Help on Minisplit Sizeing

[dnwong]

I have a newly built attached garage with 2 floors above it. The garage level is 18x29x12 feet high. The walls have r-19 and ceiling has r-38.

The two floors above are currently unfinished. Second floor will be 18x29x9ft high with r-19 walls and r-38 ceiling. The top (third) floor will be an attic space. 18x16 with r-30 cathedral ceilings.

Questions:
1. Should I have 1 or 2 mini-splits? The garage will be heated or cooled when I am there.
2. What size BTU will I need to given 1 unit vs 2?

 

[theoldwizard1]

You will need 2 "air handlers" (up stairs, down stairs), but only one compressor. Additional fans would be good.

Sizing is beyond my capability. Buyer beware. Not all mini-split are good at heating, especially below freezing. Ask questions. Learn how to read the specs.

 

[mrpizza]

The Carrier MG series heat pump mini splits will deliver 100% of rated heat down to -5 degrees on some of the models. We typically size them for the heating load, not cooling. Have someone perform a heat load calculation on your entire structure. You'll be looking for what is called a multi split system.

 

[justinjoyal]

2 air handlers for sure.

Sizing is hard: cold climate? Lots of windows/sun?

For a "regular" garage this size with few windows and insulated doors, I'd say a 15k unit would be good for the lower level and a 12k for the upper level.

A proper heat load calculation would be needed for exact sizing.

 

[Jackfre]

Be aware that many multi's will not work without at least two evaps hooked up. You are looking at a 3 story load and I assume you will do one evap on each floor. When I chose my system for the house I was going to go for a triple. When I looked at the pricing I found I could buy a 25 seer/12hspf single and a 16 seer/9.5hspf dual for the same money as a triple. The hi eff. unit is in the main living area an the duals are in the bedrooms and get limited operation. In my experience with mshp's they consistently exceed expectations, so I would rather slightly undersize them and push them. If the building is well built I wouldn't go over a 15 in the main area.
I was just up in OR this past week, which is a very big mshp market and talking with some contractors. They were saying that a study done in the NW is saying that oversized mini-splits are showing high energy consumption. I don't have the report yet, but am trying to get my hands on it. It makes sense to me as long run times are the most efficient. Starts and stops raise the stress on the equipment and the bill.

 

[justinjoyal]

I was just up in OR this past week, which is a very big mshp market and talking with some contractors. They were saying that a study done in the NW is saying that oversized mini-splits are showing high energy consumption. I don't have the report yet, but am trying to get my hands on it. It makes sense to me as long run times are the most efficient. Starts and stops raise the stress on the equipment and the bill.

True.

People think they can oversize their inverter mini-split as they please because it will simply "throttle down" but that's not true. Problems or incomfort will occur.

 

[mrpizza]

As mentioned, dont expect the inverter drive (if equipped) to solve all the problems. If its too big you will end up with humidity issues in the summer.

Get the heat load and size from there, thats how we do it where I work.