Mini split AC in uninsulated garage workshop

[BrettS]

So I realize that this is really off topic here, but there are a lot of smart people here and I’m hoping to get some good advice. If you have any suggestions of better forums for this question I’d be happy to hear them as well.

In any case, living in Florida, I have no basement, so I’ve been using part of my garage as a workshop. I’ve recently been spending a lot of time in there working on a few projects and working in a non climate controlled workshop in florida is pretty miserable during most of the year, so I want to do something about that, and I think a mini split is probably the answer. The only thing that gives me a little concern is the fact that the exterior walls and ceiling are not insulated. The garage door is insulated with R8 fiberglass batts. The exterior walls are cinderblock and one exterior wall is north facing, so luckily it doesn’t get a lot of sun. The garage door is west facing and there is a small section of south facing wall. (The rest of the south facing wall as well as the entire east facing wall are attached to the house and are insulated).

I’m not planning to run the AC 24/7, but rather just for a few hours now and then when I’m working on a project in the garage. I realize that with the lack of insulation that this won’t be the most energy efficient thing ever, but I’m trying to figure out exactly how much of an impact it will make. I don’t want to just be throwing money out the window every time I do run the AC.

Given that the walls are cinderblock it would be quite an effort to insulate them. I’d probably have to frame something out, insulate it, then install sheet rock, which is way more work than I’m really hoping to do. I do have access to the attic above the garage, so insulating the ceiling would be less effort, but I question exactly how much it would help, given that heat rises anyway. Since I live in Florida and the garage doesn’t get cold enough in the winter to require heating I’m thinking I might be OK without insulating the ceiling.

So all that said, does anyone have any thoughts on this? Should I really go through the time and effort of fully insulating the garage just for a few hours of cooling now and then? Is it worth insulating the garage ceiling when I won’t have to worry about heating the garage at all?

At this point the temperatures are starting to fall, so I’m probably not going to be getting the mini split until spring, so I have a few months to figure things out and insulate if I must.

Thanks for your thoughts

 

[roblab]

If I thought it would save me money, I might glue up styrofoam sheets (half inch thick would do the job) over the cinderblock. Cardboard over that would extend its life, but it's a lot of effort for little return.

For no more than you'll be using it, I'd bet you could just turn on the mini's (I have two) just to dehumidify and cool it a little when you're out there. You wouldn't be doing any long spell of cooling, I'd imagine, just the amount of time you're working in the garage. That's not a lot of time.

 

[ReddyLeaf]

When our house heat pump AC went out a couple winters ago, I was financially short and needed a quick, cheaper fix than full replacement of the house AC system. Went with a 120V portable heat-AC unit. Couldn’t get one to fit into a window, so went with one on rollers similar to this: Honeywell 15,000 BTU Portable Air Conditioner with Heat and Dehumidifier in White-HJ5HESWK0 - The Home Depot

It works pretty well to cool about 400 sq ft (kitchen-dinette), with some carryover into the dining and living room. Something similar should work for a garage. It’s not a massive amount of heat/cooling that would come from a 240V minisplit, but for me it was crucial to be able to plug directly into 120V 15A circuit without requiring an electrician or permit.

 

[BrettS]

If I thought it would save me money, I might glue up styrofoam sheets (half inch thick would do the job) over the cinderblock. Cardboard over that would extend its life, but it's a lot of effort for little return.

I hadn’t considered that. The foam might get damaged a bit, given that it will be installed in the garage, but that might be an option if I decide to go the insulation route.

For no more than you'll be using it, I'd bet you could just turn on the mini's (I have two) just to dehumidify and cool it a little when you're out there. You wouldn't be doing any long spell of cooling, I'd imagine, just the amount of time you're working in the garage. That's not a lot of time.

And yeah, that’s what I was thinking. I wouldn’t be trying to run it full time. I would just need it for the time I’m working in the garage.

 

[BrettS]

When our house heat pump AC went out a couple winters ago, I was financially short and needed a quick, cheaper fix than full replacement of the house AC system. Went with a 120V portable heat-AC unit. Couldn’t get one to fit into a window, so went with one on rollers similar to this: Honeywell 15,000 BTU Portable Air Conditioner with Heat and Dehumidifier in White-HJ5HESWK0 - The Home Depot

Thanks for the reply. I had considered going the portable AC route, but in my research I discovered that they are actually pretty hugely inefficient. A mini split or a window unit has the condenser outside, so it uses outside air to cool the condenser coils. But with the portable units the condenser is inside, so it uses the air that it just cooled to cool the condenser, and perhaps worse, it then pumps the now heated air outside. But that air needs to be replaced with something, so it will wind up sucking hot humid outside air through the cracks and crevices in your house to replace all the cool air that it’s pumping outside.

