Mini-Split in Insulated Metal Building - Single or Dual Fan Unit?

[CV428]

I am working through insulating a 24x50x12, doing 1.5" polyiso w/ a 0.75" air gap and encapsulated closed cell bubble as an interior skin for now - debatably R28-32 range. All air gaps vent upward to the roof ridge vent behind the insulation for moisture control. I'm going to do the frameouts for either a single or a dual interior fan/evap unit. Calcs say 36k btu should be sufficient for ~58-60f in the winter and ~78-80f in the summer (just to take the edge off the extremes, Zone 8B here).

-Does anyone have a ~1200sqft garage with a single fan/evap mini-split unit?
-Does the single fan unit keep up decently? How much of a temp gradient do you see across the building?
-Do you use other fans to help circulate?
-Do you have the condenser unit mounted to the building frame and do the vibrations cause excessive noise?

 

[jmdirk]

I can provide some input, though not exactly the exact same case.

I just finished (yesterday) installing a 18k BTU on my 30 x 40, insulated with R20 faced insulation. I also have radiant in floor heat with a 18k BTU boiler, so the mini split is more to supplement heat when it is super cold out and to help maintain temps during peak electricity cost times. So I can't really comment on the temp gradient across the building. But regardless, I would do fans anyway, just to help with air circulation and take the edge off when it's warm, but not necessarily warm enough to need AC. I have two 60" fans.

The single 18k BTU mini split was able to raise the temp from 18.2C to 21C in about 2.5 hours. Outdoor temps went from -1C to -5.5C during that time. Radiant flooring didn't run during that time, but the slab was at temp so it would have been contributing to the heat.

I ground mounted the condenser on a stand. My building has columns spaced at 10' I didn't feel comfortable having it just screwed through the sheet metal. If I was standing on the other side of the wall from where the condenser is, I could just barely hear it running. Mounting it to the wall and the sound of vibrations will be highly dependent on the resonant frequency of the building and how and where the outdoor unit is mounting and whether or not you can isolate the mounting brackets from the building and/or the condenser from the brackets.

 

[ericm]

Not knowing what temps the OP might get kinda makes sizing recommendations useless.

I have three outside units mounted on walls. On the garage (2x6 studs) with a Daikin I can hear the outside unit spin up from inside. On my shop that's pole barn with insulation and interior metal liner panel, I can't hear the Mitsubishis running. I don't know if the difference is the outside units or the building construction.

 

[theoldwizard1]

Calling them a "fan unit" is not correct. The correct term would be "air handler".

2 air handlers at diagonally opposed corners, along with 2 exterior compressors, will give to the best balanced temperature. Single air handler per compressor is also more efficient. It will also be lower operating cost because during certain times of the year, you will only need to run one of them.

 

[TobeyA]

I'm in Houston. I have a 30x50x14. Spray foam insulation on the walls (average is about 3" thick) and batt-type insulation on the underside of the roof (open rafters; unknown R value). I have a 36k Btu Mr. Cool. One air handler more or less centered on the long wall near my lift with 2 wall-mounted fans at opposite corners for circulation. In the worst part of the summer, it struggles to keep it close to 80 in the heat of the day. The shop gets shade early and late, but full sun from 11-4 in August. But it keeps the humidity down, and all I have to do is step outside and back in to feel cool again. If I had to replace it, I don't think I'd go bigger. My memory is that there was a significant cost increase to go the next size larger. I don't bother using it to heat the shop in the winter. It doesn't stay cold long enough down here. I did go the first summer with the insulation and fans but no mini-split. It was miserable.

My unit is on a slab, not attached to the building. I never notice it running. And I don't notice a significant temperature gradient with with fans running.

