Mini splits in a 40x60 pole building

[jsd245]

So I built a 40x60 pole building workshop . 14' ceilings. 2 12x12 R16 garage doors, two entry doors I put those green hinges on the garage doors so they seal pretty tight. (For a garage door) . No windows. Insulation in ceiling is blown in r38 and the wall insulation is r19 covered with 1" iso foil faced foam with all the seams taped. Slab is not radiant but has 2" of insulation under it and around the perimeter.

I decided to use 2 18k btu mini splits even though it looked like they were a bit undersized. My reasoning was it is a garage and I'm not trying to keep it 70 on the winter or 68 on the summer. And I did a fairly good insulation package.

I am in CT so we have fairly cold winters and a dozen or so really hot and humid days over 85 degrees during the summer. I figure heat on very cold days would be the biggest challenge. Not AC.

Well we've had some pretty cold days- 18-20 degrees overnight. Just above freezing during the day. These two units are having absolutely no problem keeping the garage at 65 degrees. In fact when I just had one in it seemed to be doing it fine.

The only knock is I have them mounted fairly high on the wall. I wanted to keep the mice out of them and the snow off of them. Mounted on a pole barn wall the transmit a fair bit of noise and vibration to the interior. Not really an issue for me, this is a garage, I'm not sleeping in it, and the benefits having them mounted like that particularly the mouse issue far outweighs that one minor annoyance. I've had to replace one of the units in my house because mice were living in it the winter and ruined it. They only get used for ac in the summer.

 

[PoorUB]

no comment, other than I would slide the outdoor units away from the building a s much as you can on those wall mounts. Give the air a better chance to flow around the unit.

 

[jsd245]

They are out as far as they go. Seems to be the design on the brackets. I've installed 7 of these in my two houses all of them using wall brackets (usually closer to the ground) and they all are similar. The are actually spaced out a bit more because I used the super strut to hang them.

 

[manwithtools]

That's good to hear about the performance. I'm planning on the same approach, dual 18k units for my 32 x 48 x 14. I'm going with 2" closed cell foam on the walls and 3" on the roof. We will need more cooling days than heating, but we do get some cold days / nights in the winter.

 

[jsd245]

Yeah seems it should be fine. I think heating is more challenging. It was 18 degrees last night. So my temperature differential is 47 degrees keeping the building at 65 which was no problem. Vs. a very hot 95 degree day (which is pretty much as hot as it gets here) where I'm trying keep the building at 75 degrees. Plus I'm pretty sure Heat pumps are more efficient cooling than heating.

While your theoretical R value is actually less on spray foam ( I guess that would be roughly R20 in the roof and R15 in the walls vs my R38 and R26), the far superior air sealing it will provide probably more than makes up the difference. I just couldn't swallow paying the premium for spray foam.

 

[racecougar]

Likewise, I've been nothing but thoroughly impressed by the 24k unit I installed on my building. It's nearly silent and has had no trouble heating/cooling a 30'x60'x13' building since April '23, even when the outdoor temp is on the wrong side of 0-degrees F.

Heating your garage, what's your setup.

I have a 28x32 garage with second floor via attic trusses 1) Reznor UDAS 60k btu propane 2) 18kbtu mitsubishi heat pump mini split on 1st floor 3) 9kbtu mitsubishi heat pump in attic room 4) pex embedded in concrete but have not yet gone beyond that. All works great....I can work out there...

www.garagejournal.com

 

[ericm]

This is great information. What location (roughly)?

I was planning 2 24k units for the 56x40x14 conditioned part of my shop. In southern Oregon where its rare to get below 20 degrees F but it can peak over 100 in the summer. 2x24k is probably overkill.

 

[jsd245]

I’m in eastern ct. once in a while we will get below zero overnight but usually not more than one or two days per year.

 

[TheClaw]

I'm in Chicago and planning a new 16x36 garage. I'm a little turned off by the initial costs of the mini-splits when I can go with a simple 10,000W +/- electric overhead heater and window style ac. What's the breakeven on spending the higher upfront on the mini-splits?

 

[jsd245]

I'm in Chicago and planning a new 16x36 garage. I'm a little turned off by the initial costs of the mini-splits when I can go with a simple 10,000W +/- electric overhead heater and window style ac. What's the breakeven on spending the higher upfront on the mini-splits?

No idea. But I installed them myself and am in $2700 for both. You're pulling over 40 amps on a 10000 watt heater so that's a hell of a lot of power. Plus you'll be $1100 into air conditioners for 36000 btus window units.

 

[ericm]

What's the breakeven on spending the higher upfront on the mini-splits?

It's impossible to say. It depends on how well sealed and insulated the space is, the inside and outside temperatures, and the cost of electricity. But unless your power is really cheap, or you don't heat or cool much, the mini split probably pays for itself reasonably soon.

 

[jsd245]

Plus don't forget there is a 30% tax credit on the mini splits provided you get the ones that are efficient enough.

 

[TheClaw]

Yeah, I looked at them again. I probably only need a 9,000-12,000 BTU sized unit. Those are more reasonable than I originally thought. Can likely find one in the $600 +/- range. Thanks @ericm and @jsd245

 

[American Locomotive]

Congrats on joining the few that actually understand that most people have way oversized HVAC systems! If you have decent insulation, it takes surprisingly little to heat a house.