You can see the inefficiency if you look closely at the BTU ratings. The unit you linked to says 15000BTU in big letters, but if you look at the specification sheet you can see that it’s only rated for 10800BTU with the SACC test. The difference is that it is a 15000BTU unit, but in the real world where it is using the cooled air to cool the condenser coils it only effectively is able to cool at the lower 10800BTU rate. So essentially you’re paying to run a 15000BTU air conditioner, but only getting 2/3 of the capacity, or 10800BTU’s out of it.

Some companies make two hose portable AC units so they can use one hose to suck in outside air to cool the condenser coils and the other hose to exhaust the warm air and that will work around this issue, but the two hose units seem to be more expensive and harder to find.

In my case, I think I’m better off with the mini split anyway. I have no window in the garage, so I will need to go through the cinderblock wall. With a portable unit I would need to cut one (or two) 5 - 6 inch holes through the wall for the vent. With the mini split I’ll just need one 2.5” hole for the line set. Additionally, the cost isn’t significantly more. A 12000BTU pioneer mini split is only $700-$800 and I’m planning to do most or maybe even all of the install myself.

It works pretty well to cool about 400 sq ft (kitchen-dinette), with some carryover into the dining and living room. Something similar should work for a garage. It’s not a massive amount of heat/cooling that would come from a 240V minisplit, but for me it was crucial to be able to plug directly into 120V 15A circuit without requiring an electrician or permit.

In my case the AC will be installed near my breaker box, so it will be easy to run a new circuit for it. They do make 120V mini split units, although I don’t believe that they have a plug… they still are expecting you to run a dedicated circuit for it.

 

[David_Cary]

Insulation is used to slow heat transfer. Florida is not that hot. Keeping an area 75 degrees when it is 90 degrees outside is not a huge insulation problem. Insulation is really helpful when it is 20 degrees out and you are trying to keep things 70 degrees inside. Not saying that insulation is worthless, it just isn't the big issue.
What you mostly need is air sealing and reducing solar load.
Air sealing a garage is very hard to do well but work on what you can.
Running a heat pump for a few hours will do very little to help. Assuming you are at steady state, the garage is 90% humidity and 100 degrees. The first few hours of run time will drop the humidity to 70% and not change the temp much at all. Then the garage door opens and the humidity jumps to 90% very quickly.
Now if you can keep the garage door closed, then you can make it comfortable. But don't have any illusions that you are able to run it for a few hours and save a lot over 24/7. Once you get the humidity down, keeping it down is not hard if you are well air sealed. Every time the garage door is opened, you will probably lose a kwh in 20 sec.
I have a mini in my primary bedroom (ducted). I did it because we like to sleep colder than the rest of the house needs to be. It is well sized - 1 ton for the suite. It can reduce temp about 1 degree per hour. Usually it is around 71 at 3pm when it starts running to get the bedroom down to 65/66 by 9 pm. Now - the doors are not air sealed from the rest of the house (and not always closed right at 3pm). Mind you, humidity is never over 55%.

 

[BrettS]

Insulation is used to slow heat transfer. Florida is not that hot. Keeping an area 75 degrees when it is 90 degrees outside is not a huge insulation problem. Insulation is really helpful when it is 20 degrees out and you are trying to keep things 70 degrees inside. Not saying that insulation is worthless, it just isn't the big issue.
What you mostly need is air sealing and reducing solar load.
Air sealing a garage is very hard to do well but work on what you can.
Running a heat pump for a few hours will do very little to help. Assuming you are at steady state, the garage is 90% humidity and 100 degrees. The first few hours of run time will drop the humidity to 70% and not change the temp much at all. Then the garage door opens and the humidity jumps to 90% very quickly.
Now if you can keep the garage door closed, then you can make it comfortable. But don't have any illusions that you are able to run it for a few hours and save a lot over 24/7. Once you get the humidity down, keeping it down is not hard if you are well air sealed. Every time the garage door is opened, you will probably lose a kwh in 20 sec.
I have a mini in my primary bedroom (ducted). I did it because we like to sleep colder than the rest of the house needs to be. It is well sized - 1 ton for the suite. It can reduce temp about 1 degree per hour. Usually it is around 71 at 3pm when it starts running to get the bedroom down to 65/66 by 9 pm. Now - the doors are not air sealed from the rest of the house (and not always closed right at 3pm). Mind you, humidity is never over 55%.

Thanks David, that was super helpful and really exactly what I was looking for without really knowing what to ask. I just had no idea what to realistically expect when I try to run the unit part time and that really gives me a good answer.

I will work on air sealing the garage. I think for the most part it’s not too bad... there is weather stripping around the big overhead door, but there are a couple of small spots where I can see light coming through, so I’ll try to get those spots sealed up. But the big issue is a 4x2 attic hatch that is just completely open. I‘ll work on getting a tight fitting cover for that.

