Splitting headache from Mini-split sizing

[G-ManBart]

I know this thread is like so many before it, and I read a number of similar threads, but I'm still undecided, so I apologize for starting yet another mini-split thread.

I have a 40x64 pole barn (metal roof and siding) with a 12' lean to off the back (long side). I've built a partition wall to create a 24x40' shop that will be heated and cooled. The slab under the shop has 2" foam board insulation, and 900' of Pex tubing in three loops. The three exterior walls are getting R19 fiberglass with liner panels over the top. The ceiling has steel liner panels, a vapor barrier and then fiberglass insulation that's fairly thin (4' wide, 32ft rolls that made installation easy) with no specified R value, but probably around R11 or so. I plan to put enough blown insulation over the top to add R25 to the existing insulation. The partition wall is R13 fiberglass with 1/2" OSB on the far side. I haven't installed the interior overhead door, but it will also be steel on both sides, and insulated at least as well as the exterior OH door.

So, that leaves me with a 24x40' shop with 13'4" ceilings. There are three exposed walls facing North, West and South, and all three have low E windows (36x36"). The West wall has an insulated 10Hx12W overhead door...steel on both sides, R13 give or take.

It will be a while before I get to the radiant heat, so I thought I'd get a heat pump unit that will at least give me some heat until I get the radiant set up, and for times in the Fall and Spring where I don't have the radiant heat on, but get a cool day where a little heat would be nice.

My plan is go go with a single zone mini-split, but the online calculators are giving me wildly different cooling requirements....from under 10K Btu up to nearly 30K Btu. I'm leaning towards 24,000Btu, but wonder if 30K might be smarter? I'm looking at Mitsubishi units, so they're all inverter models that should prevent me from worrying about having too much capacity leading to inefficiency.

I'm also stuck going back and forth between the P series and M series. I plan to use radiant for when it's really cold, but it's nice to have a backup just in case something happens, and it regularly gets well below zero here in the Winter.

For reference, we've had stretches here in the Summer where it was well over 100*F every day for days on end, so it's not nearly as cool as many people might assume.

Thoughts?

Since we all love pics...here's one after hanging the ceiling and getting the lights back up. I took it to show the lighting in the middle of the night. The second was recently when I sheeted the other side of the wall, and the third was when I started on the interior walls.

 

[patrickoneal]

I'm no expert, but 24k sounds about right for cooling. Being that mini splits don't typically have electric backup heat, and capacity drops as the outdoor temperature lowers, you should probably size it for the heating load. I think you'll need more BTUs for heat than cooling, and an inverter unit will slow down to lower capacity when you don't need it running full blast. The 30k M-Series drops to 9800 BTUs in low speed cooling mode, so you'd be in no danger of overcooling and failing to dehumidify. If I were you, I'd go with the 30k if you've calculated 30k BTUs to be enough in the winter. I can't imagine the cost is much more than the 24k, and with an inverter there's really no disadvantage to a small amount of overcapacity other than initial cost.

You'll also be able to heat up or cool off the shop faster with a larger unit, if you only heat or cool sporadically when you're working out there instead of maintaining it year round.

The only other thing I'd suggest is to blow in more insulation than you mentioned. You said R-25 over R-11 for R-36 total, but the R-11 is just a guess. It might be better, might be worse. Blown insulation is cheap and easy, may as well get it up to R49 if you have the room for it. Cellulose has a higher R-Value per inch than fiberglass by a good margin, so if headroom is a problem you can get more insulation value in the same space with cellulose. Cellulose is a good deal heavier than fiberglass, so that's a concern if your ceiling isn't constructed to hold much weight.

 

[Fueler]

Know what you mean about the calculators. I think most of them are aimed at decently insulated homes w 8 ft ceilings.

Anyway,
FWIW I am cooling very nicely a 1400 sq ft, 12ft Open ceiling pole barn with 32000 btu mini split. I have machines running during the day. I am also no where near as well insulated as your place. Maybe later this year to complete my roof line upgrade.

Took it down to 68 yesterday but the nice part the humidity inside was at 52%. Outside it was 82 with 95% due to more rain coming and going.

Weather this year has been soggy and cooler than normal.
I am waiting for the worst of the worst of our normal gagging humidity to hit for a serious test.

Despite your smaller sq foot I would go with 24k as a minimum. 3 ton would be my preference though. It seems to me, right or wrong, you can't over do it for a large shop area that work is being done in. If you just sit and admire toys then it won't take much of a unit to keep up.

