Help me choose a mini split

[stm317]

I've got a 32x48x10 pole barn that I'm mostly done framing, and ready to finish insulating, but figured I needed to plan out a mini split layout first.

Location: Central Indiana. Temps range from sub-zero to triple digits.

Building details: 32x48x10, but I have scissor trusses, so the peak of my ceiling is around 12'6". Walls have 1.5" rigid polyiso Foam + R19 fiberglass for something in the neighborhood of R27. Ceiling will be blown in cellulose targeting R45, but depth is limited near the eaves thanks to the trusses.

I don't need it to be perfectly 72F year round, but if it could be 60+ in the winter and 75ish or better in the summer that sounds pretty good to me.

Most of the sizing calculators seem to have me right on the edge of maxing out a 24k unit, so I'd like some input from you all regarding sizing. From there, is there a significant advantage to a multi zone unit if it's conditioning a single, large space? Or should I just stick with a single wall mount unit? Thanks in advance for the input!

 

[GRivera]

That's a large space - I can't see a 24K being enough to heat it. Mine is a 30x40x12' with scissor trusses, peak at about 14'. R19 walls, R38 ceiling. This is my experience so far with the 36K Mr. Cool: https://www.garagejournal.com/forum/showpost.php?p=8335238&postcount=43

Full thread: https://www.garagejournal.com/forum/showthread.php?t=432165

Mr. Cool 36K unit: https://iwae.com/shop/36k-btu-16-se...with-wifi-smart-kit-wall-mounted-ha18422.html

 

[Yankeefarmer]

I have a 36k Fujitsu multi-split conditioning our great room (36’x24’ with lots of glass) via a 24k head and our master bedroom via a 12k head. I also have a MRCOOL Advantage 18k Unit in my 28’x 28’ detached garage shop. I’m on the CT/MA border so dealing with similar temperature ranges as you. These are my observations and experiences that may help you:

The amount of windows through which the sun can heat your space has a significant effect. Even though our great room is very well insulated and tight, the 24k unit is maxed out cooling on plus 90 degree days. By comparison, my almost windowless shop is easily cooled by the 18k MRCOOL.

The 24k head in the house can really discharge the cold air- even on the hottest days, the cold air can be felt moving 30 feet away. I run the fan on Auto. The MRCOOL unit in my shop has a more anemic fan which, even on high, can’t be directly felt 20 feet away.

Our Fujitsu unit is capable of heating at temps below 0 F, while my MRCOOL unit does not generate enough heat to be useful if the outside temp is much below 32F.

Our Fujitsu unit was sized and installed by an HVAC company. I got 3 quotes- all offered similar sized units, but different configurations of single or multi-head units. I am glad I didn’t go with the guy who said 18k was big enough for our great room. Heating with it was not a consideration, as we heat the room 24/7 with a wood stove.

I sized and installed the MRCOOL. I recognized that I would need a larger unit for heating demands than for cooling demands, and decided to size it only for cooling rather than for heating capability. The shop has a propane furnace for heating. With outside temps above 35F, the MRCOOL can provide all the heat I need, so I’m happy with it.

In your situation, I’d want at least two heads for conditioned air distribution. Whether you do it it with multiple condensers or a multi-zone condenser is a choice my bidders didn’t agree on. Window area can be a big consideration, and higher grade equipment can make a difference as well.

 

[stm317]

That's a large space - I can't see a 24K being enough to heat it. Mine is a 30x40x12' with scissor trusses, peak at about 14'. R19 walls, R38 ceiling. This is my experience so far with the 36K Mr. Cool: https://www.garagejournal.com/forum/showpost.php?p=8335238&postcount=43

Full thread: https://www.garagejournal.com/forum/showthread.php?t=432165

Mr. Cool 36K unit: https://iwae.com/shop/36k-btu-16-se...with-wifi-smart-kit-wall-mounted-ha18422.html

I've been watching your install with great interest, but I guess from the title I thought you'd gone with a 24k. Didn't read carefully enough and see that you went with a 36k. We have similar buildings, but as you noted I've got about 1000 more cubic feet to condition (rough estimate). I should end up with a little more insulation than you, but my temps are a bit more extreme than what you see too. Anxious to hear more about how yours is working as you continue to use it!

I have a 36k Fujitsu multi-split conditioning our great room (36Â’x24Â’ with lots of glass) via a 24k head and our master bedroom via a 12k head. I also have a MRCOOL Advantage 18k Unit in my 28Â’x 28Â’ detached garage shop. IÂ’m on the CT/MA border so dealing with similar temperature ranges as you. These are my observations and experiences that may help you:

The amount of windows through which the sun can heat your space has a significant effect. Even though our great room is very well insulated and tight, the 24k unit is maxed out cooling on plus 90 degree days. By comparison, my almost windowless shop is easily cooled by the 18k MRCOOL.

