2 story garage, 2 zone mini-split sizing

[deanw]

Hopefully, this is a new spin on the old question.

Structure: Garage is 22x26, 2x4 framing, T1-11 siding. Just had the old truss roof ripped off and a 2nd story added on; 2x6 framing, 2x12 rafters. Located in in the Northwest corner of Oregon- commonly in the 30-90 degree range; records are probably from 15-115.

Insulation: Downstairs I'm furring out the walls to 5.5" deep and will insulate to R-21; the 16x8 garage door is somewhat insulated. Upstairs, R-30 in the floor joists, R-21 walls, R49 (maybe a little higher) roof.

The goal is to use upstairs as office space and downstairs as garage / shop space so doesn't need to be as "comfortable"- heat to 55, cool to 80. I have a handful of wildly different system quotes for a mini-split; from an 18,000 BTU condensor unit with 12k+12k heads, to a 28k condensor with 12k (down) and 18k (up) heads, to 36k condensor and 2x18k heads, with suggestions (only one of which I got a quote for) for separate condensors (sized to the heads, either symmetric or different sizes- 9/18, 12/18, 18/18) in case one zone wants heat and the other cooling. (I get what they're saying, but I can't see how that'd be an issue in my case where I expect the garage area to always be hotter in the summer and colder in the winter). None of the quotes were significantly less than $10k. I'm handy and have friends that have DIY'd a mini-split to code, so that's the direction I'm headed.

Anyway, at this point I figured there's no harm in soliciting opinions from people who actually live with these solutions, if there's anyone else in the same rough situation.

 

[mepstein]

Why do you need to insulate the second story floor joists. I’m just asking since I have a very similar space but don’t insulate the second story floor.

 

[dcg9381]

Why do you need to insulate the second story floor joists. I’m just asking since I have a very similar space but don’t insulate the second story floor.

I have a similar setup to the OP. It's basically living space over a 3-car + garage. We spray foamed the floor joists to "enclose" the living space. As the OP indicates, the garage itself will not be the same level of "heated space" as the living space above it, so we treat them as separate spaces.

It's probably "less important" in a heating situation as heat rises.. But as peak temps are 90+ degrees, he's going to want floor insulation in that space if he doesn't want cool air leaking into the garage through the floor... Also, cars "heat soak" so - they're great as big pieces of heated metal in the summer.

OP: I found also that turn key multi-head mini splits were out of my price range. In fact, I found that multi-head mini-splits were more expensive than two indepedent mini-split systems.... I've done about 4 DIY now and I'm no HVAC guy. I'd budget less than $3k DIY for two units.

 

[deanw]

Re- insulating 2nd floor- good question.

Building code requires it because it's a "garage" downstairs; I also have to put up Type-X fire rated sheetrock (the 5/8" stuff that's a PITA to work with) on the garage ceiling. (Probably do OSB for the walls though.)

But besides that, I intend to let the garage get hotter (summer) and colder (winter) than the upper floor.

As for cars being a source of heat- I don't drive much... mostly it's motorcycles and power tools down there.

The contractor that did framing was going to do the mini-splits; he's bailed out and written me a check for the $2500 he was holding for the purpose, so that's about my budget. (not counting required disconnects, breakers, wiring, and my time).

 

[justinjoyal]

loadcalc.net is your friend

Otherwise you are just guessing and if you have no hvac experience you might not be satisfied with the outcome.

 

[PoorUB]

The upstairs will cool on 12,000 BTU , one good for well below zero. The garage should too. As for heat that is where you may wantto go to a larger unit as most start to fall off as the outdoor temps get colder.

It isn't as simple as saying 12,000 or 18,000 for heating. You need to find the specs for each unit and what the heat ouput is at a certain temp. As it is your only source of heat you need to look for units that heat well below zero. There is a difference between heating below zero and actually producing usable heat as many of them will heat to just below zero, but the capacity is about 1/2 as it would be in warmer temps. Some mini splits will produce near full capacity -10F. You will want a hyper heat model. You might want an 18,000 for both floors simply for the heating capacity. An other option is to buy a lower priced mini split that might not heat in severe cold and add some electric baseboard.