Mini-split - sizing, questions

[ururk]

I started a project several years ago (https://www.garagejournal.com/forum/threads/barn-workshop-build.296935/) and am in the home stretch of my build. I'd like to condition the space, and my original plan was radiant so I put pex in the concrete floor, ran a natural gas line from the house. I don't have so much invested in the pex/gas line that it would not be a problem to abandon (temporarily or permanently) that plan.

While I've used the space for some projects, there have been a few times during the year where it's unusable and I had to abandon my second flooring project as it got too cold in January:

January-March (too cold)
Occasional days during the summer it's too uncomfortably warm inside (I have a dehumidifier, so humidity is kept in check)

I have 600 sq ft on the second floor, 700 on the first
R40 ceiling
R20 walls
Insulated 9x9 roll-up door with seals (no wind blows through it, but it's only 1" thick)
7 windows
One insulated man door
Well air-sealed (but not tested)
11' ceiling first floor, 10' ceiling (average) on second

100A breakered subpanel off main 200A house panel

From doing a manual J it seemed like a 36K unit, two zone was my best bet - floor unit on the second floor, wall-mount on the first. Was unclear if the second floor should have a smaller unit than the bottom one.

I know an HVAC technician via family friends who will do the final install steps for me - he's done mini-splits before and will review the final unit I pick out before I buy one.

So... my questions

1. I want to run the linesets on the inside (box them in wood) and only exit my building where the outside unit is - should I be concerned about condensation on the inside? Alternatively I have extra sheetmetal from my siding I could use as a cover, but would prefer to minimize cuts/holes in the siding. I no longer have a flashing brake so it's going to be a hassle to form it.
2. I realize only I can make the decision for myself, but is it better to go with a cheap unit and know it will likely be cheaper to replace than repair, or go with a "good" brand - Mitsubishi/Fujitsu? Mitsubishi has those branch boxes which complicate the install.
3. Some units have two fans on the outside unit at this size range - does the extra fan make it more efficient?
4. I'm in southeast Michigan. I'm interested in running it during the colder months (ie, Nov-March) in heat mode, and cooling just to take the edge off during the summer. Curious what this ends up looking like in practice. Do I "start" the system the morning I intend to work outside? Do I run it all winter at a cool temperature (ie, 50, 60) and bump it up when I go out to work?
5. Someone suggested SS unistrut for a base rather than manufacturer bases - worth the effort?
6. Is a "whole-house" surge suppressor sufficient to protect the unit, or are there inline units I should put in?
7. It looks like these units need to be on 30/45A breakers - is this for inrush current or do they draw this much power continuously?

I'd like to order soon - if I can order the unit and plan the install locations I can get the concrete pad in ahead of colder days.

Thanks!

 

[theoldwizard1]

First, anytime you install more than 1 or 2 air handler on the same compressor you lose efficiency. It costs a lot more to install 2 systems, but you will save money in the long run (5 yr, 10 yr?). The 2 fan on large systems are necessary to get rid of/capture the "heat". At 700 sq ft (20x35) you are going to need 2 air handlers to get reasonable distribution.

Mini-splits are not the best for raising the temp quickly. If you set the thermostat at 45° and you need 65° to be comfortable it will take awhile. Some people have an auxiliary heat source (propane well heater) to get the temp up quickly. Also if you have any plumbing how are you going to protect during an extended power outage ? Simplest thing to do is drain the water.

The units will not draw near 30/45A. 100A is plenty for your garage. Worst case would be compressor and plasma cutter and someone else firing up a welder.

 

[ururk]

First, anytime you install more than 1 or 2 air handler on the same compressor you lose efficiency. It costs a lot more to install 2 systems, but you will save money in the long run (5 yr, 10 yr?). The 2 fan on large systems are necessary to get rid of/capture the "heat". At 700 sq ft (20x35) you are going to need 2 air handlers to get reasonable distribution.

Since I want to condition both upstairs and downstairs, would this mean one system for above with two handlers, and one for below with two? I can't imagine needing four air handlers but I'm willing to research.