Oh yeah, definitely get the heat pump version. It dropped to 65 last night. I had mine on automatic. When I came in this morning it was spitting out some heat. Check the thermometer. Though I didn't need it to be it was holding it at 70 degrees. Nice to know that works. It is a nice day so I shut it down.

I have tube radiant that kicks **** but if this heat pump deal helps knock down my LP bill it's a win/win.

 

[zmaxmotorsports]

Id go with a 2t with a cuple of heads myself.

 

[G-ManBart]

I'm no expert, but 24k sounds about right for cooling. Being that mini splits don't typically have electric backup heat, and capacity drops as the outdoor temperature lowers, you should probably size it for the heating load. I think you'll need more BTUs for heat than cooling, and an inverter unit will slow down to lower capacity when you don't need it running full blast. The 30k M-Series drops to 9800 BTUs in low speed cooling mode, so you'd be in no danger of overcooling and failing to dehumidify. If I were you, I'd go with the 30k if you've calculated 30k BTUs to be enough in the winter. I can't imagine the cost is much more than the 24k, and with an inverter there's really no disadvantage to a small amount of overcapacity other than initial cost.

You'll also be able to heat up or cool off the shop faster with a larger unit, if you only heat or cool sporadically when you're working out there instead of maintaining it year round.

The only other thing I'd suggest is to blow in more insulation than you mentioned. You said R-25 over R-11 for R-36 total, but the R-11 is just a guess. It might be better, might be worse. Blown insulation is cheap and easy, may as well get it up to R49 if you have the room for it. Cellulose has a higher R-Value per inch than fiberglass by a good margin, so if headroom is a problem you can get more insulation value in the same space with cellulose. Cellulose is a good deal heavier than fiberglass, so that's a concern if your ceiling isn't constructed to hold much weight.

Good points....I'm glad I'm not the only one that thinks 30K or even 36K isn't unreasonable.

I'm not overly worried about weight with the steel ceiling, so you're right...won't hurt to go thicker on the insulation.

 

[encantofred]

i have a genie scissors lift in my garage as well. it was one of the best things i bought....

i use it all the time. i have a 20' ceiling.

btw, i did r30 in my 2x8 walls (then osb sheathing, 1" foam and stucco) and r38 in the ceiling blown in fiberglass and without any conditioning, it stays high 80's here in the summer of az when it is 115 outside.

tom

 

[G-ManBart]

Know what you mean about the calculators. I think most of them are aimed at decently insulated homes w 8 ft ceilings.

Anyway,
FWIW I am cooling very nicely a 1400 sq ft, 12ft Open ceiling pole barn with 32000 btu mini split. I have machines running during the day. I am also no where near as well insulated as your place. Maybe later this year to complete my roof line upgrade.

Took it down to 68 yesterday but the nice part the humidity inside was at 52%. Outside it was 82 with 95% due to more rain coming and going.

Weather this year has been soggy and cooler than normal.
I am waiting for the worst of the worst of our normal gagging humidity to hit for a serious test.

Despite your smaller sq foot I would go with 24k as a minimum. 3 ton would be my preference though. It seems to me, right or wrong, you can't over do it for a large shop area that work is being done in. If you just sit and admire toys then it won't take much of a unit to keep up.

Oh yeah, definitely get the heat pump version. It dropped to 65 last night. I had mine on automatic. When I came in this morning it was spitting out some heat. Check the thermometer. Though I didn't need it to be it was holding it at 70 degrees. Nice to know that works. It is a nice day so I shut it down.

I have tube radiant that kicks **** but if this heat pump deal helps knock down my LP bill it's a win/win.

That sounds like a good setup. For a few years I won't be working in there every day, but ultimately I plan to spend many hours per day working in there after I retire. That's why I'm thinking the extra money spent on insulation, and a better mini-split makes sense. I think I'll be running the radiant heat full time once it gets to late Fall, but before that, the heat pump would be enough to keep it reasonable, as well as in late Spring after the real cold is done.

 

[G-ManBart]

i have a genie scissors lift in my garage as well. it was one of the best things i bought....

i use it all the time. i have a 20' ceiling.

btw, i did r30 in my 2x8 walls (then osb sheathing, 1" foam and stucco) and r38 in the ceiling blown in fiberglass and without any conditioning, it stays high 80's here in the summer of az when it is 115 outside.

tom

You won't believe what I paid for the Genie 2646...$1K from my neighbor, and everything works! I did have to put a couple of batteries in it, but I just used two 100AH 12V batteries in series to get the 24V....runs all day no problem. It's the best $1K I've spent in ages, and I already have three standing offers to buy it when I'm done with the barn

 

[bzinsky]

The 30k M-Series drops to 9800 BTUs in low speed cooling mode, so you'd be in no danger of overcooling and failing to dehumidify.

this is precisely what is happening to my mini splits, I have 9k btu units cooling 600sq ft apartments. No matter how hot it is, they never seem to blow cold air, they just run the fan all day long, and they are maintaining 74 degrees all day everyday. No insulation on the exterior walls, except block and a few layers of this and that which adds up to r12.

