mini-split sizing for upstairs

[jpcjguy]

Hi all,

So my wife got a new job - leaving teaching. That is another story. Anyway we need to make more room for a home office as I currently work 90% at home also.
The plan is to condition the upstairs of the detached garage for me and then the 2 days she is at home she can use the desk in the corner of our bedroom that I currently use. Guess I have to clean and organize now!
I still need to build the outside staircase access but can use the inside pulldown that I currently use to get up there. The building is fully insulated but not conditioned at all - the temps in there are not bad year round. I live in Richmond, VA with only end of Jan/Feb/early March being the coldest around 48-50 degrees in the garage regardless of outside temp (11-70 degrees) and the warmest being 80ish degrees in August September when it can be 95+ outside. Really follows the slab.
There is no insulation between the first floor and the second. The upstairs is a room truss setup that is the following dimensions:
Length: 31' 5"
Width: 13' 10"
Height: 8'
Knee wall height: 5' 5"
Span at 8' height: 5' 11"

So the area is about 434 square feet so I think a 12k would do it - or because the downstairs is not conditioned (yet) should I move up a size?
I think the only area is near the ceiling on the end wall with the tall storage racks (opposite the door wall). The only thing that makes me pause if the upstairs is ever converted to a hangout space that wall would be great for a project screen. But I am probably overthinking this. Don't think above the door would work as that is only 12.5" from the door trim to the ceiling.
My other thought was a system that is ceiling registers (green squares in one pic)- but that would mean a traditional air handler setup (which I could put in the knee wall area.) or maybe that mrcool ceiling mount system which is an 18k unit (but probably $$$).
Anyone else have any ideas or thoughts? Trying to buy once/cry once.

 

[pcmeiners]

18k????????
Doing an 1080 sqft garage, 10 ft ceilings, 2 large roll ups, standard insulation, with a high efficiency 12k, which could easily be done with a 9k

Agree you should organize if your wife is working there.

 

[jpcjguy]

18k????????
Doing an 1080 sqft garage, 10 ft ceilings, 2 large roll ups, standard insulation, with a high efficiency 12k, which could easily be done with a 9k

Agree you should organize if your wife is working there.

No - I will be working there - no way would she want to sit among the tools - while I would love it

Smallest I saw was a 12k BTU 22 SEER MrCool DIY Ductless Heat Pump Condenser - 4th Generation - with 25 ft Line Set

 

[pcmeiners]

There is a 9k, 12k will not hurt, if you keep the fan on low, that would be less noise than a 9k

 

[PoorUB]

9K should do it, but i might drop a 12K in there any way. I doubt the price is much different, plus the low "speed" on them is probably the same. The 12K will probably run 4k output when it catches up.

My only question is how are the roof trusses insulated? 4" inches of fiberglass? 6"?

 

[jpcjguy]

9K should do it, but i might drop a 12K in there any way. I doubt the price is much different, plus the low "speed" on them is probably the same. The 12K will probably run 4k output when it catches up.

My only question is how are the roof trusses insulated? 4" inches of fiberglass? 6"?

I had the attic trusses scabbed with an additional 2x6 in the room section so even with my baffles (which are 1/2" XPS spaced 1.3" from the roof deck), I still had plenty of room for batts. That should get me around R30ish, with no air washing from soffit to ridge vent (which really hurts fiberglass insulation

 

[Bert_]

You couldn't buy a mini split that's too small for that space.

The difference between a 9 and a 12 was less than $100 and nobody stocked the 9 when I checked a few months ago.

 

[brewchief]

You can buy an indoor unit that basically sits on the floor instead of a high wall mount, if you search low wall mount they will pop up. I've never seen one in person or installed one but they are an option that might have some merit in your case.

 

[PoorUB]

I had the attic trusses scabbed with an additional 2x6 in the room section so even with my baffles (which are 1/2" XPS spaced 1.3" from the roof deck), I still had plenty of room for batts. That should get me around R30ish, with no air washing from soffit to ridge vent (which really hurts fiberglass insulation

Well insulated.

Set a bag on ice on the table, that should do it!

I am certain a 9K will do it, but if you cant find one I would go with a 12K, but I would try hard for a 9K.

When figuring AC for a home I would start about 600 sqft per 12K BTU and adjust up or down depending on insulation and other factors. You are pretty well insulated, other than the floor, an only 400-something sqft.

