1-2 Zone or 2-1 Zone Mini-Splits?

[Jim Becker]

12,000 BTUs per ton ... I thought the terms were interchangeable.

They may be, but Mini Splits seem to be marketed by BTU. 'Just an observation, that's all.

[Jim Becker]

Two single zone units seem the way to go. I haven't decided yet whether 2 ton or 3 ton mini-splits would be best. The shop is 30' wide by 60' long but over 20' high at its highest point. I would like to equally space the units along the 60' length at about 8' high. In other words the units would be about 15' from the ends with a 30' space between. See the first photo below showing the wall that they would be placed on. The second photo shows the mezzanine beyond and the high exposed ceiling. And yes, the shop will eventually be insulated, probably with fiberglass batts.
Attachment 496872Attachment 496873

I can tell you that in my 24x36x10 shop with open rafters (spray foam envelope), the 24K BTU unit I have is fine...except...when things get down in the lower temperature range for heating. There are two reasons for this...the conditioned space is actually larger than would typically be the case for an 860 sq foot space due to the height of the space (so it pushes the limits of a 24K system and the fact that I installed the single indoor unit on a short wall at the back which makes for a little bit of an air distribution mis-match. I likely would have been better served with two separate units at either end for performance as there is a 1-3 degree difference back to front. I'm not unhappy with how I did things because it fit the budget for sure, but I could be "happier" if I had been able to implement things a little better.

So with your height, I think you do need to do the 36K BTU systems to best handle the extremes. And unlike with traditional heating systems, you will not suffer from "oversize" when things are milder because of the intelligence and variable-speed compressor functions that come with a modern Mini Split.

[Jim Becker]

At least on the AC side if your unit is too big it won't run long enough to dehumidify the air, resulting a cold, clammy, rust-inducing environment.

True for traditional HVAC heat pumps, but Mini Splits are a much more intelligent animal with variable speed compressors.

[Steve Mathews]

I can tell you that in my 24x36x10 shop with open rafters (spray foam envelope), the 24K BTU unit I have is fine...except...when things get down in the lower temperature range for heating. There are two reasons for this...the conditioned space is actually larger than would typically be the case for an 860 sq foot space due to the height of the space (so it pushes the limits of a 24K system and the fact that I installed the single indoor unit on a short wall at the back which makes for a little bit of an air distribution mis-match. I likely would have been better served with two separate units at either end for performance as there is a 1-3 degree difference back to front. I'm not unhappy with how I did things because it fit the budget for sure, but I could be "happier" if I had been able to implement things a little better.

So with your height, I think you do need to do the 36K BTU systems to best handle the extremes. And unlike with traditional heating systems, you will not suffer from "oversize" when things are milder because of the intelligence and variable-speed compressor functions that come with a modern Mini Split.

Thanks for the vote of confidence. The 2-36K BTU systems wasn't entirely my idea. My situation is somewhat similar to Keith Rucker's shop except for the climate. He's in southern Georgia. The video of his installation is linked below for anyone interested.

[Jim Becker]

Yea, while 24K BTU is generally acceptable up to 1000 sq ft, that's likely with the assumption of a "normal" height ceiling which neither you nor I have. Mini splits definitely "outperform" expectations, but they are not total magic and there is going to be a limit to the amount of extra variance they can handle. In my case, 24K BTU works ok but would likely be better as a 36K BTU system or a combination of a 12K BTU and an 18K BTU. You know what they say about hindsight....

One other observation...the challenge is likely greater in the heating season because heat rises, making it important to be able to "overachieve" to force the "level" of the warm air lower and lower. In the cooling season, the cooler air is going to sink to the bottom. That's what creates the challenge with a big, open, tall space. Next "winter", I may experiment a little with trying some additional air movement with a fan to hopefully even things out a little if I can come up with an electrically efficient way to do that.

[Steve Mathews]

...One other observation...the challenge is likely greater in the heating season because heat rises, making it important to be able to "overachieve" to force the "level" of the warm air lower and lower. In the cooling season, the cooler air is going to sink to the bottom. That's what creates the challenge with a big, open, tall space. Next "winter", I may experiment a little with trying some additional air movement with a fan to hopefully even things out a little if I can come up with an electrically efficient way to do that.

Good point Jim. There is another YouTube video where a guy combined a gas fired shop heater to supplement his mini-split on really cold days. I plan to give that concept more thought. Our 2 story house has 3 HVAC systems, one for the main downstairs, one for the upstairs and another for an area off to the side of the house. In the winter we're heating the downstairs only and the reverse during the summer. On really cold days the downstairs HVAC system really struggles to maintain the thermostat setting. I think the entire system was incorrectly sized to cover only the area that it served, meaning the downstairs system for the downstairs only, etc. It should've been sized to cover the entire envelope IMHO.

[Jack Frederick]

Roger makes what to me is the deciding point. The Multics typically do not operate down to the low temps available on the singles. Check the specs on that as my info may be somewhat dated but a few years ago that was the case. In the cooling mode the air louvers are set to blow air across the ceiling where it will settle into the space. In heat mode, those same louvers tilt almost straight down to get the heat to the floor where it will spread as it rises. Putting the jointer or TS or other large gear in front of the unit can really stall things. Prior to going with a gas supplement I’d see how the MS did for at least a couple months of the heating season. If you do go gas I’d suggest a Rinnai DV wall furnace. I have the 38 model in my 1000 sq ft shop and it walks away with the job. I put that one in as it was a pro type from Rinnai at the time. Prior to that I used the EX-22 and it did the job as well. Rinnai’s are pricey, but the vbent is included and you can install the unit yourself in about 2 hrs.

[Jim Becker]

Gas supplemental is not a practical solution for me. No gas and I'm not affording another propane installation beyond what I had to do for the generator. It would actually be less expensive to add another small mini-split!

[Derek Meyer]

I got a quote for a Fujitsu 18,000 BTU system to be installed in my garage. Head unit on exterior wall, exterior unit directly behind it on the ground. Maybe 10 feet of line total. Electrical availabe from the panel 20 feet away. The unit is a low temp (can heat with an outdoor temp down to -15F), inverter style rated at 20 SEER. They quoted $7550 for the system installed. I thought that was kind of high but it is in line with quotes from other installers in this area.

[Tom M King]

https://www.ecomfort.com/Fujitsu-18L...MaAk7aEALw_wcB

[Carl Beckett]

The single biggest determinant of heating/cooling size needed is INSULATION. There used to be a free online tool to figure out detailed heat loads - called slantfin

Having said that if your space is similar to 'others' in how it is built (size, insulation, number of windows, outside temp ranges) then sizing can be generalized with rule of thumb.

[Steve Mathews]

While searching for 36k BTU mini-splits I was able to find Mitsubishi Mirage complete units on sale for a little less than $1900 ea. They are 16 SEER and come precharged. Is this one of those situations where you get what you pay for or is this a good deal. At this price I may just go ahead and do it myself.

[Jim Becker]

At 16 SEER, they sound like they may be older models but at that price for your workshop, totally worthy of serious consideration.

[Steve Mathews]

I just received the last of 4 quotes from contractors to install 2-36k BTU mini-splits. They are $5600, $11k, $18k and lastly $28k. I think the cheapest quote was from a non-licensed contractor and is the one that offers the Mitsubishi Mirage units for $1800 ea. as a DIY project. MrCool 36k BTU units seem easily obtained online for around $2800 ea. Very interesting.

[Jim Becker]

That's quite the range of costs!!!!