Contractor undersized mini split in garage, trying to figure out solutions

[dockdoc]

We built a detached garage a couple years ago, and had the second floor as an in law suite which we recently finished off to make it livable. As part of that, we had a dual-zone mini split installed, with one head in the garage itself, and the second upstairs for the living area. It's a mitsubishi 24,000 btu with two 12,000 btu heads. Each area is a little over 600 sq ft.

With our hot summers, the upstairs unit is not keeping pace with the heat. It can get it down to about 75-76, but no lower and that's without us even occupying it yet.

The HVAC contractor is working with us on it, and wants to upsize to a 30,000 BTU system with two 15,000 heads. However, the lines for that system are bigger, so we'd have to install all new lines outside the building (the existing lines are hidden in the ceiling / walls because it was a new build).

I'm trying to figure out if there's any other solution that would make sense. Like for example, should we consider just upsizing the outdoor unit and adding a second mini split head upstairs? Like a 6000 or 9000 BTU unit? That would bring us up to, say, 18,000 BTU upstairs (more than enough) but I'm not sure if it would be too awkward having to units... would have to make sure the temp on both was the same or they'd be competing with each other, right? We also have an ecobee wired to the one head, so that would no longer be functional...

Other thoughts?


EDIT: Appreciate all the suggestions. Lots of folks commented that the insulation must be inadequate... I don't know, but even if it is, I'm not about to rip out the entire project! The HVAC co is giving me a pretty good deal on upsizing the system to a higher capacity, I think we just didn't properly account for the large open stairwell. Gonna go that route, I'll report back.

 

[justinjoyal]

What are the temperatures like during summer ? Is the building insulated ? How high are the ceilings ?

 

[mike93lx]

Is it only on the hottest days it can't keep up or is it consistently not enough?

I keep my house set at 75 and find it quite comfortable. How much cooler are you trying to get it?

How is the insulation?

 

[Mike007]

Is the space well insulated? If not, is there any way to add more? What is the outdoor temperature when you are only seeing 75-76?

 

[pcmeiners]

What are the insulation R numbers on your insulation, walls and ceilings and the minisplit model numbers?
With a 1080 sqft, 10ft ceiling, 2 garage door garage, my 12k unit has no issue at 95 degrees.

 

[dockdoc]

What are the temperatures like during summer ? Is the building insulated ? How high are the ceilings ?

We're in NC - usually 2 - 3 months of consistent highs in the 90's, with 'real feel' temps in the 100's. Lots of sun exposure. Ceilings are 8'3" at max, but it's dormer'd so there are lower areas.

Building is spray-foam insulated, professionally done.

Is it only on the hottest days it can't keep up or is it consistently not enough?

I keep my house set at 75 and find it quite comfortable. How much cooler are you trying to get it?

How is the insulation?

It's basically on the hot days, but that's really all summer for us. We're not trying to get it much below 75, but would like to be able to get it down to 72-74 when needed. My bigger concern is that 75 is the coolest it can possibly get, with zero people, no doors opening/closing, etc. When you start introducing some heat and movement, it quickly jumps up a few degrees and can't cool back down.

Is the space well insulated? If not, is there any way to add more? What is the outdoor temperature when you are only seeing 75-76?

I believe it's very well insulated, very tight envelope. No way to add more, everything's buttoned up at this point and before they did the spray foamed every last nook and cranny anyway. Outdoor temps above 90 and it can't get below 75, and again that's with zero people in the room, no electronics running etc.

Also - when the downstairs unit kicks on, that's when I notice the biggest issue - the upstairs unit must get some extra capacity when it's the only one running, but when both are running simultaneously, the temperature upstairs starts creeping up... 77, 78 degrees.

 

[housewolf]

I have/had almost identical situation. I have a Mits 18K upstairs and a 12 in a section of the garage. The unconditioned part of my garage is uninsulated. We had 6” of foam sprayed under the floor and now I can get it like an icebox even with 105+ outside temps. The upstairs is foam insulated roof and gable walls.

ETA - 750 sq ft no dormers but I wish I’d done two

 

[bluedog225]

I’d go for the easy solutions first. Home Depot Henry’s cool roof elastomeric if the aesthetics work. Will drop the heat load quite a bit.

Light exterior paint if that works.

And window tinting. Good stuff will keep out a lot of heat.

Maybe the installer will go half on a window unit for redundancy.