 

[jsd245]

3 degrees here last night garage was set for 62. No issues.

 

[jsd245]

So after the first month of running this- keeping the 2400 square foot garage between 62 and 68 depending upon what I was doing there, this adds about $350 a month to my electric bill. Now CT has one of the most expensive electric rates in the country. Right now about $.31 per kw/hr so I a not real unhappy with that given the size I am heating. Yet I don't think propane would be cheaper. Propane is $3.80 a gallon right now. As a comparison it's costing me about $300 a month in propane to keep a 1500 square foot, pretty well insulated beach house at 55 degrees.

 

[racecougar]

Right now about $.31 per kw/hr...

You should be really unhappy about that part. Ooof!

Looking at my personal utility cost/usage tracking spreadsheet, the avg cost per kWh here near St. Louis over the past 24 months has been $0.122, including all taxes/fees. The average monthly bill for my all-electric 2800 sq ft house and 1800 sq ft shop over that time was just $115.59. I couldn't imagine a $350/month electric bill, not to mention a $350/month increase to whatever the existing electric bill is. Man.

 

[theoldwizard1]

First, I think you are smart installing 2 units ! Individual units are always more efficient that than one compressor running 2 air handlers.

Second, how is the heat distribution ? Do you have extra fans ?

If it is one big "room", I would have considered putting one unit on each 60' wall.

 

[theoldwizard1]

Mounted on a pole barn wall the transmit a fair bit of noise and vibration to the interior.

I wondered about that ! Anyone have a suggestion ?

The only solution I see is place a couple of 6x6 post in the ground and mount the brackets to those posts.

 

[jsd245]

You should be really unhappy about that part. Ooof!

Looking at my personal utility cost/usage tracking spreadsheet, the avg cost per kWh here near St. Louis over the past 24 months has been $0.122, including all taxes/fees. The average monthly bill for my all-electric 2800 sq ft house and 1800 sq ft shop over that time was just $115.59. I couldn't imagine a $350/month electric bill, not to mention a $350/month increase to whatever the existing electric bill is. Man.

Yeah it's CT. It's expensive but worth it (IMHO) I'll never live anywhere else. My typical bill for a 2800 square foot house that has no electric heat- oil heat (well really wood since I heat whole house with wood) but a well pump electric dryer and stove/ oven runs about $250/Month in the winter. Electric is abnormally high right now because there is a very high public benefits charge which makes up about 25% of the bill. It has to do with a deal to keep the one Nuclear Power station in CT running. Supposed to only last 10 months but we will see.

 

[jsd245]

I wondered about that ! Anyone have a suggestion ?

The only solution I see is place a couple of 6x6 post in the ground and mount the brackets to those posts.

It's a garage. My suggestion would be to ignore it. Half the time I'm running something noisy enough to need hearing protection anyhow. Ground mount would eliminate it but then you have a path for mice to get in. 6x6 post would create the same problem.

 

[jsd245]

First, I think you are smart installing 2 units ! Individual units are always more efficient that than one compressor running 2 air handlers.

Second, how is the heat distribution ? Do you have extra fans ?

If it is one big "room", I would have considered putting one unit on each 60' wall.

It's one big room. Both mounted on one wall, and I'm not running any other fans. Heat distribution is no issue. I just shot the wall with an IR between the two units and it's the same temperature as the wall across the room. I shot the ceiling it's about 3 degrees warmer than the wall chest height. These mini splits circulate the air very well. I mounted the indoor units about 9 feet up. I am putting a lean to on the other side and didn't want one of the units over there.

 

[jsd245]

First, I think you are smart installing 2 units ! Individual units are always more efficient that than one compressor running 2 air handlers.

Second, how is the heat distribution ? Do you have extra fans ?

If it is one big "room", I would have considered putting one unit on each 60' wall.

My thought with two was redundancy. If I have a problem with a condenser there is a second one running. Plus in the winter when one goes into defrost the other one is running. I think you're right on the efficiency part. Most of the multi heads didn't qualify for the tax credit. I suppose if you were paying big bucks for Mitsubishi the multi heads are efficient enough.

 

[jsd245]

Month two with some really cold nights- about the same - about 1100kw for the month. Was 2 below a couple nights ago. Not typical for CT. Garage stayed warm- so so far no issue with heating capacity.

 

[TxSteve]

You must have the 'cold rates' version of the heat pumps. That's pretty good.

I live in North Texas and have 2 24k non cold rated units in my 40x50 12' walls building. A/C is more important to me than heat. I think your building is probably better insulated then mine (2" closed cell on all walls/ceiling), but my 2 units keep it cool in the hottest days and so far - nice in the winter. We've had a couple of weeks in the low to mid 20s and they are doing ok keeping it at 62. The air they are producing isn't exactly 'hot' but it's warmer then outside ;-)

You might want to turn the thermostat down a little to save some $. I bet you won't really notice it. I started at 68 and slowly lowered it until I felt uncomfortable then put it back up a little. Once I get in the barn and start working, I'm comfortable. Plus - just put your workbench in the flow of air and you'll have warm air blowing over you while you work...