Mini-splits are not the best for raising the temp quickly. If you set the thermostat at 45° and you need 65° to be comfortable it will take awhile. Some people have an auxiliary heat source (propane well heater) to get the temp up quickly. Also if you have any plumbing how are you going to protect during an extended power outage ? Simplest thing to do is drain the water.

I ran a water line but haven't connected it. I will have to think about this - I've been monitoring the temperature inside since 2019. The coldest it's gotten in the barn is 20 degrees - while this is below freezing, there have only been 23 days since 2019 where it dipped below freezing. More likely - instead of a power outage - I'd expect a heating equipment malfunction. For that, I can install some heating tape as a precaution (I'm not running water all over, just where it comes out of the floor). If I don't get to the HVAC before this winter I'm going to put some water bottles in the barn to see if they freeze, it'll be a fun experiment.

The units will not draw near 30/45A. 100A is plenty for your garage. Worst case would be compressor and plasma cutter and someone else firing up a welder.

It's just me. For my worst-case power consumption, I'd be running:

vacuum/dust collection
3D printer
CNC machine (essentially DWP611 + steppers)
another power tool
stereo

I probably won't get a welder, although I really would like to some day.

 

[ururk]

Mini-splits are not the best for raising the temp quickly. If you set the thermostat at 45° and you need 65° to be comfortable it will take awhile.

Also wanted to ask what you mean when you say "quick". Are we talking a day, half a day? General rule of thumb is OK. Both floors will remain fully open, and my expectation would be 6 hours to reach temp. Is that unreasonable?

 

[Mzungu]

I think the 36K will be sufficient. As theoldwizard1 said you lose quite a bit of efficiency by having a two zone unit. For myself I would install 2 x 18k single zone units, one down and one up. I have the same setup in my house in climate zone 6 which has less insulation than your shop (built in the 70's). I have a floor unit downstairs and a wall mount upstairs. I went with the cheaper Aura diy units from Senville and am quite happy with their performance. On the coldest days (-24-33 celcius) I sometimes supplement with my woodstove. Their warranty is also good and with you having an HVAC technician to do the final hookup and testing the warranty will be valid.

 

[yeldogt]

Without getting into the specifics. There are too many variables to simply state that duel head systems are less efficient ... they difference in efficiency ratings has to do with many factors. You can't just say .... the 12K units are "X" each and the duel 24 is "Y" . So the one is better ... all the smaller compressor models have higher ratings at optimal output. What is true is you don't want multi head systems that are not branch boxes set up where heads are off ..... since refrigerant is always flowing to all the heads ... there will be a loss on the heads that are off. Also you can get long lines and that's a loss as well.

As far as single point heat and cooling -- that's going to depend on the building. The tighter the building the better. I heat and cool one of my outbuilding with a single point cabinet heater and 24k AC -- it's 1700sf.

All that said --- you need to do a heat load. Is the first floor ceiling insulated?

Unless you have cheap electric -- since it seem you have NG piped to the building that is going to be less expensive.

My guess is you will not use the mini splits at the same time -- I would do two separates. You want to make sure the line - sets are insulated. I always burry mine in the walls -- insulated the pipes in an insulated wall. No problem. I hard pipe the drains with PVC.

You need to do that heat load .... also -- heat pumps are sized for maintaining temps not rapid changes. My 1700 sf can be heated with 15k of electric heat ... that's maintaining it ... not getting it to 65 when it's 35 degrees

 

[ururk]

Electric rates out here are $0.11 / kWh.

Building envelope is insulated - in-between floors is not.

If I install the mini-split at the rear of the building - where I'd prefer - it will be an 8 foot run and a 20 foot run (at most).

I air sealed the entire envelope, but there are some areas that might let air in - around my insulated rolling door (not overhead/garage door) - but I have brush seals installed.

Every time I run the Manual J I end up just under 36K (combined) - the upper floor is typically less than the lower floor.