help determining size of a ptac HVAC unit

[Jbridge337]

Hi, I am trying to get a general idea of what size unit I will need for a 20x30 outbuilding located near Atlanta. I realize that there are typically specifics needed (ie. window surface area etc.) I can provide more information if needed if someone has the time and knowledge to help with this. I believe either a 12,000 btu or 15,000 btu is what I would need but I'm trying to make a somewhat educated decision from this point. I knew I should have saved those textbooks from 25 years ago!

Thanks,

Jim

[dbhost]

I can tell you what I am doing, or planning on doing, and how well it works on the Texas Gulf Coast, probably a little tiny bit hotter and more humid than Atlanta... But probably not by much...

Space to heat / cool. 2 car garage. Dimensions 18 feet square, 8 foot 6 ceiling height. 324 square feet, or 2754 cubic feet.

Garage doors double insulated with 2 layers of R-5 foam board, 1" pipe wrap duct taped to the joints to prevent drafts. Rubber gasket between doors and frame. Door that remains partially opened for mini wall with A/C unit has sides insulated with "insulation snakes" made from pink fiberglass baffles and sewn into old sheet material. Top segment gap insulated with built to fit giant snake. (DONE). Mini Wall insulated with R13 pink fiberglass (DONE)
2x4 stud construction walls insulated with R13 pink fiberglass and finished with Pegboard instead of sheet rock. 1x2 trim where tape joints would be on sheet rock.
Ceiling. Decking removed, Stacked 2x6 construction (no joke, my ceiling joists combine to be 2x12"!, goofy wind storm requirement, but the walls are only 2x4!), Double layers of R19 pink fiberglass, decking reinstalled.

The sheetrock in my garage is trash, and I want to expand the decking in my attic, so this project will coincide with that project...

The heat gain was RADICALLY reduced when I installed the foam board and split pipe insulation on the garage door. It's not great, but it is manageable. With the mini wall and the snakes in place, it takes a 10K BTU Goldstar Window Unit, placed at floor height, 1 hour to cool down my shop to 75 degrees on a 94 degree Houston day...

Heating here isn't THAT much of a problem, but I do periodically need heat, for that I use a 1600watt Lakewood Oil Filled radiator. Not a good solution to your 20x30 space, but a couple of those with box fans moving the warmed air around would make it awfully comfortable at a low setting on the radiator...

I have a friend whos FIL has a shop your size, he has a 10K BTU window unit on each end of the shop, and the same exact radiator I have with an oscillating fan that moves the air, cooled, or heated around the shop. He has been running it that way for 10 years or so... His shop is always more comfortable than my office!

So long winded stupid answer, if you are going to use a PTAC unit, then maybe a Carrier 52M 15K BTU unit is enough, but you will heat / cool only one side of the shop, you will need SOMETHING to move your air around... You would be better off with a multi component approach...

[Jbridge337]

Thanks, I will be installing an air cleaner in the space which I think will help move the air fairly well. I hadn't looked at that aspect of the air cleaner, but seems like it would help.

Jim

[maxparot]

My shop is a bit larger at 800 sq ft with 10' ceilings. Mine is well insulated and fully sheetrocked and painted and has a insulated RV door. I have a 14400 btu window unit installed through the back wall with 2 52" ceiling fans to move the air around. I can maintain 75*F when it is over 110* outside but trying to cool the place down once it is hot takes hours. Based on my experience if you also have 10' ceilings the 15 kbtu would be about right but if your ceilings are lower then the 12 kbtu would be better.

[Jbridge337]

Thanks Max.

[dbhost]

jbridge,

Maxparot has the perspective of a similar sized shop, which is great. HOWEVER, he is coming from Arizona, where a muggy day is 30% humidity....

Georgia being a southern state, is going to get the hot, wet air off of the Gulf of Mexico, Ignore the advice about the 12K BTUs being enough. It takes some of that cooling energy to suck the moisture out of the air, so unless you are going to be using a dehumidifier as well... Upsize your cooling a bit from what the desert folks are doing...

Hey, if he WANTED to he could use a swamp cooler and be good with it...

Lord I miss living in Tucson...

[Jbridge337]

Thank you dbhost. Just a tad more humid here.

[LCHIEN]

There is a science to calculating cooling needs.
A building cooled to say 75 degrees is a leaky bucket - BTUs of temperature leak out all over and must be replaced by the A/C unit supply at least equal (to just maintain the temperature balance) and preferably 2x to be able to cool it down if it starts hot.

