Sizing mini split for my new garage with cathedral ceiling

[DEllis]

I'm looking for some help sizing a mini split for my new garage. I have an ongoing build thread here if people are interested. Construction is now underway after almost a year of planning.

https://www.garagejournal.com/forum/showthread.php?t=442443

Garage internal dimensions are 24' deep by 23' wide. The ceiling is cathedral with R-38 batts and the walls will be 2x6 with R-19 batts.

The interior will be drywall finished.

Below is a 3D Diagram of the space.
* The height of the wall is a 9'5" at the door and 15' at the peak.
DEllis-Diag-Car-up-wpfc by duncanrellis, on Flickr

Screen Shot 2020-02-02 at 5.02.14 PM by duncanrellis, on Flickr

Other notes:
* The garage door faces East. There are no windows or doors on the southern end of the building.
* The garage door is 10' x 8' insulated.
* Two 36" wide insulated exterior doors.
* One 36" x 36" double pane insulated window.
* Floor is 4" concrete with foam insulation on the foundation wall and 1 ft of interior foam on the perimeter of the slab.

I've looked at a couple of sizing charts, but the cathedral ceiling complicates it for me. I've been looking at sizes between 18,000 BTU and 24,000 BTU.

I live in SE Pennsylvania, which I believe is in climate zone 5. Winters here rarely drop below 0F and the summers sometimes creep into the 90s.

My plan is to condition this space year round so that my tools don't rust.

Here's one unit I was considering.
https://www.acwholesalers.com/LG-LS180HSV5/p81405.html

And another.
https://www.acwholesalers.com/LG-LS243HLV3/p106690.html

 

[ticklechicken]

You're in the right range with the 18k and 24k units. For the small cost difference, I would definitely pick the 24k. That size will pay off when you pull in a hot car on a summer day.

 

[nadogail]

May you have good results from your labors.

 

[Black Oak]

Without knowing how the LG units stack up in heat mode , I wonder if you have any other heat source . Some are better than others . On the AC side of things , 18k - 24k will do great . Decide your heat requirements and keep that in mind ,as you do research . Some of the low end units don't heat real well .

 

[SALIV8]

I would be wary of buying LG.

There are horror stories regarding parts availability and more.

 

[fastevo9]

i purchased 1 pioneer unit and 2 mr cool units. the mr cool units are amazing and cheap.
my garage is 21x23 and i use an 18k btu in hot nyc summers. you can order direct from home depot and get the unit on sale for around $720 bucks. the unit comes with a 15 foot line set. if you need a longer line set then you may have to look elsewhere.

 

[Jackfre]

I would not go over a 15kbtu unit for that size space. I would place it to the rt of the man door blowing toward the large door.

 

[dcg9381]

I'm surprised with the 18k-24k rating. I'm in Texas. I'm cooling 2400 sqft with 2 x 24K Daikins. My ceiling walls are 16', but my insulation isn't as good (R15) and I have 3 big doors that aren't very insulated.

We did do a 600 sqft boat house with an 18k unit that really wasn't enough - however, it wasn't insulated and basically leaked through the floor... Fixing it involved installing insulation.

Here's what I don't know - my main function is cooling. Heating is not an issue. Perhaps your calculations are based on being able to heat that space with those high ceilings? If so, I don't have any experience with it - but I would caution you too look at performance curves of the units you are considering in "heat" mode. Heat pump efficiency has gotten much better, but it's not at all unusual to see units drop by 50% BTU capacity as temps dip below freezing... Note, there are "hyper heat" models designed for cold climates.

My advice for "go head and over-BTU" in regard to ductless splits has been criticized on this forum.. That's OK, but I continue to sorta give that advice. As these units have inverter technology and variable speed fans, I'd rather put one in that is too large versus one that is too small. I'd not see the same of traditional ducted HVAC.

 

[DEllis]

Had an HVAC company out last week because we're also converting an attic and wanted to discuss some options. They suggested that a two head unit might be better with some ceiling fans to ensure heat is not trapped up in the ceiling.