Seems a shame to tear out a completed install.

 

[justinjoyal]

What is the R-value of the insulation ?

Are you sure your system is performing as it should ?

 

[dockdoc]

I’d go for the easy solutions first. Home Depot Henry’s cool roof elastomeric if the aesthetics work. Will drop the heat load quite a bit.

Light exterior paint if that works.

And window tinting. Good stuff will keep out a lot of heat.

Maybe the installer will go half on a window unit for redundancy.

Seems a shame to tear out a completed install.

Thanks I'll look into those options. We do have a large section of flat / membrane roof, could I basically paint that elastomeric stuff on top of it? It's black now, so I imagine that could help a lot.

Yeah I'm reluctant as well... but also 12000 BTU just doesn't seem like enough. When I talked to the HVAC subcontractor recently he told me that if he had known what we were using the space for he would have recommended 15000 BTU for sure (slams head into wall).

What is the R-value of the insulation ?

Are you sure your system is performing as it should ?

I honestly don't know for sure, but I believe it's adequate, and regardless... no way I could possibly add more.

Yes, system has been professionally inspected and is performing at max capacity.

 

[Mike007]

If the space is truly well insulated, I'm surprised it's not performing a little better.

 

[Shiftless]

The large flat roof that is black is a fantastic solar heat collector. With no attic air space as a buffer, the few inches of foam insulation is overwhelmed with full sun in 100+ degree weather.

Changing the color seems to me like a good place to start.

 

[mike93lx]

If the space is truly well insulated, I'm surprised it's not performing a little better.

I'd agree. My 3rd floor guest suite is poorly insulated, with a large south facing roof, and the 18k ducted system is significantly oversized. Plus I am not very far north

 

[mike93lx]

Yeah I'm reluctant as well... but also 12000 BTU just doesn't seem like enough. When I talked to the HVAC subcontractor recently he told me that if he had known what we were using the space for he would have recommended 15000 BTU for sure

That sounds like a BS cop out to push some of the blame to you

 

[bluedog225]

1000% Henry’s eleastomeric on a flat roof. Great stuff.

Of the Henry’s options, get the most expensive one (but not the silicone).

Goes on easy, water based. Two coats. You can put your hand down on the roof surface on a 110 eff day. Extends the life of the flat roof.

And I agree that it sounds like the unit seems like it should work for that small space.

 

[pcmeiners]

You should find out how well insulated the attic is, doubt it is enough, doubt it is near an R40 or more. You could blow insulation in, even if you need to cut Sheetrock for an opening. Also, if you are turning the minisplit on an off, your building materials are soaking up your AC until they are fully cooled, particularly your concrete and stored tool, car etc, leave it running.
To add, temporarily the garage 12k unit is also cooling my entire attached home.
Something is not right.

 

[thunderalley3]

When I changed the size and layout of a West facing roof last year I went from black roof rubber to white and it really only covered/affected on room but I can honestly say that it reduced the temperature in that room by 10 degrees. It would always feel warmer even when the A/C was on but now it is no different than the other rooms in the house and when the A/C is not in use it get's no warmer than the rest of the house.

 

[pcmeiners]

If changing roof color gets that much temperature difference you could use more insulation.

 

[mike93lx]

If changing roof color gets that much temperature difference you could use more insulation.

It's spray foamed already. Almost certainly no room for more insulation

 

[PoorUB]

It's a mitsubishi 24,000 btu with two 12,000 btu heads. Each area is a little over 600 sq ft.

I have a hard time believing 12,000 BTU will not cool a 600 sqft area that is properly insulated.

Is this area enclosed, or can air from some other area easily migrate into. For example, a 600 sqft open loft area adjoining a cathedral ceiling in a larger room.

 

[dockdoc]

I have a hard time believing 12,000 BTU will not cool a 600 sqft area that is properly insulated.

Is this area enclosed, or can air from some other area easily migrate into. For example, a 600 sqft open loft area adjoining a cathedral ceiling in a larger room.

It's an enclosed space, except that the staircase coming up into the room is an open stairway. I.e. the room has 8' ceilings, but the staircase has a ~16' ceiling or something, the width of the stairs. So that's probably not helping.

