48kBTU mini split for 750sq ft detached?

[barnparty]

I know, I know.. another mini-split sizing thread!

Now that we are past that, curious about the thoughts of all you non-experts and experts alike.

Looking to put in a mini split heat pump in my two story 750 square foot detached garage. All relevant specs I could think to list are below. My cousin is in the HVAC business and ran my specifics through manual J and came back and recommended a minimum 4 ton unit. Seems massive for this size out building, even with insulation not being excellent. I was anticipating 2-3tons. Based on thespecs do you think this is accurate?

• Northeast Ohio
• Shaded
• 750 sq ft, two story (so sized for 1500 sq ft total)
• T1-11 siding with R7 board insulation on first floor
• Insulated 16ft garage door
• Double metals doors with windows on first floor
• Single metal door with window on first floor
• Concrete slab on grade
• 2 single pane windows on first floor
• 2 single pane windows on second floor
• R-13 insulation on second floor
• 9ft walls on both floors

Anyone using a mini split with a similar building? Is 4tons way oversized or just right? At that size I'll end up with a 3 zone unit most likely, would put 2 on first floor and one on second. I won't be heating/cooling this 24/7/365, only will run heat as needed when I'm working out there in the winter or hanging out at my bar. For cooling, it usually stays pretty cool in the summer months, but I could see it being useful for 6 weeks out of the year. Primarily to help with humidity.

 

[vavet]

Your climate is different than mine, but my 768 SF garage has an 18k unit and does fine with it down to about 20F. Lower than that it runs constantly just to maintain and doesnt even really do that overnight,

 

[fitter30]

30 years ago 400 sq ft per ton. Sorry get a new in law. One that can run a load. 3 zone unit have to run at least two all times. Im in the ozarks 1900 sq ft modified A frame have two two ton systems 3 head each run the house with one unit till it gets to 5° ac 100°

 

[mike93lx]

That is nuts. I cool my entire first floor (1600 sq ft) with 2.5 tons in VA.

Wildly oversized unless you are running servers or a grow farm in there

 

[Steve W.]

My shop is 720 sq ft on the bottom level. Attic trusses upstairs give a 10x30 open area, but the storage area in the trusses is all open. I don't have my mini-split installed yet, but it has been spec'ed out at 18k for each level. R-13 in all walls and ceiling. Two small double-pane windows upstairs. Four larger double-pane windows downstairs, a 10x7 insulated overhead door and a 36" man door.

.

 

[barnparty]

My shop is 720 sq ft on the bottom level. Attic trusses upstairs give a 10x30 open area, but the storage area in the trusses is all open. I don't have my mini-split installed yet, but it has been spec'ed out at 18k for each level. R-13 in all walls and ceiling. Two small double-pane windows upstairs. Four larger double-pane windows downstairs, a 10x7 insulated overhead door and a 36" man door.

.

This is very similar to my layout but insulated better with R-13 being in all the walls. I appreciate the feedback. I'm getting a second opinion on it today.

 

[PoorUB]

I would think 12,000 would heat and cool the garage and 18,000 for the upper level. Maybe 36,000 tops. 48,000 seems a bit much.

One needs to keep in mind for the garage level there is conditioned space above it, not a 125 degree attic. I used to cool my 900 sqft garage with a 12,000 BTU window unit. It would keep up just fine, but don't turn it on late in the afternoon on a 95F day and expect it to cool the shop down. With condidtioned space above it most likely would have done it.

The only reason I would oversize to to make sure they heat in the winter. You really need to consider where the capacity falls off in colder outdoor temps, then decide.

All heat pumps lose capacity as the outdoor temp drops, some fall off at 20F some drop of a -30F.

 

[barnparty]

So the second opinion I got also said 4 ton for how it was currently insulated. If I put R13 on the walls and ceiling downstairs, then I could go down to 3 ton (right in line with what Steve was spec'd for his similar building). The price difference on a 3-4 ton unit is around $800. I'd rather just size up regardless of if I re-insulate in the future or not.

He also mentioned an electric garage heater (radiant style) as an option, but warned to expect a major jump in my electrical if I were to use consistently for the month. Given my estimated use of less than 40 hours a month, this option has me intrigued but I need to do some more research. I could get one of these for around $1000 and install would be much easier. Obviously, no AC with this option but heat is the primary need.

 

[mike93lx]

Sizing up an a/c is generally badz but most (all?) good.mini splits are inverters, so they can throttle, making it much less of an issue

 

[PoorUB]

IMO, 48K is too large, even with the poor insulation.

For example, my house was built in the 50's and the insulation in the walls is poor, like a 1-1/2" bat of fiberglass, 930 sqft. I cooled it with a 12,000 BTU window unit until I put in central air and went with a 1-1/2 ton as that is the smallest available.

I would bet you can cool the garage level with 12K BTU and the upper on 18K BTU, 30K BTU total, but hey, I worked in HVAC for 22 years so what do I know!

In my experiance 90% of contractor oversize equipemnt. Nobody complains when it heats or cools and keeps up, but undersize a system and you have trouble, so oversize it!

 

[Mzungu]

Your insulation is way below the recommended for your climate zone 5, which is why the 4 ton unit seems excessive. Based on the parameters you gave, the manual J could easily be accurate for the heating requirement of 48k btu.

https://insulationinstitute.org/wp-content/uploads/2015/12/OH-2009.pdf

if the use is intermittent I would probably consider a different method of heating as the time it will take to bring it up to a comfortable temperature will be considerable.

 

[Jackfre]

Unless you are going to add insulation I’d not get a big mini-split for it. I’d suggest a Rinnai EX38 LP or NG. It will heat the space and you can get a mini-split in the future sized for the cooling load.