Duncan

 

[DEllis]

Had an HVAC company out last week because we're also converting an attic and wanted to discuss some options. They suggested that a two head unit might be better with some ceiling fans to ensure heat is not trapped up in the ceiling.

Duncan

Got the quote back from this company.

1. One Mitsubishi MXZ-3C30NAHZ2 – 30,000 BTU hyper heat / heat pump outdoor unit
2. Two Mitsubishi MSZ-GL15NA – 15,000 BTU indoor wall mounted unit

$10,916

 

[SALIV8]

Wooohhh

That’s why I diy’ed mine.

 

[DEllis]

Wooohhh

That’s why I diy’ed mine.

I edited the post to lower the price a bit. The contractor emailed back and said they'd accidentally added an extra outdoor unit into the pricing system.

 

[yeldogt]

Did they do a load for the spaces ? How big is the attic ?

That's in the ball part for that equipment ... I installed the same unit outside in two different houses with 3 x 12BTU heads. 13k or so

It's important to have a load done and match the outputs ... especially with the multi heads. As you see above I have 36k BTU on a 30BTU unit.

Nothing out there will match that equipment -- it's the best. Hyper heat compressor and the duel split head with eye. If you felt energetic you could always place a DIY unit in the garage and have the other done for the attic.

With labor costs what they are -- installing cheaper equipment gets you cheaper equipment ... same labor costs for each.

 

[DEllis]

Did they do a load for the spaces ? How big is the attic ?

That's in the ball part for that equipment ... I installed the same unit outside in two different houses with 3 x 12BTU heads. 13k or so

It's important to have a load done and match the outputs ... especially with the multi heads. As you see above I have 36k BTU on a 30BTU unit.

Nothing out there will match that equipment -- it's the best. Hyper heat compressor and the duel split head with eye. If you felt energetic you could always place a DIY unit in the garage and have the other done for the attic.

With labor costs what they are -- installing cheaper equipment gets you cheaper equipment ... same labor costs for each.

There's no attic, it's one large space. The footprint of the garage is 23' deep by 24' wide. There's 2 36" insulated exterior doors, and one 10x8' insulated garage doors The space is 10' high at the lowest, and extends to 16' at the ridge.

R-38 in the ceiling and R-19 insulation in the walls. I don't know if the first contractor did a proper load calculation.

thanks,

Duncan

 

[yeldogt]

OH -- So the second head it not going in the attic you spoke of ?

You do not need 2.5 tones for a 550sf building. In PA -- how tight is the new large door.

When doing AC with high ceilings -- the goal is to cool the space under 6' .. you don't run fans and mix the hot air above into the space. That's why old houses had high ceiling .. to take the heat up.

 

[DEllis]

OH -- So the second head it not going in the attic you spoke of ?

You do not need 2.5 tones for a 550sf building. In PA -- how tight is the new large door.

When doing AC with high ceilings -- the goal is to cool the space under 6' .. you don't run fans and mix the hot air above into the space. That's why old houses had high ceiling .. to take the heat up.

Yes, it's all one space. What you said makes perfect sense. The door is a new vinyl door with R-8 panels. Sealed to the outside with weather strip.

thanks,

Duncan

 

[yeldogt]

I guess no NG and you mention the unit for heat as well ...

They may have sized it for heat ... and if you said you did not want to condition all the time you would need a larger unit.

 

[DEllis]

I'm looking to use the unit for both heat and cooling. I plan to condition the space year round, although maybe not at the same level as the house.

Duncan

 

[DEllis]

Mitsubishi contractor number two is recommending an 18k hyper heat unit

 

[yeldogt]

With spray foam and maintaining temps in PA -- I start with 12K (1 ton) per 1k sf. Go up from there. The old school way of doing a guess is 500sf per ton of cooling -- that always resulted in oversizing unless an old building.