Lots of comments about the insulation being inadequate, maybe. I sure don't know, and can't prove that it is or isn't. It was done by a professional spray foam insulation company that supposedly knows what they're doing. All I can tell you is every square inch of that envelope was spray foamed, so there's no point worrying about it because the only way to add more would be to tear out the entire job and start over... and I guess just rebuild the entire building with a different framing structure to allow for more insulation? I don't know - I appreciate the suggestions but there's simply nothing I can do about that now.

I will look into the white paint on the roof, especially if it can prolong the life of the membrane.

Other than that; I think we're just going to have them upsize the system to 15,000 BTU capacity and hope that solves the issue. I know it's a very rough estimate, but from what I've read you multiply the square footage of the room by 25 to get a rough estimate of the capacity needed. 600 sq ft * 25 = 15000 BTU/h. Doing that math backwards, 12000 BTU/h should cover around 480 sq ft. I think taking into account a sun drenched building in a hot climate we're probably operating on a thin margin, so hopefully bumping up in size will get us some more comfort. We'll see I guess, likely won't know til this time next year as it'll be cooling down soon.

 

[mike93lx]

That open staircase is a killer. All the cool air is dropping to the first floor and being replaced by any warm air on the first. I bet if you put a door at the top of the stairs, it would change everything

 

[Mr onetwo]

plus one on Mikes suggestion....door to separate the spaces.I have a white metal roof...the color makes a huge difference...paint that sucker white.12K should work fine with those changes.Get a Midea U shaped inverter AC unit for the really hot days...$399 https://www.midea.com/us/air-condit...MI5YHQir_tgAMViXxvBB07PAzBEAAYAiAAEgIXyfD_BwE

 

[justinjoyal]

It's an enclosed space, except that the staircase coming up into the room is an open stairway. I.e. the room has 8' ceilings, but the staircase has a ~16' ceiling or something, the width of the stairs. So that's probably not helping.

There you go..

Can you put a door at the bottom of the stairs ?

 

[mike93lx]

There you go..

Can you put a door at the bottom of the stairs ?

I would do the top as this is an apartment/suite. People staying there should have easy access to close off their space

 

[welder4956]

How about some pics of the attic area over the living space? I'm having a hard time envisioning how the spray foam looks up there. Is it a flat roof over the living space with sheetrock ceiling and spray foam filling the cavity between the sheetrock and the roof?

 

[dockdoc]

That open staircase is a killer. All the cool air is dropping to the first floor and being replaced by any warm air on the first. I bet if you put a door at the top of the stairs, it would change everything

Yeah I think you're right, and this is probably underrated... We really can't add a door at the top without doing some significant renovations. There's a half wall at the top of the stairs, so that would have to be closed off, which would block a window... would take a lot of thought and work. One easy thing to do is put some blinds up on the windows at the bottom of the stairs - they're west facing and bring in a ton of afternoon sun, so just reducing that might help a bit.

How about some pics of the attic area over the living space? I'm having a hard time envisioning how the spray foam looks up there. Is it a flat roof over the living space with sheetrock ceiling and spray foam filling the cavity between the sheetrock and the roof?

I'll throw some pics on here later but what you said is basically right. It's a gabled roof with a dormer on each side. In the middle of the room there is a little void above the sheet rock that's got foam insulation, and as you get out to the dormer ends there is basically foam sandwiched between sheet rock and the roof sheathing.

I will say I've measured the temperature of the ceiling in various areas and it's not particularly warm even on hot days.

 

[Mike007]

How about sealing the door opening with some tape and some plastic to see if it would resolve your cooling issue? If it does you will at least know.

 

[mike93lx]

Yeah I think you're right, and this is probably underrated... We really can't add a door at the top without doing some significant renovations. There's a half wall at the top of the stairs, so that would have to be closed off, which would block a window... would take a lot of thought and work. One easy thing to do is put some blinds up on the windows at the bottom of the stairs - they're west facing and bring in a ton of afternoon sun, so just reducing that might help a bit.



I'll throw some pics on here later but what you said is basically right. It's a gabled roof with a dormer on each side. In the middle of the room there is a little void above the sheet rock that's got foam insulation, and as you get out to the dormer ends there is basically foam sandwiched between sheet rock and the roof sheathing.

I will say I've measured the temperature of the ceiling in various areas and it's not particularly warm even on hot days.

