The Metric System
Well-known member
- Joined
- Apr 28, 2025
- Messages
- 270
I thought my experience regarding minisplit selection/sizing might be helpful to others.
As always, your application may vary and the best reference is a detailed manual calculation for your own specific space but I still think it's good to get a point of comparison from a real-world application.
Location
Mountains of Western NC at about 2,100 ft. Geographically we're in the South but due to the altitude the climate is more midwestern. Think Cincinnati rather than Charlotte.
The space
A newly-built ~25'*55' stick-built garage that we added to our residence.
Very good. We have had it in place through worst-case cold and hot days and it maintains the space at a comfortable temp even in these edge cases. As this is a working space we typically set the t-stat less aggressively than we do in the house.
Conclusions
I did the thermal calcs myself and knowingly undersized the system vs the standard recommendations that you'd get from a minisplit manufacturer's sizing tool. This smaller unit reduced cost, installation burden, and the level of required utilities and space and still gives very satisfactory performance in a working (rather than living) space. I would not have cut the capacity so close in a residence, but it is fine in a garage where temperature requirements are loose and you don't have the added heat load from cooking/showering/etc.
As a back-up, I originally had provisions for a second identical system to be installed if we found ourselves to be short on capacity and even routed the electricity and lines in-place to make future install easy. I think it is unlikely we will ever need to do that.
As always, your application may vary and the best reference is a detailed manual calculation for your own specific space but I still think it's good to get a point of comparison from a real-world application.
Location
Mountains of Western NC at about 2,100 ft. Geographically we're in the South but due to the altitude the climate is more midwestern. Think Cincinnati rather than Charlotte.
The space
A newly-built ~25'*55' stick-built garage that we added to our residence.
- 2x 8'*20' insulated garage doors facing South
- 1x man door to the outdoors
- 1x man door to the living space
- 4x 72"*80" sliding patio doors used primarily as windows
- 1x ~3'*4' double-hung window
- Vaulted ceilings, 16' at the peak
- 2x ~2'*3' skylights on the South face
- Concrete slab, insulated with 2" of expanded polystyrene board underneath and turned down at the edges
- Provisions for radiant heat in the floor, not yet active
- Ceiling R-38 fiberglass insulation
- Walls R-19 fiberglass insulation
- Between a common wall and a loft area there is a total of about 600 square feet of insulated walls separating the garage and the living space. These were previously exterior walls but became interior walls when the addition was put on. This is an important factor, as the other side of these walls can reasonably be expected to be ~65-75F rather than whatever temperature it is outside.
- By code and by plan there is no air exchange between the living space and the garage beyond normal leakage and what we get from the man door opening.
- A no-name 24,000 BTU 21 SEER unit purchased from Costway in March of 2025 for $1,157 shipped.
- Installed by me; I work in an adjacent industry and already had the necessary tools/skills to form the pipes and pull a vacuum etc. Nothing special about the install except that I ran the refrigerant and water lines inside of the wall before drywall went on. It was a bit of a hassle to package the 5/8" suction line within the wall but not too bad.
- Indoor unit mounted on the center of a short-side wall about 8' off the ground.
- Outdoor unit mounted on the ground directly below the indoor unit. It faces West and has no discrete protection from the elements, just the structure of the building next to it.
Very good. We have had it in place through worst-case cold and hot days and it maintains the space at a comfortable temp even in these edge cases. As this is a working space we typically set the t-stat less aggressively than we do in the house.
- In Winter it maintained the space at a setpoint of 65F even when outdoor temps were about 10F. In this condition an interior temp of ~68F was the limit of the system's capacity.
- In Summer it maintains the space at a setpoint of 77F even when outdoor temps are about 95G with full sun on both the building and the outdoor unit. In this condition an interior temp of ~75F was the limit of the system's capacity.
- Experimenting with capacity limits I've found that with exterior temps between ~15F and ~85F the system can hit any plausible indoor temperature I ask for, it only falls off under what are (for this area) extreme conditions that only occur a few days per year.
- Dehumidification performance is excellent; humidity in the garage has stayed at about 50% throughout the Summer even as local RH is in the 80-90% range. There's a steady visible drip of water from the drain line, the grass in that area is growing faster as a result.
- Temp recovery has not been an issue; after opening a big garage door for ~30 minutes during worst-case weather the space recovers in about 15 minutes once the door is closed. I attribute a lot of this to the thermal mass of the concrete slab, even in its current passive configuration it is a very effective regulator.
- Even with the system blowing the length of the 55' room temperature distribution in the space is good, with (at most) 3 of variation across the length of the structure - about what you'd see from normal demand cycling. This is probably helped by the opposite end of the garage being tied into the house with a common wall, which reduces the thermal load on that side of the space and allows us to steal some heating/cooling from the living area.
Conclusions
I did the thermal calcs myself and knowingly undersized the system vs the standard recommendations that you'd get from a minisplit manufacturer's sizing tool. This smaller unit reduced cost, installation burden, and the level of required utilities and space and still gives very satisfactory performance in a working (rather than living) space. I would not have cut the capacity so close in a residence, but it is fine in a garage where temperature requirements are loose and you don't have the added heat load from cooking/showering/etc.
As a back-up, I originally had provisions for a second identical system to be installed if we found ourselves to be short on capacity and even routed the electricity and lines in-place to make future install easy. I think it is unlikely we will ever need to do that.
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