Mini-Split Sizing for Florida beach garage

[Florida Pilot]

Trying to find recommendations for air conditioning my garage...I've been looking for a couple of months but can't seem to find anything concrete for my specific situation.

Garage is 22 x 23 x 8.5'
Walls are CMU (cinder block) and garage door is insulated and there is one side door but no windows.
Ceiling is insulated with fiberglass insulation.

I live about a quarter mile from the beach and its pretty humid most of the year with the summer temps in the high 90s. I desire to maintain 70-72 degrees when I'm in the garage and around 80 when I'm not.

Most generic calculators recommend 12-14k BTU. However a few I've found to address a taller than normal ceiling along with high humidity recommend an almost 50% increase in rated BTU leaving me either getting an 18k or 24k BTU system. I'm looking at the LG LS243HLV system.

With regard to mini splits I've read that they can "throttle" themselves to continually run enabling someone to over-size the unit without the downsides of over-sizing a traditional central AC system. I'm not an expert in anyway just looking for advice from an expert or someone who has air conditioned a garage under similar circumstances.

Thanks in advance.

Mike

 

[WorthFlorida]

Trying to cool a garage in FL you pretty much throw out all normal calculations. It really depends what your trying to achieve. Do you want a living environment, just want to keep the heat down, the humidity or storing a high price vehicle? Would you be parking a vehicle in it with a hot engine?

CBS walls offer hardly any R value, the attic space above is not insulated. The insulation on the garage door is really to minimize heat transfer into the space when the sun hits the metal door. 1” of foam insulation does have R value but it’s only about 20% of the total wall space. There is a huge amount of air leaks all around the door itself and you probably see light somewhere along the bottom seal since the concrete is never totally flat. What direction does the door face?

I live in Florida and had one home with the garage door facing west and it turned into a oven late in the day. My current home the garage door faces north. Stays cooler but no cooler than the outside ambient temp.

It’s a hard one to call for sizing. Trying to maintain 70-72 degrees with a lot of air leaks you’ll have a lot of condensation.

 

[Sponsor]

 

[Florida Pilot]

Thanks for the reply. I have a flight/racing simulator (sensitive electronics) that I use in there as well as work on various projects/models that are made of wood and sensitive to humidity as well as curing materials that are sensitive to temperature. I’ll probably keep a car in there when I’m not working in the garage but I’m ok with the temperature spike that would bring.

I’m looking to keep the humidity under control when I’m not in there and keep it cool when I am in the garage which is typically at night after the kids go down.

The door faces west but is shaded from some of the direct sunlight thanks to trees. I looked in the attic space and verified that it is not insulated but I could easily add some fiberglass rolls between the joists and seal up gaps I see with spray foam insulation.

 

[WorthFlorida]

I myself always wanted to add some AC to the garage just to knock the heat & humidity down while using it as a work shop but always imperative that the wife’s car had to be inside of it. She loves to put her vehicle in the garage with a hot engine and close the garage door immediately.

Without trying to calculate heat loads etc I would guest nothing larger than 12k. The longer it runs the more humidity can be removed. In Florida humidity control is more important than temperature. Close the garage door, turn off the lights and look for light around the door frame that would be air leaks. Make adjustments or adding weather stripping. This will help more with humidity control. You’re only dealing with about 450 sq ft. and one wall is an interior wall to the house so no heat gain there. 24 k unit you’ll have a cold damp garage. However, if the unit can be throttled back, as you mention, if oversizing becomes a problem sounds like a good option.

The humidity goal is not to be over 60%. Right now my home in the Orlando area the inside humidity is 57% and outside is 90%. Temp was set to 78 since I’m out of town so I lowered down to the usually summer setting of 77 degrees to drop the humidity. The Honeywell monitors their smart thermostats and when the humidity hits 60% they send you an email of high humidity. You can program the account for lower humidity but no higher than 60% and any temperature warnings. In the winter months the temp is usually set to 75 just for humidity control. Cooler outdoor temps the ac hardly runs, therefore, the humidity gets too high.

 

[Dana]

What WorthFlorida said- no more than 12,000 BTU/hr!

A ~500' uninsulated garage could have a cooling load of about 1-ton if it leaks a lot of air. With the ceiling is insulated it's probably less than 1-ton unless you have an unusual amount of west facing window on that garage door or a constant and substantial electric load such as a rack of computer servers in there. With no windows and a CMU wall your primary heat gain is from the hot roof deck, so insulating the ceiling is going to be worth it.

Sealing up the gaps to the ceiling would reduce stack-effect driven air infiltration, and is a critical first step to any insulation solution. Six to eight inches of blown cellulose (R22-R30) would be more effective than low density fiberglass batts. Fiberglass is somewhat translucent to radiated infra-red from the hot roof deck, cellulose is opaque. Cellulose is also far more air-retardent than fiberglass, pretty much eliminating convective heat transfer between the insulation layer and the hotter attic. Fiberglass would need a top side air barrier to hit similar levels of performance. Blown insulation raked flat also fills in all gaps & voids around framing, plumbing, electrical, whereas getting the perfect fit with batts requires a lot of trimming & sculpting with a batt knife. Box stores that carry cellulose usually throw in a day or two of "free" blower rental with a minimum purchase. If your garage isn't going to meet their minimums you might consider adding 3" overblow of cellulose on any fiberglass in the main attic of the house, which is enough to fix the air-retardency and IR translucency shortcoming of fiberglass.