Properly sizing a mini-split - advice?

[Dave-H]

Ok, so it's looking like I'll go for the Mitsubishi mini-split for the garage! Now to choose the size.

This is a 20 x 20 garage, epoxy floor, rockwool batts with R15 in the walls and R23 in the cathedral ceiling. Also an insulated garage door. This garage has 7 windows in it (yes, 7) but 4 of them are quite small (18x18) and they are all very well installed low-E/Argon windows.

This garage is only 7 years old and it has good construction, not leaky considering it's a garage. However, I am realistic about how well an insulated garage door will seal, and all those windows aren't helping my situation at all.


Based on what I've learned from various sources I could do a 15k or an 18k and be ok. The online calculators seem to think that a 15k could do the job, although there is a wild range of results from those crude tools.

Meanwhile, all 3 installers who came to the house and offered to source and install the unit recommended an 18k. Those estimates were all very high, and I'm told that a lot of HVAC guys will sort of 'round up' when sizing a unit just to be safe.

Meanwhile, the 2 installers who offered to install an online-purchased unit both said that I could probably get away with a 15k unit but there might be some supplemental heat needed.

It's an interesting puzzle trying to sort this out. I have decided to go with the rockwool batts, which should stop a lot of air movement but probably won't seal up the place as well as foam or blown. But then, I spent some time poking around looking for areas to seal with a can of great stuff and to my surprise it seemed like the construction was quite tight - obviously it's hard to eyeball, though.

Then, I was reading that if you use a mini-split with an inverter it doesn't need to run at 'all or nothing' and that allows it to better handle a situation where it's oversized a bit. The idea being that if it's oversized, it can run at, say, 50% and still be effective and comfortable. I don't know if that's true or if I stated that correctly.

If anyone has any thoughts on how to approach this, I'd sure love to hear them. Thank you!

 

[Wizzard]

Go 18K. The 18K inverter units can spool down to below 7K btu. Also if you had the garage door open you want the unit to cool the garage down quickly.

 

[zmaxmotorsports]

I'd say something in the 18,000 but range,nothing bigger than that.

 

[theoldwizard1]

If you are going with a Mitsubishi Hyper Heat system you should be good. Anything else, you need to do your research on how well it performs down around 0F.

Auxillary heat is always good, even if it is just do a fast warm up. Kerosene will usually leave a smell. LP direct heat is fine for short usage.

 

[dakkon]

dave, i'm in dallas and will have a 700sq/ft garage with insulated walls and an insulated garage door... my heat is an issue, your cold is your issue...

right now i'm thinking about a 36k unit.. one of my buddies use to do ac work.. his comment was "for a garage you can never go to big" in a garage we arent trying to nessairly maintain humidity like hvac systems are for indoor applications... i'm wanting enought thermal capacity to overcome the ambient temps as well as a vehical...


have you thought of a traditional hvac system? horizontal mount maybe? that is one option i'm considering.

 

[Dave-H]

dave, i'm in dallas and will have a 700sq/ft garage with insulated walls and an insulated garage door... my heat is an issue, your cold is your issue...

right now i'm thinking about a 36k unit.. one of my buddies use to do ac work.. his comment was "for a garage you can never go to big" in a garage we arent trying to nessairly maintain humidity like hvac systems are for indoor applications... i'm wanting enought thermal capacity to overcome the ambient temps as well as a vehical...


have you thought of a traditional hvac system? horizontal mount maybe? that is one option i'm considering.

I am loving the minisplit setup - quiet and efficient which is perfect for my use. You would be surprised how hot Denver gets in the summer - it's high and dry and we have plenty of 100+ days.

I am still torn between the 15k and the 18k but will probably go with the 18k just to be sure.

 

[chrispyny]

Definately get the 18. Like others said, with inverter technology, that 18 can get down a little below half of what its rated for. So on cooler days, it can run as slow as if it were a 7 or 8k btu. Thats LOW. And on hot days, i'd MUCH rater have an 18 than a 15. And once the garage reaches set temp, the unit can kick all the way down to a 7 or 8k btu again.

I'd hate for you to get a 15, pay all that money, and watch it struggle to do anything in the Co heat. It's a no brainer.

 

[Jackfre]

You should look at the heating curves on the 15 and 18. If cooling is your primary game I would go with a 12. I haven't looked at the 15 & 18 as I cannot access Mitsu's site deeply enough, but I bet the output between the 15 and 18 in heat mode is very, very close. Wizard, do you have that comparison info?

 

[Dave-H]

It's just about time to order and I'm still struggling with this!

I have spray foam insulation in the garage, so the insulation is decent. Then again, I have 7 windows cut in, and a 16x7 garage door (it's supposed to be r19 but it's still a garage door!).

Using loadcalc.net I get numbers along these lines:

Total Btu's Cooling 4363
Sensible Load 3963
Latent Load 400
Total Btu's Heating 54756

Now I'm trying to interpret that:

The Mistu 15k unit has cooler power of: 6,450 - 19,000 Btu/h and and heating anywhere from 5k to 24k depending on outside temp.

That 'total BTU's' heating number is confusing. If I get the 15k, is it possible that the lowest cooling output isn't quite low enough, and the highest heating isn't high enough? I must not be understanding this

At this point I'm starting to wonder if the 12k would be fine for me. The 15k seems nice to get a little extra power if needed, but if the minimum setting on the 15k is still a bit too high, that's a bummer.

Very confusing, this stuff!

 

[theoldwizard1]

• What is your goal for temperatures in winter and in summer ?
• Were you going to turn the system off or set it back over night/when you are not inside ?
• I assume that all 3 systems are Mitsubishi Hyper Heat, correct ?
• And the real kicker, what is the price difference between all 3 ?

I haven't been to Denver in over 20 years, but IIRC, the winter temps were not super cold (like Fargo) and the summers could be hot, but cooled off at night.

If you go small, take some of the money you saved and buy a GOOD auxillary heater, like a "Sun Stream" 70,000 BTU Kerosene Heater.

This one has gotten a lot of good reviews on YouTube because

• it is quieter than almost all torpedo heaters
• it dos not smell (because it actually uses a fuel injector)
• you can burn diesel or kerosene

 

[Dave-H]

• What is your goal for temperatures in winter and in summer ?
• Were you going to turn the system off or set it back over night/when you are not inside ?
• I assume that all 3 systems are Mitsubishi Hyper Heat, correct ?
• And the real kicker, what is the price difference between all 3 ?

I haven't been to Denver in over 20 years, but IIRC, the winter temps were not super cold (like Fargo) and the summers could be hot, but cooled off at night.

If you go small, take some of the money you saved and buy a GOOD auxillary heater, like a "Sun Stream" 70,000 BTU Kerosene Heater.

This one has gotten a lot of good reviews on YouTube because

• it is quieter than almost all torpedo heaters
• it dos not smell (because it actually uses a fuel injector)
• you can burn diesel or kerosene

I don't have a specific goal for winter/summer, just general comfort. I would say I'm not all that picky about temperature so it doesn't have to be anything extreme. I doubt I would turn the unit off over night very often unless it was very easy to heat/warm the place quickly the next morning. From what I've heard, it can be more efficient to lower the temperature at night but leave the HVAC running, but I'm not sure about that.

All the systems were hyper-heat, yes, and about $1000 difference in cost between them. It's not so much the cost that makes me want to size the unit well, it's the idea that a right-sized unit would run a longer cycle and have more stable temps in the room with less humidity and a lower elec. bill.

Originally I figured a slightly oversized unit would be fine because it can run below capacity, but trying to understand the online manual J (for what it's worth) made me worry that the lowest cooling on the 15k was actually a bit too high!