 

[toyotadriver]

I think 48k is WAY oversized. I would suspect you need about 2-2.5 tons. You need a Manual J calculation to be closer than our guesses.

 

[jcarapet]

Looked at multiple online calculators for 1980 square foot 14 foot wall shop I'm building and they all came around 4 tons. Buddy put in a 18k for 650 square feet with multiple holes into attic and it's working fine.

if you have a multi head unit you could leave one running full time but I would reconsider.

 

[purplezr2]

Sizing may have to do with the heavy turn down in efficiency that mini-splits have when used for heating.

I have a 2 ton in my garage which is about 1500 sqft. Garage has a 16X8 and 10X8 overhead doors, 3 fiberglass man doors, and 35 sqft of windows. 2x6 construction with 3"+ of spray foam and 30+ inches of blown fiberglass in the ceiling.

The unit cools with no issues even when the outside temp is 90+, I can have it 66-68 in the garage. On the flip side when it comes to heating when it gets below 20F out, it will barely keep up. It is only about 65-70% efficient at that temperature.

 

[dcg9381]

The unit cools with no issues even when the outside temp is 90+, I can have it 66-68 in the garage. On the flip side when it comes to heating when it gets below 20F out, it will barely keep up. It is only about 65-70% efficient at that temperature.

I think it's oversized, I cool 2400 sqft in Texas, R-13 insulation, 3 big *** doors, with 48k BTU (2 units).
Like /r/purplezr2 says, my units are NOT built for cold, when it's below 30 their heating capacity drops very very quickly, pay attention to their performance curve in the cold.

 

[barnparty]

Thanks everyone for all the feedback. I just ordered a King Electric 10kW ECO2S+ garage heater. It equates to 34k BTU. Just to clarify, I had a Manual J calculation done (by two different professionals) and both said 4 Ton minisplit needed for current insulation to get heat to a comfortable temp. I picked up this King Electric from Lowes for around $1k (minisplit was 5x that) and can easily install myself. Going to see how much use I put it to this winter, and what the costs look like. I estimate at most I'll use it 15hrs a week. Then I'll evaluate if I want to upgrade the downstairs insulation to R30 and go minisplit down the road. Given my needs are primarily heat for the near term, I decided this was the best route for now. Worst case, I'm sure I could resell this unit and recoup most of the cost.

I really only "need" AC right now for maybe 4-6 weeks out of the year. I like to open my garage door in the summer when possible, as I have a long driveway with a nice view. Looking into full garage door screens next year to combat bug season. Long term, if I turn the upstairs into an office, I'll re-evaluate the minisplit as I'll definitely need efficient year-round heating/cooling up there. I think if I just want heating/cooling for the upstairs, I could get away with 1.5 ton minisplit no problem.

 

[PoorUB]

Just to clarify, I had a Manual J calculation done (by two different professionals) and both said 4 Ton minisplit needed for current insulation to get heat to a comfortable temp.

I am shocked by that! You place sounds better insulated than mine and I can cool my 900+ sqft with a 12,000BTU.

Like I mentioned earlier, I would figure 12,000 BTU for the garage and 18,000 BTU for the upstairs. 20 years of HVAC experience.

When I put AC in my house I told them to put in 1-1/2 tons or 18,000 BTU. The guy could not explain why I needed 24,000, (his recommendation!), when my 12,000 window shaker was keeping the house cool.

Same with the furnace, guys kept telling me 60,000 BTU and I wanted to go with a 45,000. My mod furnace runs at 24,000 BTU on low fire and will heat the house until -30F on low fire.

Guys over size equipment all the time!

 

[mike93lx]

Just to clarify, I had a Manual J calculation done (by two different professionals) and both said 4 Ton minisplit needed for current insulation to get heat to a comfortable temp

Did they look at the space or go off a common set of specs that you provided?

I was quoted over a grand for a load calc on my house, so I'm wondering what you got (presumably for free if two did it).

 

[barnparty]

Did they look at the space or go off a common set of specs that you provided?

I was quoted over a grand for a load calc on my house, so I'm wondering what you got (presumably for free if two did it).

They looked at it in person. I just had a local HVAC place give me a free quote and a guy put the specs into an app on his iPad. Honestly it wasn't anything more advanced than the free online manual J calculator that people reference here. Is a load calc more advanced than that?

Being a ductless system with the minisplit, I think that's one of the reasons they'll oversize it. Want to try to avoid that when you have duct area to consider. Also, the big reason for the oversize requirement was to reach comfortable heat levels. For AC it was sufficient at 2 tons.

Regardless, I think if I ignored them and went with just 2 tons it would have been OK but not the best. I feel good spending less for now with this electric unit, which I think will give me a much faster warm up time than the mini split would've. Also I can still put it on an ecobee smart thermostat and remotely pre-warm. If I do put a split in down the road, this will be able to remain and be used as supplemental heat.

 

[mike93lx]

They looked at it in person. I just had a local HVAC place give me a free quote and a guy put the specs into an app on his iPad. Honestly it wasn't anything more advanced than the free online manual J calculator that people reference here. Is a load calc more advanced than that?

Being a ductless system with the minisplit, I think that's one of the reasons they'll oversize it. Want to try to avoid that when you have duct area to consider. Also, the big reason for the oversize requirement was to reach comfortable heat levels. For AC it was sufficient at 2 tons.

Regardless, I think if I ignored them and went with just 2 tons it would have been OK but not the best. I feel good spending less for now with this electric unit, which I think will give me a much faster warm up time than the mini split would've. Also I can still put it on an ecobee smart thermostat and remotely pre-warm. If I do put a split in down the road, this will be able to remain and be used as supplemental heat.

Yes, a real load calc is involved.

Dimensions, insulation, exposure, windows, adjoining spaces, ductwork, etc. Took me a couple hours to pull everything for my house together and I had plans to work off off