be run constantly or only when occupied. A shop/garage is not a house.

If only ran while occupied a larger unit will get the temperature down quicker, while a "normally sized" unit may never get to temperature before the shop is no longer occupied. Most shop activities also generate excessive heat unlike a house.

My detached garage has an oversized A/C unit, and an oversized forced air oil burner. I can get the garage to a comfortable temperature in under 30 minutes whether 10F or 90F outside. I do not run the A/C or heat unless I am out there.

Since I only run the A/C or heat when out in the garage working, I can't imagine installing a normally sized unit and be frustrated that it never gets to temperature while I'm out there.

 

[rjacobs]

I just put a 12k BTU Toshiba inverter stand alone(not a window unit) in my garage... It has a "dry" mode for dehumidification only. Do any of the small 6k btu window units include a dehumidification mode?

 

[Terry D]

Hence the reason I asked if it was going to be run constantly or only when occupied. A shop/garage is not a house.

If only ran while occupied a larger unit will get the temperature down quicker, while a "normally sized" unit may never get to temperature before the shop is no longer occupied. Most shop activities also generate excessive heat unlike a house.

My detached garage has an oversized A/C unit, and an oversized forced air oil burner. I can get the garage to a comfortable temperature in under 30 minutes whether 10F or 90F outside. I do not run the A/C or heat unless I am out there.

Since I only run the A/C or heat when out in the garage working, I can't imagine installing a normally sized unit and be frustrated that it never gets to temperature while I'm out there.

But wouldn't a normally sized unit be the correct size, if sized correctly.

 

[MDJAK]

Not derailing the OP, but quick question.

I started reading and thought 5-6000btu would be about right. Oversized ac won't pull enough humidity out of the air.

I realize you are in Texas, but, I run an 18,000 on the main floor of my house which is about 1k sqft with lots of windows. It's doing fine with a high of 97* today.

A friend of mine who is a builder once told me you can expect a 20 degree drop with central AC. So with 97 outside, are you able to cool down to 70 or lower if you wanted to?

 

[Bert_]

Hence the reason I asked if it was going to be run constantly or only when occupied. A shop/garage is not a house.

If only ran while occupied a larger unit will get the temperature down quicker, while a "normally sized" unit may never get to temperature before the shop is no longer occupied. Most shop activities also generate excessive heat unlike a house.

My detached garage has an oversized A/C unit, and an oversized forced air oil burner. I can get the garage to a comfortable temperature in under 30 minutes whether 10F or 90F outside. I do not run the A/C or heat unless I am out there.

Since I only run the A/C or heat when out in the garage working, I can't imagine installing a normally sized unit and be frustrated that it never gets to temperature while I'm out there.

Problem with too much oversized is that it might drop the temperature quick but it won't pull much humidity out. You end up needing to set the temperature even lower to be comfortable.

 

[Bert_]

Not derailing the OP, but quick question.

A friend of mine who is a builder once told me you can expect a 20 degree drop with central AC. So with 97 outside, are you able to cool down to 70 or lower if you wanted to?

The 20° is from return to supply. Not outside to inside.