Sizing a mini split

[tec508]

I'm sure this is common question. I kept getting vastly different answers to it so I'm hoping the GJ collective can weigh in and help me.

I'm getting ready to order a dual zone system for my detached 3 car with a loft. (1 zone on each floor).

Interior dimensions are 35'x28' 11' ceiling height. Upstairs is the same but with a pitched roof.

Building is getting batt insulation downstairs, spray foam upstairs.

Hoping to keep it around 75F and dry in the summer months, with the capability of getting a little colder in there. Mainly taking the edge off and getting it dry is my concern but being able to get it nice and cool for working on projects would be a major plus.

Upstairs use is undetermined, but when finished will probably be a multi-use room/pool room/something..

I was thinking a pair of 18k heads with a 36k condenser would be fine. Some are saying that's too big, some are saying it's perfect (including my misguided calculations and research), a small percent are saying it's under sized by a lot.

 

[MattT]

I think it should work for cooling. You could maybe go a little larger downstairs on account of the garage doors.

If you also want to use the mini for heat you could be undersized there.

 

[finn]

Check the price and performance specs of two totally separate units instead of a dual head mini split.

Two different hvac companies recommended two separate units instead of a dual unit.

Full disclosure: I have a triple head Mitsubishi in my house...

 

[justinjoyal]

Where are you located?
Insulation R-Value?
Windows/doors?
Sun exposure?

 

[tec508]

Where are you located?
Insulation R-Value?
Windows/doors?
Sun exposure?

-Connecticut.
-R-19 on the walls downstairs, R-30 on downstairs ceiling, R-38 on sloped ceiling (upstairs).
-Downstairs has 3 9' doors (insulated), 1 person door. 2 windows (1 on each side of the building 30").
-Upstairs has 2 windows 30".
-Building is partially shaded almost all day (the rear of the building). Mostly surround by trees.

 

[tec508]

Check the price and performance specs of two totally separate units instead of a dual head mini split.

Two different hvac companies recommended two separate units instead of a dual unit.

Full disclosure: I have a triple head Mitsubishi in my house...

Did they specifically (or do you know) why it makes sense to use 2 separate condensers over 1?

A 36k BTU condenser with 2x 18k heads seems like it would be pretty similar to 2x 18k condensers with 2x 18k heads. I'm not an HVAC engineer just applying logic to it.

 

[justinjoyal]

I would probably do 2 x 12 or 12+18 on a 24k condenser. I would run a load calc and get a feel of the place but thats what im thinking.

18+18 seems fine if you plan on opening those doors while cooling. Might even need more. It all depends on how you are going to use that garage.

 

[finn]

Did they specifically (or do you know) why it makes sense to use 2 separate condensers over 1?

A 36k BTU condenser with 2x 18k heads seems like it would be pretty similar to 2x 18k condensers with 2x 18k heads. I'm not an HVAC engineer just applying logic to it.

No, but looking at the specs, most of the high seer units are single heads, and, in some cases, units with a seer of 22+ qualify for a substantial rebate from the electric utility, so that may be part of it.

In my case, the layout of the house lends to simpler, shorter line sets with multiple condensers.

Two different contractors recommended multiple condensing units instead of multiple heads. On of those contractors did my original multi head installation for my home unit.