Other than those issues the garage should be reasonably well air sealed. As I said above, there is no insulation in the ceiling, but there is drywall between the garage and the attic and it’s in good shape.

I figured that I would need to keep the overhead door closed when I was working in the garage and wanted to keep it cool.
I do regularly use the overhead door since I keep my car in the garage as well, but I usually move my car to the driveway when I’m working in the garage to give me extra space anyway, so keeping the door closed when the AC is on shouldn’t be a big problem.

I certainly wasn’t expecting to turn on the AC and have it instantly drop to 75, but it sounds like it may take more hours than I was expecting to get the temp down. Even if that’s the case, I should be able to work around it. If I know I’ll be working in the garage, then I can turn the AC on in the morning and give it time to dehumidify and keep things cool.

I also wonder if it makes sense to get a bigger AC. My garage isn’t very big… it’s a small 2 car garage at about 400 square feet. I was looking at a 12000BTU AC, which they say is typically good for up to 550 square feet. I wonder if it might make sense to set up to 18000BTU’s instead.

Thanks again for your post, that was definitely very helpful

 

[ReddyLeaf]

Thanks for the reply. I had considered going the portable AC route, but in my research I discovered that they are actually pretty hugely inefficient. A mini split or a window unit has the condenser outside, so it uses outside air to cool the condenser coils. But with the portable units the condenser is inside, so it uses the air that it just cooled to cool the condenser, and perhaps worse, it then pumps the now heated air outside. But that air needs to be replaced with something, so it will wind up sucking hot humid outside air through the cracks and crevices in your house to replace all the cool air that it’s pumping outside.

Everyone’s situation is different and I initially tried to go with the 240V minisplit, but just couldn’t make it work. I researched the portable inefficiency issue and decided that it would still work ok for me. I opened the unfinished basement window and let the heat come in where I wasn’t, and put cold air into the kitchen where it was needed. Worked great until about 105F, then needed to move it into the bedroom at night to sleep.

Some companies make two hose portable AC units so they can use one hose to suck in outside air to cool the condenser coils and the other hose to exhaust the warm air and that will work around this issue, but the two hose units seem to be more expensive and harder to find.

Yes, a friend has one of those in Seattle, Wynter brand, and likes it. It wasn’t very expensive, though prices have risen a lot. The biggest problem here is that the wildfire smoke gets sucked into the house. The year that I needed it, I got lucky, and the smoke wasn’t bad until late September (when it was cooler).

In my case, I think I’m better off with the mini split anyway. I have no window in the garage, so I will need to go through the cinderblock wall. With a portable unit I would need to cut one (or two) 5 - 6 inch holes through the wall for the vent. With the mini split I’ll just need one 2.5” hole for the line set. Additionally, the cost isn’t significantly more. A 12000BTU pioneer mini split is only $700-$800 and I’m planning to do most or maybe even all of the install myself.

In my case the AC will be installed near my breaker box, so it will be easy to run a new circuit for it. They do make 120V mini split units, although I don’t believe that they have a plug… they still are expecting you to run a dedicated circuit for it.

This is the biggest difference between our situations. Not possible for me. Installing it properly might have cost $10,000+ (Yes, I know that sounds excessive, but it’s an old house with major issues that would require 150ft trenching diagonally across the entire backyard, complete panel replacement, possible rewiring the whole house, and then the “normal” installation) and I just didn’t have the skills, money or time to deal with it. I just needed to temporarily defer major expenses until I could get the whole house heat pump replaced.

 

[BrettS]

This is the biggest difference between our situations. Not possible for me. Installing it properly might have cost $10,000+ (Yes, I know that sounds excessive, but it’s an old house with major issues that would require 150ft trenching diagonally across the entire backyard, complete panel replacement, possible rewiring the whole house, and then the “normal” installation) and I just didn’t have the skills, money or time to deal with it. I just needed to temporarily defer major expenses until I could get the whole house heat pump replaced.

Yeah, sounds like you definitely made the right decision there.

 

[Ampster]

I agree with everything others have said. With regard to the OP"s comment about not needing insulation in the ceiling I wanted to discuss that. The heat will rise but depending on the situation do not discount the effect of heat radiating from the roof. A quick inexpensive fix might just be reflective film in the attic to prevent heat radiation..

 

[BrettS]

I agree with everything others have said. With regard to the OP"s comment about not needing insulation in the ceiling I wanted to discuss that. The heat will rise but depending on the situation do not discount the effect of heat radiating from the roof. A quick inexpensive fix might just be reflective film in the attic to prevent heat radiation..

That‘s a good point. I hadn’t thought about that. It might be worth insulting the ceiling after all. I feel like that may be as much effort as reflective film.

 

[Ampster]

I feel like that may be as much effort as reflective film.

I think the material cost might be different. If you do insulate the ceiling you may want insulation with reflective foil.