My guess is 18k is still oversized even with wanting needing some extra capacity -- that unit can produce almost 20k in cooling ... pull up the specifications. It's really high output in HP mode at 40 degrees for heat -- it will do the full 18k at -5.

The hypers really pump it out ...

I'm doing an 18k in the kitchen of my new place ... the turn down on the 15k is amazing. That's the lowest output. I was worried about parties and a big stove ... the fact that we don't use the place full time.

You don't need two heads in that space ..... I thought you were using the second head in the attic.

 

[DEllis]

With spray foam and maintaining temps in PA -- I start with 12K (1 ton) per 1k sf. Go up from there. The old school way of doing a guess is 500sf per ton of cooling -- that always resulted in oversizing unless an old building.

My guess is 18k is still oversized even with wanting needing some extra capacity -- that unit can produce almost 20k in cooling ... pull up the specifications. It's really high output in HP mode at 40 degrees for heat -- it will do the full 18k at -5.

The hypers really pump it out ...

I'm doing an 18k in the kitchen of my new place ... the turn down on the 15k is amazing. That's the lowest output. I was worried about parties and a big stove ... the fact that we don't use the place full time.

You don't need two heads in that space ..... I thought you were using the second head in the attic.

Thanks for such a detailed reply. It’s been really helpful.

 

[yeldogt]

Thanks for such a detailed reply. It’s been really helpful.

Have you thought of spray foam ?..... more $$ to insulate ...less to heat and cool and smaller equipment.

I have a 1600sf spray foam building that is not taxing an 18k ....but -- it's very tight... like a thermos.

What happens as you do better insulation -- the building holds it's temp longer and you end up maintaining a more stable temp ... so you actually need smaller equipment.

If you look at the Mitsubishi specifications -- the 15 and 18 have outputs that are not all that different ... the 15k will go lower so it's better suited for more stable outside temps .. the 18 for slightly more delta and greater swings.

The 18 will give you more reserve ...

 

[DEllis]

Have you thought of spray foam ?..... more $$ to insulate ...less to heat and cool and smaller equipment.

I have a 1600sf spray foam building that is not taxing an 18k ....but -- it's very tight... like a thermos.

What happens as you do better insulation -- the building holds it's temp longer and you end up maintaining a more stable temp ... so you actually need smaller equipment.

If you look at the Mitsubishi specifications -- the 15 and 18 have outputs that are not all that different ... the 15k will go lower so it's better suited for more stable outside temps .. the 18 for slightly more delta and greater swings.

The 18 will give you more reserve ...

The time to make that decision (spray foam) is already past. The ceiling installation and drywall is already installed.

 

[starckie]

I'm about 30 miles south of you with a 22x21 attached garage with almost 10ft ceilings and an east facing door and I'd also recommend a Mr Cool DIY.

I have an 18k unit and have it set to 72 degrees 24/7 and have no problems at all maintaining that temp. And this is in a garage built in the 70s with leaky single pane windows and poorly insulated (almost no insulation in the attic above and likely minimal in the side walls) and the indoor unit not optimally placed. Impact to the electric bill has been minimal and I have the 2nd gen version which is rated lower seer than the new ones.

Within reason it would be hard to oversize one of these units. On a traditional heat pump because they are on/off an oversized unit might hit desired temp before the humidity is removed. On the MR Cool and other inverter style mini splits it will continue to run at a lower speed maintaining temp but removing the humidity. Also agree with using one indoor unit - it is plenty powerful to launch the air 20ft across the garage

For the price difference and the vaulted ceilings I would recommend the 24k. If you can drill a hole and use a wrench you can do it yourself and save a ton of $. I believe Ingrams Water and Air in Kentucky is the distributor for these units (the bracket i ordered through HD came direct from Ingrams) Give them a call or chat with them on their website - my previous sales person was Doug.

 

[DEllis]

HVAC company two which I think actually ran a load calculation came back with a recommendation for an 18K hyper heat unit.

