[Shop-talk] mini-split or window unit ?

[Arvid]

My experience in Minnesota.

TL;DR; Had a reliable HVAC company install a Mitsubishi mini-split. YMMV.

I recently had a 24 x 36 building built behind the house. Lower level is 10ft high. It has a gambrel roof so there is a 12 x 36 x 9 upper level area. R30 in the walls and R40 in the roof. One 4 x 8 person door and one 8 x 9 garage door. The two long walls were framed with window air conditioner openings. No windows were actually installed but the exterior siding and interior walls could easily be modified by removing the insulation to create an opening for a window air conditioner. I am assuming it would have taken one or two days to convert the opening, install the window unit and seal everything back up again. I did all of the electrical in the building and had installed 220v 20amp outlets right next to both of the windowless openings in anticipation of window units.

Primary winter heat is provided by a natural gas boiler with in-floor radiant heat.

The "what size air conditioner?" needed to be addressed. Much research in the 9000 to 18000 BTU cooling capacity showed that 9000 are were more efficient than 12000, 12000 better than 15000, and 15000 better than 18000. My calculations suggested 12000 of cooling would do the trick.

In that capacity window units seemed to be anywhere from $400 to $900, with mini-splits being in the "considerably more" category. One point for the window units.

"What about efficiency?" I found several mini-splits in the 12000 size that were considerably more efficient than any similarly rated window unit. One point for the mini-split.

Next was to look at the possibility of using the heating capabilities. More calculations and it looked like the electrical power used by the mini-split would be less than the cost per BTU of natural gas until the outside temperature got down to about 35f. Yes, there are lots of winter days in Minnesota where the high for the day is less than 35f but there are also many spring and fall days where 35f is the low so it seemed like a reasonable solution. The 15000 and 18000 units would have provided more heat but the trade-off was less efficient cooling. 12000 still seemed to be the reasonable solution given the boiler as primary winter heat. Another point for the mini-split.

Finally, "DIY or have someone do it?" I had narrowed it down to a Mitsubishi 12000 mini-split. Certainly not the cheapest but I liked the reviews and both the heating and cooling efficiency. I looked at several on-line places that would provide all of the "stuff". The price came to around $2800 and I would have to come up with a way to evacuate the lines to make it all work. I then called the heating contractor that has serviced our home for the last 30 years and asked if they installed Mitsubishi (that is all they will install, which seemed to be a good sign) and how much would it cost. He came out and looked at the building and said "$3200". My response was "When?". "A few days after you have installed the electrical disconnect on the outside wall."

I had already installed the 220v for the window unit so it only took a day to drop the wiring down and out the wall to an external 30 amp disconnect. The following Monday the HVAC contractor and a helper showed up at 7am. My job was to point to where I wanted the indoor unit. They drilled a hole in the wall, pushed the lines and wiring thru the hole and mounted the inside unit. They brought a stand for the outdoor unit and put the unit on top of it. Routed the lines and wiring to the unit and placed a nice cover over that mess. Attached the unit to the disconnect I had installed and then proceeded to evacuate the lines. Shortly thereafter they were showing me how to use the remote control and handing me all the paperwork.

It was now 9:50am and they wanted to get some breakfast before their next job. I just started doing 'in the garage things" rather than 'to the garage' things.

Yes, I probably could have save a lot of money by going with very inexpensive (possibly 'disposable') window units but even with the 'already have the framing work done' it would have been a hassle and a couple of days work for me (to be repeated again in xx years). But I would not have had any heating ability (which has been working quite nicely this spring as the boiler has not come on for many weeks) and would be paying more during the cooling season based on the reduced efficiency of the window units.