I used to cool my 900+ sqft house with a 12K window shaker. As long as it was on before it got hot it would keep up so the heat load was probably right at 12K. Start when the house was 80F inside and it wouldn't catch up until over night.

I put in central air, a 1-1/2 ton unit and put the window shaker in my oversized two car garage and it cooled it to, as long as it started when the garage was cool..

 

[thammel]

From my personal experience, 9k will work for heating easily. But for cooling, unless you close off the entrance to the ground floor, you will lose a LOT of cool air flowing right down to the first floor. You could rig up a simple foam board closure when you have the cooling running.

 

[jpcjguy]

From my personal experience, 9k will work for heating easily. But for cooling, unless you close off the entrance to the ground floor, you will lose a LOT of cool air flowing right down to the first floor. You could rig up a simple foam board closure when you have the cooling running.

Good idea! Did not think of the cool air "falling" down to the lower level.

 

[mike93lx]

Howdy neighbor.

I'd do either a 12k mini. Or just install a window and use one of the nicer heat pump inverters. 9-12k is about $400-500, plus you get some added light in the space.

You can get mini splits that have in ceiling cassettes, so it wouldn't force you to go ducted split, but it gets away from the DIY systems.

I think I would go for a window unit in that space

 

[jpcjguy]

Now regarding lineset routing, if I do that end wall above the shelving. Luckily, where it will be mounted is above the ceiling of the lift bay area so it will be in "open attic space".
This gives me 2 options for routing to the outside - referencing the finshed picture below:

Green - is to come back into the crawl area (from the lift bay attic space) and go out the raised heel portion of the wall (now look at the insulation picture and you can see the yellow triangle space that connects the 2 "attic areas". The

Purple - stay in the lift bay attic area and go out through the soffit - but is that even allowed? Would be nice to not have to put a hole in the wall! The third pic shows the scissor truss area with the purple line representing the lineset and how it would run down to the soffit

 

[jpcjguy]

Howdy neighbor.

I'd do either a 12k mini. Or just install a window and use one of the nicer heat pump inverters. 9-12k is about $400-500, plus you get some added light in the space.

You can get mini splits that have in ceiling cassettes, so it wouldn't force you to go ducted split, but it gets away from the DIY systems.

I think I would go for a window unit in that space

Howdy neighbor! Unfortunately there is no windows upstairs - window unit is not an option. I thought of the ceiling cassette but that would be more $$$, no DIY and generally more difficult install.

On another note - seeing that it looks like you are a mustang guy - 93lx - how well do you know the 5.0? I have a CJ7 with a 347 stroker from a 93 Cobra that I am trying to get back on the road and I don't know much about these motors (AMC guy originally)

 

[mike93lx]

Howdy neighbor! Unfortunately there is no windows upstairs - window unit is not an option. I thought of the ceiling cassette but that would be more $$$, no DIY and generally more difficult install.

On another note - seeing that it looks like you are a mustang guy - 93lx - how well do you know the 5.0? I have a CJ7 with a 347 stroker from a 93 Cobra that I am trying to get back on the road and I don't know much about these motors (AMC guy originally)

How about a through wall by the door?

Its been a long time since I had a mustang and I was never anything better than a **** less hack, to be honest.

At this point, I'd LS swap

 

[American Locomotive]

I would think a 9,000 BTU/hr unit could easily heat/cool that place. If you get a "Hyperheat" unit, they usually can go a bit beyond their nameplate capacity if needed.

I find that for most manufacturers, their smallest units are their most efficient. Mitsubishi's 6,000 BTU/hr unit is their most efficient, followed by their 9,000 BTU. Their 12,000BTU+ units start to take efficiency hits.

 

[pcmeiners]

He is in Virginia, so a hyper heat would be overkill, unless climate change goes negative

 

[jpcjguy]

Thanks for the feedback - just called the local inspector office and talked to someone about running the lineset through the soffit and he said it would ok. Also asked about height of electrical shut-off if I mount it high on the wall and he said if it is near the unit no problem (was specifically asking about the 6'7" max height I saw in code)

 

[pcmeiners]

Hell. that room is so small and insulated you could heat it with 3 or 4 candles, and ice cubes for air conditioning.