Each square foot of ceiling, wall and floor is evaluated - the heat flow out is proportional to the temperature difference (inside to outside) and the thermal resistance in R units. There are R-unit values for windows, insulation of given thickness, doors of different construction, concrete of given thickness, wall board (sheetrock), exterior paneling, tar paper etc. The resistance thru a particular square foot of wall can be calculated as the sum of the resistance of the layers, e.g. sheetrock, fiberglass batting between 2x4 studs, tarpaper and a brick wall. Even the number for a garage door with foam insulation blocks is probably known. The heat flow is the difference in temperature divided by the resistance.

For the walls you should use the max outside temperature (like 105 for Houston, a difference of 30F, for the ceiling the differnece should be based on the expected attic temperature, and for the floor, maybe you can use 75-80 degrees, normally the ground is cooler and you will lose little thermal energy thru there. If the room is over A/C space then use 75 and if the room is over an uninsulated garagethen you'll have to insulate the floor and use maybe 100F. (temp diff of around 30 degrees.)

Add up the heat flow for each square foot (or each type e.g. if you have 800 sq ft of wall then you can figure one foot and multiply by 800) and you will determine the minimum BTUs you need. You should double it if on a hottest day you want the A/C to run 50% of the time, a good figure, which will allow your A/C to cool down a hot room yet not be oversized. Also add more BTUs if you have a bit of heat producing machinery (electronics etc) and a lot of people.

Fortunately if you look on the internet there are some programs for sale to professional installers but they also have on-line demos you can use to calculate a single installation. They'll ask the dimensins of the wall and ceiling areas and the doors and windows and the location to determine the max temperatures. They'll also ask the heat load in watts for appliances and lights and you should add about 100 watts per person, too.

[maxparot]

jbridge,

Maxparot has the perspective of a similar sized shop, which is great. HOWEVER, he is coming from Arizona, where a muggy day is 30% humidity....

Georgia being a southern state, is going to get the hot, wet air off of the Gulf of Mexico, Ignore the advice about the 12K BTUs being enough. It takes some of that cooling energy to suck the moisture out of the air, so unless you are going to be using a dehumidifier as well... Upsize your cooling a bit from what the desert folks are doing...

Hey, if he WANTED to he could use a swamp cooler and be good with it...

Lord I miss living in Tucson...

Actually I have a swamp cooler also but it only works well when the dewpoint is low and the temperature is bellow 90*.

[Tom Slick]

This site might help

http://www.warmair.com/html/acc.shtml

[Jbridge337]

Thank you Loring and Tom - great info.

[dbhost]

Actually I have a swamp cooler also but it only works well when the dewpoint is low and the temperature is bellow 90*.

Must be the CAP canal making you guys humid these days. When I lived in Tucson the swamp coolers worked all the time! I had mine cranking away keeping me to a comfy 75 degrees on a 117 degree summer day in the early 90s!

[maxparot]

Must be the CAP canal making you guys humid these days. When I lived in Tucson the swamp coolers worked all the time! I had mine cranking away keeping me to a comfy 75 degrees on a 117 degree summer day in the early 90s!

You're forgetting about monsoon season or maybe it just did have as much effect on the Tucson area.
My swamp cooler is also a bit too small for my shop. The prior owner built my shop 2 years before I bought the property from him. He made 2 mistakes on what otherwise is a really well done building.
The first is the undersized swamp cooler.
Second is a central air filter built into the ceiling that vents outside instead on returning within and retaining the heat and/or cooling.

[dbhost]

I am not sure how monsoon impacts Phoenix / Tempe / Mesa etc... But in Tucson, it hit every afternoon at about 4:00, lasted for an hour and a half to screw up the drive home, and drove the humidity up to a whopping 30%. Swamp coolers still worked great!

That was many moons ago though...

If you like the outdoors, you live in some of the prettiest country on the continent in Arizona... And this is coming from a guy that grew up in Oregon... You want desert? You got it, you want green lush mountains? You got it, you want snow on those mountains in July, no problem! (Seriously, there is a reason the White Mountains are called that!)

[maxparot]

I am not sure how monsoon impacts Phoenix / Tempe / Mesa etc... But in Tucson, it hit every afternoon at about 4:00, lasted for an hour and a half to screw up the drive home, and drove the humidity up to a whopping 30%. Swamp coolers still worked great!

That was many moons ago though...

If you like the outdoors, you live in some of the prettiest country on the continent in Arizona... And this is coming from a guy that grew up in Oregon... You want desert? You got it, you want green lush mountains? You got it, you want snow on those mountains in July, no problem! (Seriously, there is a reason the White Mountains are called that!)

I have to agree it is a pretty part of the country in many ways. My wife and I were married in the red rock area of Sedona and have made many trips to the Grand Canyon.
We both grew up on Long Island in New York. New York also has some very beautiful areas. Upstate has Mountains, forests, rivers and lakes. Then there is the coastal areas, NYC and Long Island.