Shop AC Sizing

[sandman2234]

I would think that a 1.5 would be sufficient to keep it cool, but it is going to take time to bring it down if you only turn it on when you are planning to spend time out there. A larger unit will bring the temperature down faster, but even that has drawbacks.
Cooling the shop only when you are in it, will cause condensation issues that will have a lot of rust forming on things you never expected to rust.
If you are out there intermittently, a smaller unit running all the time would be better than a larger unit running just while you are out there. Plus a smaller unit running all the time would be cheaper than running a large unit, but another advantage of running a smaller unit is the humidity. It takes time to remove humidity, and running a unit that is smaller will cost less to run to remove the humidity, but takes longer to do it.
You have to decide if you are going to run the unit full time, or just when you want it to cool the shop while you are in it. If running full time, at elevated temps, such as 78 to 80 as a normal setting, and kicking it down while your out there, you can get away with a 1.5 but if you plan on cooling it from outside temp down to comfortable in a matter of a half hour(ish), then a larger unit is going to be needed.
Running any a/c unit while you are out there, with the air circulating around you, dispite it's temp, is going to feel better than stagnant air.
Another question, why a mini-split? Why not a regular unit? A typical heat pump lifespan is a lot longer than a mini split, from what I have seen.
David from jax

 

[dodge man]

I have a 28’x30’ building that’s insulated in the walls and use a portable air conditioner. It looks like a big dehumidifier and you just run an exhaust duct to a window. I’m like you and just want to take the edge off.

 

[Fixastuff]

my unit also has dehumidifier setting but I that’s just ac running at max. lol.

 

[Sigarms]

I am thinking about a mini split HVAC system, but when I run the size calculations it comes up with 24k units.

What software did you use for a manual J?

I'd ask what was your outside design temperature vs indoor temperature, but I'm guessing no clue?

FYI, SEER rating has absolutely very little to do with what you want to look for in a ductless mini split.

 

[Sigarms]

my unit also has dehumidifier setting but I that’s just ac running at max. lol.

Dehumidifier settings on a ductless mini split is just smarter people in marketing who don't know what they're doing using cool terms to help sell the equipment

 

[Sigarms]

I would think that a 1.5 would be sufficient to keep it cool, but it is going to take time to bring it down if you only turn it on when you are planning to spend time out there. A larger unit will bring the temperature down faster, but even that has drawbacks.

Honestly, not the way a ductless mini split is designed to work.

If you're planning on turning your ductless on and off when you're in the conditioned space, save yourself the money and don't even buy one.

 

[EddieWalker]

"The R-value of bubble insulation can vary depending on the type and thickness of the insulation. Here are some general guidelines:

• Single bubble insulation: The R-value of single bubble insulation is typically around 1.0 to 1.1, which is relatively low. This is because the bubbles are not as effective at trapping air as double bubble insulation.
• Double bubble insulation: The R-value of double bubble insulation is typically around 3.5 to 4.0, which is higher than single bubble insulation. This is because the double bubble design traps more air and provides better insulation."

I looked up the R-Value of bubble insulation to see what you are dealing with. As a Contractor, my opinion is that you will be wasting your money on trying to cool this building unless you get your R-Value up to a level that makes sense. In my opining, you might see some results with R7 or R8 on the walls, and R20 in the ceiling. This is way below Code, but just a minimum for a shed or shop to actually notice it.

An empty building without AC, but insulated to R13 in the walls, and R40 in the ceiling wont even need AC in my part of East Texas. The difference between outside temps and inside the insulated building is significant. I'm personally insulating my garage to R60 in the ceiling. I've done this for clients and been really impressed by the results.

Since you don't have a ceiling, or an easy way to install insulation, Closed Cell Foam is probably your best solution to get a decent R-Value inside your building. Never use Open Cell Foam directly against the metal of a building. It holds moisture, and over time, it will cause rust. Closed Cell has about an R7 value for every inch, but after 2 inches, the R-Value per inch decreases. Four inches would be the max you can use and be able to see results, but for most people, 2 inches is the limit, with just one inch being good enough. Closed Cell Foam also acts as a vapor/wind barrier, which makes the R-Value deceiving. Even though it's rated R7 per inch, the results will feel like double that.

 

[Sigarms]

I looked up the R-Value of bubble insulation to see what you are dealing with. As a Contractor, my opinion is that you will be wasting your money on trying to cool this building unless you get your R-Value up to a level that makes sense. In my opining, you might see some results with R7 or R8 on the walls, and R20 in the ceiling. This is way below Code, but just a minimum for a shed or shop to actually notice it.

Eddie, you know what you're doing in HVAC and you may not even know it. Spot on!

I came across it years ago for wrapping uninsulated ductwork. For that application, it's a gimmick where if you want to do it right, spend the money for insulated duct wrap.

 

[GSVette]

In my 40x60x14 foot open ceiling shop I have a 36k btu Mr Cool unit installed. With R19 in the ceiling and R21 in the walls it can generally keep the shop at a fairly comfortable 72-76F when outside air temperature is in the 90’s. Prior to ceiling insulation it was almost impossible to keep the temperature above 60F in the winter or below 80F in the summer.

 

[Sigarms]

Prior to ceiling insulation it was almost impossible to keep the temperature above 60F in the winter or below 80F in the summer.

Insulation above the conditioned structure is just as important, if not more important than insulation on the side of the unconditioned structure in determining heating and cooling loads.

Doing it right, you also have your indoor desired temps vs outdoor temps to help determine exactly what size system you need.

Long story short, the biggest impact on your load is how well the structure is insulated. ALWAYS spend money on good insulation to help reduce the load.