The 24k head in the house can really discharge the cold air- even on the hottest days, the cold air can be felt moving 30 feet away. I run the fan on Auto. The MRCOOL unit in my shop has a more anemic fan which, even on high, canÂ’t be directly felt 20 feet away.

Our Fujitsu unit is capable of heating at temps below 0 F, while my MRCOOL unit does not generate enough heat to be useful if the outside temp is much below 32F.

Our Fujitsu unit was sized and installed by an HVAC company. I got 3 quotes- all offered similar sized units, but different configurations of single or multi-head units. I am glad I didnÂ’t go with the guy who said 18k was big enough for our great room. Heating with it was not a consideration, as we heat the room 24/7 with a wood stove.

I sized and installed the MRCOOL. I recognized that I would need a larger unit for heating demands than for cooling demands, and decided to size it only for cooling rather than for heating capability. The shop has a propane furnace for heating. With outside temps above 35F, the MRCOOL can provide all the heat I need, so IÂ’m happy with it.

In your situation, IÂ’d want at least two heads for conditioned air distribution. Whether you do it it with multiple condensers or a multi-zone condenser is a choice my bidders didnÂ’t agree on. Window area can be a big consideration, and higher grade equipment can make a difference as well.

I appreciate the first hand info, thanks! The shop has two windows about 32x38, and half of the man door is glass as well. But all of them are under a porch with significant overhangs so no direct sunlight comes into the building. So I'm not expecting significant solar gain/loss from those, just a bit less insulation in those spots.

I'd really like for the mini split to be my only source of heating and cooling, so my calcs have to focus on heating loads. I don't need something that can blast heat in sub zero temps, but keeping the place above freezing in those somewhat rare conditions, and 60F or better the rest of the time would be great. Cooling and dehumidification seems like an easier task, and is just gravy for me.

 

[GRivera]

I've been watching your install with great interest, but I guess from the title I thought you'd gone with a 24k. Didn't read carefully enough and see that you went with a 36k.

I thought about starting a new thread to avoid confusion but neglected to do so. I look forward to your decision and install!

 

[dcg9381]

Most of the sizing calculators seem to have me right on the edge of maxing out a 24k unit, so I'd like some input from you all regarding sizing. From there, is there a significant advantage to a multi zone unit if it's conditioning a single, large space? Or should I just stick with a single wall mount unit? Thanks in advance for the input!

Your insulation is good, best to take advice from people in your area.
I found that 2 x 24K units were more cost effective than a single 36K unit, so that's what I went with.. Like you, I was a little "experimental" on the sizing, but as I could run one unit or two, it gives me options. Indiana summers generally aren't that extreme (compared to Texas) - but there may be cases when you have to heat/cool on the extremes. Right now, it's 40 degrees, my insulation is only R15 or so, and my space is 2400 sqft x 16' walls - I'm running one unit on heat mode. That's good enough to bring the shop up to the 60s or so... In summer, when it's 98+ out, I run both units.

I'd be more wary of cool - heat pumps generally lose performance (BTUs) as temperatures drop to and below zero. Modern units can be very efficient to very cold temperatures, but they all drop capacity to some degree.

Multi-zone - I didn't do this. Expensive, complicated, and perhaps less efficient. Instead, again, I used two units. One big space as long as you move the air around (fans perhaps) you'll be OK unless you've got sub-structures (rooms) within your building.

 

[stm317]

That's a huge space to condition with two units, wow! But I guess if they're just cooling you can get away with that since cooling loads are much smaller. For my situation, where I'd want them to maintain heat in 20 degree ambient temps I'd need a bunch more btus.

It's not living space, it's a shop so I don't need a unit that can maintain interior heat @60F or greater when it's 5F outside. If it's that cold, I'd probably only be working in the shop for emergencies anyway. But keeping it above freezing all the time is pretty much a requirement, and at or near the set temp in the 60s would be the goal if it's 20F or warmer outside.

 

[justinjoyal]

You need to find out your heating/cooling loads and then go from there.

Zip code?

Is the floor insulated?

Number of windows, doors, their size and insulation?

Building orientation?

 

[stm317]

For visualization, this is basically what my shop looks like:

Zip: 46227
Floor is uninsulated slab 4 inches thick

The 2 windows are 32X36, but they're both under the porch with an 8ft overhang. Very little direct sunlight gets in, but it probably does happen in the very early morning.

The side with the overhead doors faces SE, with the porch being on the NE corner. The backside of the building gets some shelter and a little afternoon shade from a treeline.

