First Thoughts/Questions About Mini split systems

[thirdgoat]

I recently completed a 30x50 shop/garage, not used for daily drivers, but hot rods/project cars located in the Huntsville (northern) AL area.There is an 18x8 door on the 30’ gable end facing east, and a 10x8 door centered on the south side 50' wall, both doors themselves are insulated as well as the windows in the top panels. There are 4 horizontal 4x2 windows installed 8’ up on the west side that I will be installing mini blinds or window shades in shortly… while they provide a great deal of light, the afternoon sun is baking the interior in the afternoons. The windows do a great job allowing natural light in, and you need a ladder to see in from the outside! There is a single entry/man door under a covered breezeway into another garage. The garage ceiling was spray foamed, and covered with OSB. The walls are R13 under 7/8’ OSB. The ceiling has two large storage truss areas that extend 11’ towards the center of the garage from both ends. The height under the storage trusses is about 10.5’ floor to ceiling. The open center area with the cathedral ceiling is 28’ wide with a height of approximately 17’ at the peak.
Here is where I need some assistance, comments, good, bad, or do you even have a clue! I would like to install a mini-split system in the shop, likely with two zone unit, two “heads” evenly spaced along the 50’ rear wall. From what little bit I am able to guess, I would need a 36k BTU unit? I don’t need to cool the garage to 72 degrees, 78-80 degrees would be fine running ceiling fans and mostly getting the humidity out. For winter heating I have a DeWalt torpedo type propane heater I can fire up to get the shop warmed quickly, then use the mini split to maintain temps. I have two circuits for ceiling fans that have not been selected or installed yet, and I have no issues with a few fans moving air around. Does any of this sound reasonable? Feel free to make any and all comments, I can handle even the “you are clueless”. Need a starting point and direction.

Thanks!!!

 

[Toomanytools?]

Call a HVAC company they will give you a quote on what you want usually quotes are free. From what I have gathered using a single outdoor unit with two heads inside isn't as efficient as two smaller units and cost is comparable. So you might get two 18K or 24K units and have a better system. You don't want your system to struggle only to get to 78-80 degrees.

 

[jeepxj]

why isn't 2 units as efficient as a multi head unit?

 

[My Old Tools]

Based on my 30x40x12 shop with insulation, a 36k should do it. Mine is barely running and it cools my shop great.

 

[Black Oak]

why isn't 2 units as efficient as a multi head unit?

generally , the seer rating will be lower , and the cost is higher , in a 1 compressor / 2 air handler system . prices and circumstances will vary .

 

[My Old Tools]

I just run a simple box fan on the floor to create a circular flow around the shop. My 36k MrCool is mounted about 9 feet up dead center of the 40 foot wall. The whole shop is very comfortable. It ***** massive amounts of humidity out of the shop.

 

[jeepxj]

generally , the seer rating will be lower , and the cost is higher , in a 1 compressor / 2 air handler system . prices and circumstances will vary .

interesting.

what about the running cost per ton of cooling compared between 2 single 18k btu units vs a 2:1 36k btu? surely the two compressors running will have high draw negating the extra seer.

 

[dcg9381]

I found that doing multi-zone was less efficient, perhaps more complicated to install, and more expensive in many cases than getting 2 units that provide the same or better output... I really don't know why this is, but I was able to buy 2 x 24K units for the price of a multi-zone 36k unit.
In my case - we have pretty extreme temps, so I can run both units in summer and run a single in fall / spring /winter.

The other "deal" to remember - and it differs from residential HVAC is if you're not heating/cooling the shop 24/7. In these cases, it may be warranted to "over" BTU it if you need to be able to adapt temperature quickly versus waiting hours...

 

[yeldogt]

The multi units work very well when they are properly sized. I put 3 head systems in two different houses -- each place has 3 12k head on one 30k compressor.

You have to size units for the max load ... so in many situations and climates a singe will be ramped way down and not running in it's sweat spot With a multi the unit is splitting everything up .... they work best when all the heads are always on.

 

[Highbeam]

why isn't 2 units as efficient as a multi head unit?

One of the many mysteries. It's okay, you really want two separate units anyway for redundancy.

The interior wall units are also much more efficient than the other styles of interior units. Mysterious.

What really irritates me is that they haven't released a minisplit heat pump boiler to heat water for out radiant slabs in north america.

Where's my Mr Cool boiler!?!? 30k btu, 80-100 degree water temps. Nope, can't have it in north america.

 

[bzinsky]

interesting.

what about the running cost per ton of cooling compared between 2 single 18k btu units vs a 2:1 36k btu? surely the two compressors running will have high draw negating the extra seer.

Nope, the number of compressors is irrelevant, energy usage is factored into the seer rating.

 

[bzinsky]

Single zone, 9-12k btu, traditional indoor head, is the most efficient thing you can buy.

You want 30 seer? Buy multiple 12k btu’s.

The really output quite a bit more than that if you’re not at temperature extreme. IE they will pump out like 16k btu’s of heat when it’s 30 degrees F. (Dont quote me on that, but I know it’s a pretty good amount of extra btu’s than the rating.)

 

[yeldogt]

Single zone, 9-12k btu, traditional indoor head, is the most efficient thing you can buy.

You want 30 seer? Buy multiple 12k btu’s.

The really output quite a bit more than that if you’re not at temperature extreme. IE they will pump out like 16k btu’s of heat when it’s 30 degrees F. (Dont quote me on that, but I know it’s a pretty good amount of extra btu’s than the rating.)

There are losses as you increase tubing size ..... twin units trying to deliver 2x the output would be rated differently also. You would be 2x the loss and motors typing to math the single.

 

[nsula_country]

Working shop or storage and play shop?

Mini's are ok. But mini's cannot filter air as well as a split or package unit.

Here is an example of a "normal" shop filters after 30-60 days of continuous HVAC.

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Have a HVAC build thread also in signature.

In Alabama, I would suggest 4 tons.

CT