For comparison - My highest use last year was 958 kWh in August for the barn (including all hvac/welding/cutting/etc). It's in a separate meter.

As for the sound - I can't help. I put mine on the ground specifically because of others talking about the noise. I can't hear them from inside.

 

[jsd245]

Yes they are the low ambient versions - supposedly good to -15. Noise is now a non issue. One unit was making the noise- also not running quite right. It was a problem (my problem) with the installation. I inadvertently left 350psi of nitrogen in the system when I opened the charge valves to let freon in. Only realized what I did about a month later when I was questioning why it was so noisy. After recovering the bad refridgerant and weighing in a fresh charge all is good. Quiet now. It looks like there was no long term damage but we will see I guess.

 

[jsd245]

Ok so first real hot weather test. It's 91 degrees at my house now. Late afternoon (4:30) and the garage has been baking in the sun all day. The side with the two garage doors is the southern exposure so the sun hits them all day. The only units that has actually run today is the one closest to the bay doors. And it's really barely running. I can tell the other one hasn't run all day because I'd see the condensate wet spot in the dirt. As expected cooling
Is way easier and these two units which all the calculations say aren't big enough are barely running. My worst case scenario over the winter was the single digits and garage stayed at 62 no problem. So I'll call this a win.

 

[SD929]

Ok so first real hot weather test. It's 91 degrees at my house now. Late afternoon (4:30) and the garage has been baking in the sun all day. The side with the two garage doors is the southern exposure so the sun hits them all day. The only units that has actually run today is the one closest to the bay doors. And it's really barely running. I can tell the other one hasn't run all day because I'd see the condensate wet spot in the dirt. As expected cooling
Is way easier and these two units which all the calculations say aren't big enough are barely running. My worst case scenario over the winter was the single digits and garage stayed at 62 no problem. So I'll call this a win.

How is the indoor humidity when the units run so little? Do you think the combined 36k BTU is over or under sized (or both depending on season)? - I have a 40x60 going up and trying to size HVAC (I'm in a more humid environment so trying to size right right or plan for dedicated dehum)

 

[dcg9381]

How is the indoor humidity when the units run so little? Do you think the combined 36k BTU is over or under sized (or both depending on season)? - I have a 40x60 going up and trying to size HVAC (I'm in a more humid environment so trying to size right right or plan for dedicated dehum)

It's regional dude.

Look, if you split these to 2 x 18K units, there is a ton of room to split "capacity" if you want to address humidity, not even considering that most of these unit have a humidity sensing mode. Just run one.

Size of your barn is one factor. Insulation is another. Climate is another. Doors, windows, etc. You get the idea. This is why I LOVE splitting them into 2 x 18K units. I'm in TX, I split into 2 x 24K units for a 40x60 and it's hot as balls here. Steady stream of water if I'm running both.

If you need to heat, I know nothing about that other than pay attention to what you are buying. It hits 25 degrees here and my shop can't hold 55 degrees inside. That's fine for the 1 day out of every 3 years it happens. But you may want a hyper-heat unit.

 

[dscheidt]

How is the indoor humidity when the units run so little? Do you think the combined 36k BTU is over or under sized (or both depending on season)? - I have a 40x60 going up and trying to size HVAC (I'm in a more humid environment so trying to size right right or plan for dedicated dehum)

Minisplits have good turndown. I don't remember what the OP has, but for an 18K unit, the minimum output is usually around 5k btu. That's low enough that unless the unit is hugely oversized it's going to have good run times, at minimum output. That's best case for dehumidification, and also for efficiency. Many 18K units are essentially 24K units limited by software, so it's common to find 24k btu units with very similar minimum outputs. One of the benefits of splitting a system into multiple units, you can get a turndown rate that's incredible, and gets better as systems get bigger. Split a 72K BTU load, which happens to be about the heat load of my house, into four 18K units, and you get a turndown ratio of of 14 or 15, which is quite a bit better than the low temp hydronic (to replace a conventional cast iron boiler) I'm looking at a proposal for. And we could actually make that work, because it could differentially heat or cool the bedrooms at night. It would cost more, and I like radiant heat, but I wonder what the breakeven point would be. I'd also have to do something about water heating (probably a heatpump electric), which would make it cost even more.

 

[jsd245]

How is the indoor humidity when the units run so little? Do you think the combined 36k BTU is over or under sized (or both depending on season)? - I have a 40x60 going up and trying to size HVAC (I'm in a more humid environment so trying to size right right or plan for dedicated dehum)

Just fine. One unit was running at a pretty low level and a good drip of condensate coming out. Almost 100 degrees today . Not typical at all- we get one or two days a year like that. Both units running when I got home at 7:00 . But the compressors were not cranked up at all . Now - if I was opening and closing the garage doors all day long I imagine they would have to crank right up.

regarding size size id say just right, two 18k units seems perfect. But again - i insulated pretty good. Not any windows except in the two garage doors and the entry doors. Garage doors are pretty tight too with those green hinges.