MSZ-FH18 / MUZ-FH18

~7,500 for the install.

 

[justinjoyal]

No more than 18k for sure !

Whoever recommended 2.5ton of cooling lost it...

Btw, I would not run ceiling fans when cooling. Let the heat get trapped up there, no need to cool it.

Oh and 7500$ for a 18k single zone thats nuts!

 

[Yankeefarmer]

In Carlisle, PA, just like here in New England, your sizing should/will be dictated by heating requirements, not cooling requirements. Contractors, rightfully so, I think, will not want to risk getting a call from you on a zero degree day saying your building is too cold. And these inverter units turn down pretty well.

If your mini split will be your only heat source, err on the larger side. And plan to install a ceiling fan to redirect hot air downward when heating. We use that concept in our our cathedral ceiling’s great room in conjunction with a wood stove and it is very effective.

FWIW, my 900 sq ft old shop is poorly insulated and my 18k MRCOOL Advantage (non-hyperheat) is adequate heat when the outside temp is above 30-35 F. It loafs along in cooling mode on 90 plus degree days.

 

[DEllis]

No more than 18k for sure !

Whoever recommended 2.5ton of cooling lost it...

Btw, I would not run ceiling fans when cooling. Let the heat get trapped up there, no need to cool it.

Oh and 7500$ for a 18k single zone thats nuts!

I thought so too. After receiving three quotes, two from Mitsubishi "Diamond Star" recommendations I'm amazed at how all over the place they are.

 

[yeldogt]

Got it -- no spray foam .....

Don't be surprised as you go through the process. This is a common problem ..... prices all over the place and three different recommendations ... sometimes more. Often the first quote will be matching what is in the house and the next will be some guess. People are busy now ... and the week coming up they will be more so.

Installs are all over the place ..... did you make some provisions for the unit when you built? electric .... placement of the compressor. Running electric from far away can add tot he cost .... distance from compressor to head .. etc...

Do you have natural gas -- how is the house heated?

In PA there is a need to size for heat .. so that my be driving the size. It is hard to figure in some spaces ... a leaking door or poorly insulated one can change everything.

7.5k seems high .... is there something difficult about the install ?

 

[DEllis]

Got it -- no spray foam .....

Don't be surprised as you go through the process. This is a common problem ..... prices all over the place and three different recommendations ... sometimes more. Often the first quote will be matching what is in the house and the next will be some guess. People are busy now ... and the week coming up they will be more so.

Installs are all over the place ..... did you make some provisions for the unit when you built? electric .... placement of the compressor. Running electric from far away can add tot he cost .... distance from compressor to head .. etc...

Do you have natural gas -- how is the house heated?

In PA there is a need to size for heat .. so that my be driving the size. It is hard to figure in some spaces ... a leaking door or poorly insulated one can change everything.

7.5k seems high .... is there something difficult about the install ?

There is a 100 amp sub panel in the garage 50 ft away so the electric should be pretty simple. New construction so I think the install is straight forward.

The heat unit and compressor can go on the same wall. No obstructions

We are electric only so this unit needs to be the sole source of heat.

I’m thinking about asking the first contractor that recommended the dual head install if they could quote an 18k unit

 

[yeldogt]

I'm doing three units in my new place -- all internal as the building goes up. I'm above the 5k per that is typical .. but they are difficult installs.

The 18k Mitsubishi hyper w/ pan heater is not cheap --- it's also the largest available with the split outputs and eye. 7.5 sounds high to me.

It's a shame you did not run power to the location when you built -- or even had the lines run in the walls But -- if it's a head on the wall out and down to the compressor. Even at 3500 for all the parts ... that's 4k to install.

 

[theoldwizard1]

I would want two air handlers AND a couple of fans !

 

[justinjoyal]

The place is only 25x24... Absolutely no need for multiple heads.

Ceiling fans for heating season only.

