How to properly size a mini split

[Jeff May]

How is the correct BTU determined when purchasing a mini split?
I was told to many BTU could be worse than not enough.
So,
If my shop is 26' x 26' with 8' ceiling.
What size should I be looking for?
Thanks,

 

[justinjoyal]

You need a cooling and/or heating load calculation.

www.loadcalc.net

Or type « cooling load calculation » into Google.

That’s what will guide you.

Basically: cubic footage, insulation, doors/windows, sun exposure, inside/outside design temperatures, etc.

 

[kelpaso1]

I have a 24x26 shop and use a 18,000 BTU unit. Works fine. Nice and toasty 65-70 degrees 24/7 all winter. Cost about $30-35/month in electricity.

 

[Jackfre]

For 500 sq ft in Maryland assuming good construction, doors and insulation, I'd go with a 12 kbtu unit. I would not go above a 15 in any case. Residentially, back east, northeast that is I would rule of thumb 600 sq ft/ton. You have to be careful of heat loss/gain programs. I know the Manual J has a 15% duct loss correction that you have to figure in. I used to use Fujitsu's heat loss program which has those things backed out, but that is behind their firewall and for contractors only.

 

[Jeff May]

Thank you everyone for the advice. ..

 

[SALIV8]

I have a 24x26 shop and use a 18,000 BTU unit. Works fine. Nice and toasty 65-70 degrees 24/7 all winter. Cost about $30-35/month in electricity.

wow that energy usage/savings is amazing. What outside air temps are you guys seeing?

What brand and model do you have?

 

[Dkramer]

Over sizing a mini split is not the same as over sizing a ducted unit. You can get away with over sizing an inverter mini split, as they will vary their output. But you will cost more to purchase a larger unit, so you do want to get close, and try not to undersize, as it will cost more to operate because of the longer run times. An inverter mini split is unbelievably cheap to operate. I have a Daiken with four wall units heating my house. My total yearly average power usage for my all electric 1600 square foot house is around $150 dollars a month.

 

[Renfrick]

Isn’t rule of thumb 550 sqft per ton for a/c


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[Dkramer]

Isn’t rule of thumb 550 sqft per ton for a/c


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That will depend on several factors, where you live and how insulated the house is are only a few. Areas that reach 100 will need more than areas that only hit 90. But it would probably work good enough for a garage.

 

[Renfrick]

True ,, but it’s a rule of thumb that will put you in the ball park without much math.
So your house is 1600 sqft
You probably have . 3 ton max


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[Toomanytools?]

You could just get a HVAC company to come out and give you an estimate.

I just had a quote done for 24'x40' 10'6" flat ceiling on half so 12x40, other side open attic. Walls R21, ceiling R30 min, 5 windows 3'x2', one man door. He spec'd a 2 Ton Daikin single head unit. I'm in eastern Washington Summers get over 100 for a week or two, winters can have a couple weeks of temps in the teens.
Hope that helps.

 

[rburke65]

Defamer....that $150 figure dos not really tell us anything. Dos that in louder water heating, dryer, cooking, ho many people, N Gas, ...? Lots of factors missing.