I use blackout cellular shades in our bedrooms and they make a huge difference. Even light filtering or room darkening should help too

 

[PoorUB]

That open staircase is a killer. All the cool air is dropping to the first floor and being replaced by any warm air on the first. I bet if you put a door at the top of the stairs, it would change everything

Yeah I think you're right, and this is probably underrated... We really can't add a door at the top without doing some significant renovations. There's a half wall at the top of the stairs, so that would have to be closed off, which would block a window... would take a lot of thought and work. One easy thing to do is put some blinds up on the windows at the bottom of the stairs - they're west facing and bring in a ton of afternoon sun, so just reducing that might help a bit.

The cool air falls down the stairs and the warm air from the other area comes up. You are not cooling 600 sqft, you are cooling the adjoining area too. I question if 15,000 BTU will cool it.

Curtains or shades on the windows help a bunch. We have a west facing living room window. On a sunny day, in the afternoon with the curtains open the room gains heat like crazy. It can be 15 degrees warmer than the rest of the house. We try close the curtain in the afternoon.

Another thing that will help is a cloth awning over the windows out side. It will keep the afternoon sun from blazing in.

 

[txvwnut]

Even after you close off the upstairs your still a little light on BTU's. My shop is the same SQFT as yours and 36,000 BTU is what my calculator came up with.

 

[mike93lx]

Even after you close off the upstairs your still a little light on BTU's. My shop is the same SQFT as yours and 36,000 BTU is what my calculator came up with.

The entire first floor of my house doesnt need 36k btu. If you need that much for 600 ft, you are completely uninsulated, in a wildly hot climate, or doing the calc wrong.

 

[PugetDude]

Put a heavy drapery across the door opening at the top of the stairs. I agree that the stairs are operating as a thermal chimney.

 

[txvwnut]

The entire first floor of my house doesnt need 36k btu. If you need that much for 600 ft, you are completely uninsulated, in a wildly hot climate, or doing the calc wrong.

36,000 BTU is for the total combined of both floors. In my case 18,000 BTU for each floor. The OP was quoted a 24,000 BTU unit for both floors of his dwelling and 12,000 BTU for each floor.

 

[dockdoc]

The entire first floor of my house doesnt need 36k btu. If you need that much for 600 ft, you are completely uninsulated, in a wildly hot climate, or doing the calc wrong.

I think he might be saying that the total building is 36k btu? If you do the basic math of 25 sq ft / BTU, that's 30000 BTU for 12000 sq ft. So still high but not crazy (I realize that math is very rough and not a load calc).

 

[mike93lx]

36,000 BTU is for the total combined of both floors. In my case 18,000 BTU for each floor. The OP was quoted a 24,000 BTU unit for both floors of his dwelling and 12,000 BTU for each floor.

My bad. Misinterpreted that.

 

[Jackfre]

Add a 9 or 12 single head if you have the panel space. It would be about double the seer rating of the multi. Taht way the contractor doesn’t have to screw around with the recovery, etc and you end up with redundancy.

 

[PoorUB]

Even after you close off the upstairs your still a little light on BTU's. My shop is the same SQFT as yours and 36,000 BTU is what my calculator came up with.

The 24,000 BTU he has should have been fine if there was no air migration from other areas. Two 600 sqft areas shouldn't need any more than a total of 18,000 BTU in most cases. When I was sizing and installing HVAC and mini splits in most cases I would a have put a 12,000 BTU head in that space without hesitation, but I would have asked a few questions about room usage.

If you have two floors of 600 sqft each I am certain it cools it easily! I would have pt maybe 24,000 BTU in your situation. I have 18,000 BTU in a 900 sqft shop and it will cool it down in a couple hours on a 95F day, or 20 BTU per sqft.

 

[Noltz]

That open staircase is a killer. All the cool air is dropping to the first floor and being replaced by any warm air on the first. I bet if you put a door at the top of the stairs, it would change everything

Agreed. Our lunch room at work (flat roof, similar layout to OP) has a 12k window unit and about 800 sq ft. Old building, limited AC, and the window unit can struggle on the hottest days. But if we close the door upstairs it can drop from high 70's to low 70's in a few hours. Put a door on that staircase.
Edit; I'd also check out "Reflective flat roof". There's a few options for reflecting the heat before you have to deal with it.

 

[theoldwizard1]

Short of replacing EVERYTHING, disconnect the unit upstairs. Install a SECOND compressor and new air handler upstairs.

Blocking the stairwell will help a lot.

EDIT : Move the second air handler downstairs, as far away as possible from the first. Most modern systems have a way to "slave" to units together.