Infact, I first installed the splits on the 3rd floor, which is 6 mini splits. Those 6 mini splits kept the entire 12k sq ft building cool, defy's logic.

I have guys leaving doors and windows open constantly while work is being done. It still doesn't phase the units.

As for my advice to the thread, I'd go with one high efficiency 9-12k unit. They seem to be the more efficient compared to the bigger units. I'd be very interested to see the limits of a 9k unit, and judging from what I've seen it could handle a much bigger space than yours.

disclaimer - I am far from an authority on this subject, these splits are the only ones I have ever used.

 

[Jackfre]

this is precisely what is happening to my mini splits, I have 9k btu units cooling 600sq ft apartments. No matter how hot it is, they never seem to blow cold air, they just run the fan all day long, and they are maintaining 74 degrees all day everyday. No insulation on the exterior walls, except block and a few layers of this and that which adds up to r12.

Infact, I first installed the splits on the 3rd floor, which is 6 mini splits. Those 6 mini splits kept the entire 12k sq ft building cool, defy's logic.

I have guys leaving doors and windows open constantly while work is being done. It still doesn't phase the units.

As for my advice to the thread, I'd go with one high efficiency 9-12k unit. They seem to be the more efficient compared to the bigger units. I'd be very interested to see the limits of a 9k unit, and judging from what I've seen it could handle a much bigger space than yours.

disclaimer - I am far from an authority on this subject, these splits are the only ones I have ever used.

BZ, I would strongly discourage use of these during construction. Things will get so crapped up you will have difficulty getting them cleaned out. Pull the disconnect and let the work continue.

 

[Zeke]

I don't know jack squat about A/C but that's one hell of a nice building!

 

[Trey T]

Couldn't help you, man, but are you using non-PT lumber at the base of the wood frame? If so, that's a big no-no; you're just asking for termite trouble especially if there's no barrier b/t the concrete and the pine.

 

[G-ManBart]

Couldn't help you, man, but are you using non-PT lumber at the base of the wood frame? If so, that's a big no-no; you're just asking for termite trouble especially if there's no barrier b/t the concrete and the pine.

Everything that touches the concrete is all PT. The non-PT boards you see are just girts (some people call them purlins, but I was told that is really the term for roof supports) for the metal panels, and are above the concrete foundation.

The posts are all PT and they connect to the foundation with Perma-Column brand Sturdi-Wall brackets. The horizontal 2x4s are raised up just a bit over the foundation as you can see in the picture below. This was before I added the internal girts for the inside steel panels:

 

[G-ManBart]

I don't know jack squat about A/C but that's one hell of a nice building!

Thanks...I had a couple of years to think about it before we could start building. We had to wait for the bank to repossess the house next door so we could buy it, knock it down, combine lots, then start building. I'd do some things differently next time, but so far I'm still pretty happy...it's getting there!

 

[patrickoneal]

this is precisely what is happening to my mini splits, I have 9k btu units cooling 600sq ft apartments. No matter how hot it is, they never seem to blow cold air, they just run the fan all day long, and they are maintaining 74 degrees all day everyday. No insulation on the exterior walls, except block and a few layers of this and that which adds up to r12.

Infact, I first installed the splits on the 3rd floor, which is 6 mini splits. Those 6 mini splits kept the entire 12k sq ft building cool, defy's logic.

I have guys leaving doors and windows open constantly while work is being done. It still doesn't phase the units.

As for my advice to the thread, I'd go with one high efficiency 9-12k unit. They seem to be the more efficient compared to the bigger units. I'd be very interested to see the limits of a 9k unit, and judging from what I've seen it could handle a much bigger space than yours.

disclaimer - I am far from an authority on this subject, these splits are the only ones I have ever used.

I wouldn't expect a 9 or 12k to heat or cool a space as large as his very well, especially with the high ceilings, or in low temperatures.

I think they all run the indoor blower constantly to stir the air to get an accurate reading for the thermostat.

