Help with mini split sizing

[tmshort]

I have a 24x36 stick built shop, gambrel roof with a 24x36 upstairs.
Standard 2x4 construction, insulated, etc. The upstairs gambrel roof is finished and insulated to the ceiling with R-19 batts. Pella windows on either end upstairs, 2 man doors downstairs and 2 single bay insulated overhead garage doors. No windows downstairs.

I want to install Mr Cool DIY mini splits (I am thinking one on each floor), and am struggling with sizing. I’ve run many online calculators, and the BTU range I get varies by a factor of 10!

A couple of other data points:
- I am in central Indiana
- I keep it heated to ~45F all winter with a 10kW electric heater currently. It will maintain 65F, but runs a fair amount
- I currently have a very old small 120V window unit upstairs, running 24/7. It keeps the temps 70-75.
- I had an HVAC guy out for an estimate, he quoted a 2.5ton heat pump for the entire space (this was forced air, not mini split). This would imply 30k BTU, but I cannot tell you what kind of (if any) formal calculations he did.

Thoughts / recommendations?

Thanks in advance.

 

[Black Oak]

I've got 670 sq. ft. in mine and have similar insul. as yours . I'm very happy with 12'000 btu in Arkansas . Without a load calc , I'd say 18,000 each floor . 2 single heads will likely be less $$ than a 3-Ton with 2 air handlers . Shop around . You may struggle some w/ heating in the dead of winter , so keep the electric heater .

 

[SALIV8]

Do you want to size this for heat or ac?

 

[tmshort]

Do you want to size this for heat or ac?

Probably will be used for both.
Right now my focus is for AC; in the winter, I keep the shop from freezing with the 10kW unit and then fire up a propane salamander for a few minutes when I go out to work.

If the mini splits will [reasonably] economically keep it heated, I will probably keep the temps warmer in there through the winter so I can work more easily.

 

[SALIV8]

I would suggest goin higher btu than you think if you want heat. The units can throttle down for the ac usage.

I sized my systems for ac and they actually work good for heat down to about 32* F outside.

If you don’t need blaring heat then I would put your needs about what Black Oak said, assuming mr cools performance data is similar to my units.

 

[EricVonHa]

Given the basic parameters of room size, I'd be buying above an 18k mini-split system.

I did a 20' x 20' x 12' with an 18k unit and it works awesome for summer cooling.

Area is super insulated with thick closed-cell spray foam. My only downside is too much glass area and I get a lot of heat-load from the windows and sun. Thinking of tint for a future upgrade.

 

[EricVonHa]

To answer your question directly about 2.5 ton for upstairs and downstairs combined, yes, it seems accurate.

What's the plan for ducting? From the air-handler 2 huge ducts for the garage and 2 huge ducts for upstairs should do the trick.

Code is finicky on return air. Nothing like mixing carbon monoxide from the downstairs into the upstairs. No permit, no worries.

 

[tmshort]

Given the basic parameters of room size, I'd be buying above an 18k mini-split system.

I did a 20' x 20' x 12' with an 18k unit and it works awesome for summer cooling.

Area is super insulated with thick closed-cell spray foam. My only downside is too much glass area and I get a lot of heat-load from the windows and sun. Thinking of tint for a future upgrade.

Not sure what you mean by “above” - if you mean more, I am tentatively planning 18k downstairs and 18k upstairs, total of 36k BTU; according to google, 2.5 tons is 30k BTU, so I am exceeding this.

I’m not doing the forced air system. They wanted like $14k to install it, which is way too much. I don’t park cars in here much / at all, so CO is not a big concern.

Thanks for the input

 

[yeldogt]

I have a 24x36 stick built shop, gambrel roof with a 24x36 upstairs.
Standard 2x4 construction, insulated, etc. The upstairs gambrel roof is finished and insulated to the ceiling with R-19 batts. Pella windows on either end upstairs, 2 man doors downstairs and 2 single bay insulated overhead garage doors. No windows downstairs.

I want to install Mr Cool DIY mini splits (I am thinking one on each floor), and am struggling with sizing. I’ve run many online calculators, and the BTU range I get varies by a factor of 10!

A couple of other data points:
- I am in central Indiana
- I keep it heated to ~45F all winter with a 10kW electric heater currently. It will maintain 65F, but runs a fair amount
- I currently have a very old small 120V window unit upstairs, running 24/7. It keeps the temps 70-75.
- I had an HVAC guy out for an estimate, he quoted a 2.5ton heat pump for the entire space (this was forced air, not mini split). This would imply 30k BTU, but I cannot tell you what kind of (if any) formal calculations he did.

Thoughts / recommendations?

Thanks in advance.

What is the size of the AC unit upstairs? .... insulation between floors? So the window unit can keep the temps inside at 70? and the lower level is not cooled? When you say running -- is the compressor running all the time or just the fan?

What about the heater if you turned it up -- how much will it heat? That's pushing 35k BTU. In the winter you should have tried to see what it would do -- turn it up as see how fast and how much it needs to run. That would give you some better ideas.

The HVAC guy may have done a load -- using an common guess that many use it would have been over 3T

 

[tmshort]

No insulation between floors.
I need to look at the unit and see, I don't know the size - but it is small, old (>20y), and runs on a 15A 120V circuit. It will get to 70 at night but gets up to 75-76 during these 90+F days. The shop is fairly shaded on the S side, so that helps.

The 10kW unit will keep the shop at 70F but has to run a decent amount to do so.

I have no idea if he did a load - if he did so, it was after he left. He knew the rough measurements and looked the place over...