I've attached an early morning satellite pic from the South, looking North. Hopefully that might clear up some details specific to the location.

 

[justinjoyal]

I used your data. Guesstimated the OH doors' dimensions. Looks like you don't even need a ton of cooling and ~3 tons of heat.

1 x 12k + 1 x 18k if you have backup heat. You will need more for heating if you only want to rely on the heatpumps (1x12k + 1x36k for example.)

Insulation under the slab wouldve cut down the heating load quite a bit !

 

[Sanmiam]

If you are doing the install yourself I would highly recommend a company called Pioneer (highseer dot com) I have installed a few of their units in my retail spaces plus my own garage and thus far they have been rock solid (6+ years). Bought one originally because it was cheap and was so impressed I ended up with 4 others for various applications. I have had one problem where an "electrician" decided to cross the ground and a hot leg frying the board in the compressor. They ended up sending me a new board free of charge. Also, they pickup the phone and have people willing to talk with consumers on an intelligent level - something seldom seen in the trades these days.

If you are not doing the install yourself, you will be pretty much married to the brand your HVAC contractor is aligned with (unless you have already lined up an installer that will put in customer supplied equipment).

Couple of things to note:

1) I would do several heads over just 1. Better distribution of heat/AC throughout.
2) Like you, I have vaulted ceilings, although my peak height is closer to 18'. The temp difference between the ceiling and floor can be in excess of 15 degrees. I would recommend installing ceiling fans if possible to move the heat trapped up high. It has made a world of difference for me.
3) I have found that sizing these heat pumps is unlike standard forced air systems. They are variable by nature, so going too big is not a huge problem and there is a large margin for error in that direction. Going too small is definitely problematic.

My suggestion: get a 3 zone unit with 3 12K heads (assuming you are in open space). If you have a separate room for a workshop you may want to size that room a bit smaller (9K head with 2x12K in the open area). Like Justinjoyal said - you are sizing for heat NOT A/C. As an FYI - any minisplit (heatpump) is going to have trouble cooling in anything less than about 10-15deg ambient as their efficiency really starts taking a dump under 20deg outside. Having a supplementary torpedo heater for those REALLY cold days/months may be a necessity.

Good luck!

 

[finn]

I have a total of three Mitsubishi units in two different houses. The oldest is a three head unit, but we added a new kitchen and, in discussion with the hvac dealer, he said he now gererally recommends, in many cases, that you install a couple of stand alone units rather than a larger multi head unit.

The single units have higher seer ratings, or at least they did when I bought the last mini split, and two stand alone units may be cheaper in some instances.

 

[stm317]

I used your data. Guesstimated the OH doors' dimensions. Looks like you don't even need a ton of cooling and ~3 tons of heat.

1 x 12k + 1 x 18k if you have backup heat. You will need more for heating if you only want to rely on the heatpumps (1x12k + 1x36k for example.)

Insulation under the slab wouldve cut down the heating load quite a bit !

Thank you for working this up!

 

[spencer5]

Where in your building did you place your units. I have 30 x 40 building with 14 ft walls and am on the fence between a single 36k or 2 24k's

Your insulation is good, best to take advice from people in your area.
I found that 2 x 24K units were more cost effective than a single 36K unit, so that's what I went with.. Like you, I was a little "experimental" on the sizing, but as I could run one unit or two, it gives me options. Indiana summers generally aren't that extreme (compared to Texas) - but there may be cases when you have to heat/cool on the extremes. Right now, it's 40 degrees, my insulation is only R15 or so, and my space is 2400 sqft x 16' walls - I'm running one unit on heat mode. That's good enough to bring the shop up to the 60s or so... In summer, when it's 98+ out, I run both units.

I'd be more wary of cool - heat pumps generally lose performance (BTUs) as temperatures drop to and below zero. Modern units can be very efficient to very cold temperatures, but they all drop capacity to some degree.

Multi-zone - I didn't do this. Expensive, complicated, and perhaps less efficient. Instead, again, I used two units. One big space as long as you move the air around (fans perhaps) you'll be OK unless you've got sub-structures (rooms) within your building.[/Q

 

[Notgrownup]

What about a dual unit. I mean 1 condenser outdoor unit and 2 wall units inside using the same condenser? Ceiling fans will help a lot as well I presume. Not an expert here just thinking out loud.
I bought my Blueridge set up from www.alpinehomeair.com , they were really helpful. Installed it all myself and had a pro cut, flare, leak check and pull vacuum for me.

 

[Theruse]

So what is the power requirement to install a mini-split 18k or 24k unit?

 

[Notgrownup]

So what is the power requirement to install a mini-split 18k or 24k unit?

30 amps