 

[LS6 Tommy]

Since you're looking at inverter heat pump units, I'd go bigger than 2 ton. I have (2) 1.5 ton cooling only Fujitsus. One for my finished 500 square foot basement and one for my 600 square foot first floor. Your space is larger with a much taller ceiling,

Tommy

 

[G-ManBart]

Since you're looking at inverter heat pump units, I'd go bigger than 2 ton. I have (2) 1.5 ton cooling only Fujitsus. One for my finished 500 square foot basement and one for my 600 square foot first floor. Your space is larger with a much taller ceiling,

Tommy

Thanks. Based on some of the responses here, I'm thinking of a 30K unit now. You have 30K for 1,100 square feet with lower ceilings, so I'm thinking 30K for 950 square feet with higher ceilings might be reasonable. I'm definitely going with more insulation in the attic now.

I'm originally from northern NJ as well...went to HS in Sparta.

 

[thammel]

Be careful about oversizing. If too large, you will not get adequate dehumidification. Larger will get it cold, but won't necessarily get it dry. You don't want that to be the case.

Tom

 

[patrickoneal]

Be careful about oversizing. If too large, you will not get adequate dehumidification. Larger will get it cold, but won't necessarily get it dry. You don't want that to be the case.

Tom

He's looking at inverter units. The 30k Fujitsu M-Series he's looking at has a 9800 BTU capacity at low speed. Oversizing an inverter system isn't an issue unless it's grossly oversized to the point where it's too big even at low speed. The only disadvantage is if one were to spend more money for the extra capacity and the unit never runs at full speed. I had to do a lot of research, but from what I understand the efficiency actually increases at low speed as well... ie an 18 SEER 30k unit running at 12K uses less power than an 18 SEER 12K unit running at full speed. I'm still not 100% sure of that fact, as the information is hard to find.

 

[LS6 Tommy]

He's looking at inverter units. The 30k Fujitsu M-Series he's looking at has a 9800 BTU capacity at low speed. Oversizing an inverter system isn't an issue unless it's grossly oversized to the point where it's too big even at low speed. The only disadvantage is if one were to spend more money for the extra capacity and the unit never runs at full speed. I had to do a lot of research, but from what I understand the efficiency actually increases at low speed as well... ie an 18 SEER 30k unit running at 12K uses less power than an 18 SEER 12K unit running at full speed. I'm still not 100% sure of that fact, as the information is hard to find.

X2 on reasonably oversized inverter units. BTW, the efficiency increases at low speed because there is less heat of compression.

Tommy

 

[G-ManBart]

I'm learning a LOT in this thread....thanks guys!

 

[phred]

I just had the calcs done on my shop. It's 24x40 with 12' ceilings. Heavily insulated and in needs 1.65 tons to cool it. 2 tons should de you just fine.


Sent from my iPhone using Tapatalk

 

[CA_Tallguy]

As for my advice to the thread, I'd go with one high efficiency 9-12k unit. They seem to be the more efficient compared to the bigger units. I'd be very interested to see the limits of a 9k unit, and judging from what I've seen it could handle a much bigger space than yours.

The difference in efficiency in the smaller units is pretty baffling to me. Why does the SEER drop so much at 1.5+ tons, especially with multiple indoor units on the same compressor? I'm trying to decide between multiple 12K high efficiency units @ 25 SEER vs a multiple head 30K unit at 16 SEER. It seems rather difficult to even get to a 20 SEER unit at 2 tons or more.

 

[G-ManBart]

I just had the calcs done on my shop. It's 24x40 with 12' ceilings. Heavily insulated and in needs 1.65 tons to cool it. 2 tons should de you just fine.


Sent from my iPhone using Tapatalk

Thanks....that helps!

 

[Highbeam]

The calculators can never anticipate the effect of leaky overhead doors letting the cold/hot outside air blow in. Oversize by lots.

 

[bzinsky]

The difference in efficiency in the smaller units is pretty baffling to me. Why does the SEER drop so much at 1.5+ tons, especially with multiple indoor units on the same compressor? I'm trying to decide between multiple 12K high efficiency units @ 25 SEER vs a multiple head 30K unit at 16 SEER. It seems rather difficult to even get to a 20 SEER unit at 2 tons or more.

Yeah I was baffled as well when I went down this road.

I wanted multiple heads, but it was so much more expensive. Close to the price of 2 separate units.

I'd go with 2 separate ones, I think the logistics of working on the multiple head units are a bit more complicated. Also, just guessing that not many hvac techs have experience working on systems with one compressor powering two air handlers.