 

[Bobthetractor]

Unless you are expecting the doors to be opening multiple times a day or for long periods there is no reason to oversize units. I remember hot and humid days up in philly so you don’t want to short cycle and push up humidity in there. Also, while not the main purpose if you’re running grinders and such you want the air to get filtered more often.

 

[DEllis]

I wrote the contractor that supplied the quote for $7.5K and asked him to provide a more detailed breakdown of his price, citing concerns about what I estimated for the cost of equipment plus the relative ease of this installation (sub panel in same space, unobstructed mourning area, open stud walls)

Have requested a few more quotes.

If I can't get something a little more reasonable, I might consider doing the install myself. I have a vacuum pump and A/C manifold that I use to charge automotive A/C systems. Given the units come pre-charged, I feel confident I could do this correctly.

My preference for the pro install was because (1) I'd rather not spend my time doing this with my work schedule and was comfortable paying someone else, and (2) I'd like to take advantage of Mitsubishi's warranty after the install is complete. That said, I could buy a spare unit and still be money ahead.

Thanks again for everyone's feedback.

Duncan

 

[DEllis]

The place is only 25x24... Absolutely no need for multiple heads.

Ceiling fans for heating season only.

Yes, I had them wire for two fans, and was planning to use them in the winter only.

 

[yeldogt]

I wrote the contractor that supplied the quote for $7.5K and asked him to provide a more detailed breakdown of his price, citing concerns about what I estimated for the cost of equipment plus the relative ease of this installation (sub panel in same space, unobstructed mourning area, open stud walls)

Have requested a few more quotes.

If I can't get something a little more reasonable, I might consider doing the install myself. I have a vacuum pump and A/C manifold that I use to charge automotive A/C systems. Given the units come pre-charged, I feel confident I could do this correctly.

My preference for the pro install was because (1) I'd rather not spend my time doing this with my work schedule and was comfortable paying someone else, and (2) I'd like to take advantage of Mitsubishi's warranty after the install is complete. That said, I could buy a spare unit and still be money ahead.

Thanks again for everyone's feedback.

Duncan

Obviously since this is the GJ: We are dealing with handy people ... people who like to do things themselves. Many work in a trade and know someone who works with refrigeration. So .. it's not the normal crew here.

It's also a garage .. so what you may be willing to do or spend for a home is not the same as a garage.

The project I am working on now will need additional AC ... it will have a sealed combustion wall unit for heat (propane cabinet style) ... it's in the 1600sf area. The house will not finish up until late fall .. so the outbuilding/garage is taking a back seat .. I have the heat covered for this winter. I have an unused 9k LG HP unit from another old project that will work in one part of the space .. but, I'm not sure how I will go with the other larger area.

I don't have the tools (pump/ gauges etc.) -- and frankly I'm not sure I want to deal with it. As I start the rehab of the building I may just install the LG unit and buy a Mitsubishi unit and install that one as well. Have the guys doing the contract on my house do the flares and the pump down .. even if they over charge me -- I'm still ahead. That's my thinking now .. next spring I may just want to write a check

The only issue you have is the cold performance -- you need that. I'm not sure if even the 3rd Generation Mr. Cool is full at 5 degrees. My outbuilding is at the back of my property up against preserved land ... no one can see the back of it. The two compressors will not be seen. If I did do a Mr Cool .. it would be out and down. With another heat source -- I don't need the coldest operation anyway.

Sometimes it's easier to just write a check ...but -- 7.5 k is pushing it with some other possible solutions for a garage. I get that ...

If not doing a full DIY unit and going to the trouble for a full pump down .. IMO spending the 1k for the better equipment is worth it.

 

[DEllis]

Another quote received.

$5695 for an 18K Mitsubishi Hyperheat installed.

 

[yeldogt]

Another quote received.

$5695 for an 18K Mitsubishi Hyperheat installed.

The last I checked the 16k hyper w/ pans are in the high 2k -- plus the lines and other small related items ... so low 3k's .. 3500 max.