 

[aggie113]

30x40x18 garage with 2" closed cell foam all around. Started with a 24k system on the back wall but it wouldn't be able to keep in summer once the garage door opened to move a car in. Combine that loss of cold air with a hot engine coming in (friends coming over) and it would be about 85 or so during a hot summer day. Added a 36k unit (maybe overkill) this fall and mounted it on the wall near where the two post lift is. Both units on the ground (plastic 'slab'). My house's minisplit is mounted on the wall and I can hear the vibrations when it's ramped up, kind of annoying.

 

[The Metric System]

-Does anyone have a ~1200sqft garage with a single fan/evap mini-split unit?

Yes, our primary heating/cooling source is a single 24,000 low-cost unit.

-Does the single fan unit keep up decently? How much of a temp gradient do you see across the building?

Yes, it maintains a comfortable garage temp in the building even in the extremes of local outdoor temperature. I would want more capacity for an HVAC system in a living space, but my expectations for a garage are flexible.

No significant temp gradient, we have the evaporator blowing along the long axis of the building.

Note that your climate and building construction may be very different from mine.

-Do you use other fans to help circulate?

No, just the minisplit indoor unit.

-Do you have the condenser unit mounted to the building frame and do the vibrations cause excessive noise?

No, we placed ours on the ground specifically to avoid this issue. A previous house had the condensing unit on one of the metal wall-mount brackets and the resulting vibrations were noticeable, even with various types of vibration-damping feet etc. IMO wall mounting is a last resort and they should be freestanding on the ground if possible.

 

[CV428]

Yes, our primary heating/cooling source is a single 24,000 low-cost unit.


Yes, it maintains a comfortable garage temp in the building even in the extremes of local outdoor temperature. I would want more capacity for an HVAC system in a living space, but my expectations for a garage are flexible.

No significant temp gradient, we have the evaporator blowing along the long axis of the building.

Note that your climate and building construction may be very different from mine.


No, just the minisplit indoor unit.


No, we placed ours on the ground specifically to avoid this issue. A previous house had the condensing unit on one of the metal wall-mount brackets and the resulting vibrations were noticeable, even with various types of vibration-damping feet etc. IMO wall mounting is a last resort and they should be freestanding on the ground if possible.

Great info, really appreciate it!!

 

[Jackfre]

TOW1 about covers it, but do look at price of individual units vs dual unit. Compare the efficiency vs cost. Yes you will need another circuit for two but otherwise, less piping and redundancy. “I’m in Zone 8B….” ZZZZZZ….I’m familiar with Manitoba or N Carolina as a location but get kinda Zoned out on 8B.

 

[dcg9381]

-Does anyone have a ~1200sqft garage with a single fan/evap mini-split unit?

2400 sqft, 2 x 24K BTU units.

In my case, I could buy two single 24k units for less than a 48k unit OR a dual head unit. You could do 2 x 12K units, 2 x 18k units... And just run one in mild temperatures.

-Does the single fan unit keep up decently? How much of a temp gradient do you see across the building?

There is a gradient, but my units are in the same "corner" of the building. Gradients can be decreased by installing a ceiling fan(s).

-Do you have the condenser unit mounted to the building frame and do the vibrations cause excessive noise?

I've done this both ways (frame/foundation), no real noise (the units are on rubber "feet").

 

[pcmeiners]

"single or a dual interior fan/evap unit" your referring to single or multi zone mini-splits.

Single zone mini-splits are much more efficient than multi zone mini-splits. (2) 18k units should be plenty as your insulation is better than mine.

As far as air circulation, I have a 1080 sft garage heated with a 12k Fujitsu low temp mini-split, no problem (-7 to 101 °F, it blows air across the garage 30 ft, it could easily go >45ft; I am in zone 6. Heed what the oldwizard1 stated. To add, with 2 single zone splits, if one go out the other can easily keep the garage reasonable warm/cool so an emergency heat situation ( getting a rip off service charge) does not happen. For zone 8 you will need little heat but more A/C so SEER is your important number. Wall mounted outside units are less desirable than placing units on a slab due to noise transfer, also mount off the slab (>)6" to keep dirt, leaves etc from getting in the unit.