Here's some info on the ones I used that I'm very happy with.

I have the 9k units, but the 12k unit data is in there.

http://www.lg-dfs.com/art-cool-prem...z-artcoolpremier-all-09-14_20141001093226.pdf

 

[CA_Tallguy]

Yeah, in addition to the added efficiency of having two separate smaller units, there is also redundancy should one fail. I'm in deep east texas and not having a working AC is not an option.

 

[G-ManBart]

The calculators can never anticipate the effect of leaky overhead doors letting the cold/hot outside air blow in. Oversize by lots.

Another good point. There will be times when I will open the OH doors to move one of my tractors/truck/etc in or out, and I'd like to keep the time reasonable to get it back to normal temps inside. That's going to require excess capacity under normal circumstances, and a big part of why an inverter mini-split makes the most sense.

I'm not sure I can justify the expense of multiple air handlers, but I'm going to run the numbers.

 

[CA_Tallguy]

One other benefit of multiple heads or even better, multiple units, is that it may better allow having a low level of space conditioning when the building is going to be unoccupied for periods of time.

From what I can gather, individual heads/units are going to throw everything they have at trying to keep the space at the set temperature. Granted, you can probably set to heat or cool only mode to come on only when it gets really hot or cold inside. But I want to keep humidity down in the building so I might prefer to have a low BTU head running all the time and then have a larger BTU head that I switch on when I want to fully condition the space. The low BTU head should take the edge off the heat or cold and also keep the humidity down in warm weather.

I guess the drawback of a single, large multi-head unit -- should you ever wish to only use it at minimal capacity with just one of the heads -- is that the minimum BTU's it can put out is a lot higher. For example, one 36000 BTU outdoor unit I am looking at can't slow down to under 10K btu for cooling and 15k btu for heating. So I guess it might be very inefficient to try to run only a single 9K BTU head on that since the outside unit wouldn't be able to slow down to fully match that capacity.

On the other hand, a stand alone 9k BTU rated system I am looking at can run at 3500 to 9000 BTUs in cooling and 2500 to 11000 BTUs in heating. That low end capacity might be really great on mild days and nights where not much is needed to maintain a good interior temp/humidity level.

So just keep in mind that if you flick the switch on a big unit -- single or multi head -- you are generally going to be pumping at least 10K BTU's out minimum. That kind of kills the biggest benefit of the variable speed inverter systems in these units (assuming that there will be times that you don't need 10K BTU's).

 

[Jackfre]

I think individual units are preferable to multis for the reasons you point out. Redundancy is key here as well as efficiency. That said, my 6 yr old Fujitsu (16.5seer) dual operating only the single unit in the bedroom provides excellent comfort without over cooling. Single units can simply installation also. You may be able to get the individual Condensing unit closer to the evap.
Whichever way you go, I will never install a condensing unit at ground level. The Air-Tec brackets work really well and help keep the condensing unit cleaner and make it harder for critters to get into them...which brings up a story. When I was the Fujistu rep I got a call from customer who needed some help. I ran out to Sharon, MA to look at this unit, meeting the tech and the owner. We walked around the back of the house and I stopped dead in my tracks. The ground seemed to be moving. Very weird. Kinda like the Teddy Bear Picnic, this was the Garter Snake Picnic. It was a new one on me, but there were hundreds of snakes. They sited the unit poorly, on a pad between two 4'high x 4-6' wide bushes, which had snakes all over them...and the condensing unit. I asked what they intended me to do and they said the unit wasn't working and I was there to fix it. Over the years I had seen this only a couple times, but very small snakes will on occasion work their way up to the top of the condensing unit and manage to get into the top of the unit, where they crawl over the boards, short everything out cook up there. I was big on customer service, but they had uncovered my weak spot. Get them off the ground. You will be happy that you did. They ended up replacing that condensing unit. Minced snakes on the fan too.
As well I have seen ground mounted units where in a very hard rain the splash off the ground completely sealed the condensing unit fins with mud. As I recall when I was working in Beaumont and Pt Arthur, it doesn't really rain there. It is always the deluge!

 

[G-ManBart]

Whichever way you go, I will never install a condensing unit at ground level. The Air-Tec brackets work really well and help keep the condensing unit cleaner and make it harder for critters to get into them...which brings up a story.

I'm glad you said this because I was thinking along those lines...get the unit off the ground and away from critters. I just put a service disconnect in the other day (30amp service with 10/2 wire) and made sure there was more than enough room to put a mounting bracket for the condenser nearby.