Mini split BTU question

[Hroger]

I have the chance to get a nice Samsung 24000 BTU mini split unit that a buddy installed in a tiny room. The long story short is that it was too big of a unit to run in a 12 X 12 room. My shop is 20 X 25 with 9 ft walls with no windows. Will this unit be too large for my shop and create the same problems he had? or will it be fine. I have spray foam on the ceiling and walls, run 14 sets of double bulb florescent lights, a mill, a lathe and a hand full of other shop tools.

 

[Mike N]

I have a 12,000 BTU Mitsu mini split in my 20 x 30 shop and I would say that it does fine for the shop but does not get the temps to what you might want in the house. I don't think the 24,000 BTU will be too big.

 

[Hroger]

That was what I was hoping. It is hard to beat a hardly used $2K plus unit for $500. I figured it might be worth taking the chance.

 

[Falcon67]

It's a little big for that area, but not so much as to walk away from that kind of deal. I'd be on that quick before they change their mind.

 

[pseudorealityx]

What 'problems' was your buddy having?

 

[Hroger]

His room was only 120 square feet and he would have to run the heat for a while before it would cool. The cycle time was way too short to remove the humidity from the room and cool it right.

 

[JakeKohl]

I have the chance to get a nice Samsung 24000 BTU mini split unit that a buddy installed in a tiny room. The long story short is that it was too big of a unit to run in a 12 X 12 room. My shop is 20 X 25 with 9 ft walls with no windows. Will this unit be too large for my shop and create the same problems he had? or will it be fine. I have spray foam on the ceiling and walls, run 14 sets of double bulb florescent lights, a mill, a lathe and a hand full of other shop tools.

I have an 18kbtu unit in my 24x36 and it has plenty of capacity to keep it at 74 degrees during our winters (freezing at night). Garage is well insulated with foam core doors. Recovery is fast too. Like the PP said, 24k is probably too much but I wouldn't worry too much about it.

 

[RKA]

The one he's getting might not be an inverter type, which might be a significant distinction in terms of sizing. I too think the 24K might be big for that space for a non-inverter type unit.

Another point of reference, I have a similar size garage as the OP, but with 11ft ceilings. No foam, just R13 all around and well insulated doors. The 24K inverter only hits peak stride when initially lowering the temp of the space, after that the fan just runs on low to maintain temps. Typically I'm not lowering the temps more than 10-15F from outside, usually I'm just trying to remove humidity. I oversized for the heat side, but if I just needed AC, I would think a 15-18K unit would be just fine. If I was getting a non-inverter unit, I'd hesitate to go 24K, but for $500, what the heck!

 

[RKA]

Hroger, where are you located?

 

[Falcon67]

His room was only 120 square feet and he would have to run the heat for a while before it would cool. The cycle time was way too short to remove the humidity from the room and cool it right.

They do that - somebody speced in a house grade 9K BTU mini unit in one of our new buildings for a network closet that was maybe 100 sq/ft. It didn't last a year and we spent a lot of time airing out that closet when the unit failed and the UPS temp alarm went off. It was replaced and the new one uses some sort of low temp coil management thing. Works good now.

 

[Hroger]

RKA- I am in South Georgia just above the Florida line.

I decided that for $500 I might as well go for it. It has to be better than the little robot looking portable that I have right now. The (very) small tank in it has to be emptied at least twice a day to keep it running.

 

[Falcon67]

>It has to be better than the little robot looking portable that I have right now.

We have some of those 12K portables for backup in our data center. They don't work for **** IMHO. We'll shut down to barely essential systems before we break those out.

 

[RKA]

Well, I'm curious how it works out for you. Post an update once it gets a little sticky outside. Good luck!

 

[Jackfre]

I would expect you to see the same issues your friend had. The degree to which this will happen will be dependent upon heat gain in the space and as pointed out whether or not it is an inverter unit. If it is an R22 single stage unit you will end up with a cold clammy space. If an inverter the problem will be moderated somewhat depending upon how low the inverter lets it run. For what you have in it, you shouldn't get hurt on it anyway, so...good luck with it.

 

[Hroger]

The manual for it says S inverter for whatever that means.

 

[JakeKohl]

I would expect you to see the same issues your friend had. The degree to which this will happen will be dependent upon heat gain in the space and as pointed out whether or not it is an inverter unit. If it is an R22 single stage unit you will end up with a cold clammy space. If an inverter the problem will be moderated somewhat depending upon how low the inverter lets it run. For what you have in it, you shouldn't get hurt on it anyway, so...good luck with it.

Does this really happen? I hear it all the time...in fact, when I replaced my standard house unit a few years ago, a couple of contractors talked of peeling paint and such from the humidity gain if you oversize the heat pump. I oversized it (mostly because I know my house has some efficiency issues due to being constructed largely of concrete). They also told me that I was making a huge error by closing off unused rooms because I was bringing "outside temperature to lesser insulated interior walls"...which is not how thermodynamics work either (my specialty in my ME degree). There are other potential issues with closing off rooms like this - but it's not this goofball temperature idea.

It would seem to me that this incredible life-and-structure-threatening moisture build up would only happen in the MOST of extreme situations. To a large degree a heat pump / AC unit that is cooling more air in one pass is also removing more moisture on an almost equal basis. I think 90% of that whole thing is wives tale.

That said, however, it is important to properly size the unit to get the most efficiency out of it...but if you're saving 75% on the up front cost, that should entirely balance out.

 

[pseudorealityx]

Does this really happen? I hear it all the time...in fact, when I replaced my standard house unit a few years ago, a couple of contractors talked of peeling paint and such from the humidity gain if you oversize the heat pump. I oversized it (mostly because I know my house has some efficiency issues due to being constructed largely of concrete). They also told me that I was making a huge error by closing off unused rooms because I was bringing "outside temperature to lesser insulated interior walls"...which is not how thermodynamics work either (my specialty in my ME degree). There are other potential issues with closing off rooms like this - but it's not this goofball temperature idea.

It would seem to me that this incredible life-and-structure-threatening moisture build up would only happen in the MOST of extreme situations. To a large degree a heat pump / AC unit that is cooling more air in one pass is also removing more moisture on an almost equal basis. I think 90% of that whole thing is wives tale.

That said, however, it is important to properly size the unit to get the most efficiency out of it...but if you're saving 75% on the up front cost, that should entirely balance out.

Yes, I've seen it. Some more extreme than others. It's typically not 'life and structure threatening', but it can be pretty nasty... here are some common examples I've seen.

1. Operating rooms. Doctors LOVE to push space temperatures down a LOT, but don't always want to pay for the insulation and HVAC to do it correctly. When combined with poor construction techniques without proper vapor barriers and whatnot, I've seen instances of condensation on/in walls, and relative humidity levels pushing up too high, which promotes microbial growth.

2. Schools. High ventilation requirements and oversized equipment leads to relative humidity in the 70+ range. That's far enough above code that legal action can be taken, again because of the possibility of mold growth.

3. A lot of restaurant kitchens. Many restaurants are NOT designed properly, and even less are actually operated correctly, due to the owners not understanding all the various components of a restaurant HVAC system. When properly designed and operated, the kitchen is typically one of the dryer spaces in the restaurant, but if you happen to frequent many restaurant kitchens, due to poor design, it's typically the most humid space, and when combined with slightly higher space temperatures, again.... mold, mildew, etc.

4. Hotels. Not much of a structural concern, but most cheap hotels have oversized PTAC units, which is why they always feel damp and clammy, especially when the temperatures are lowered.

5. Clean rooms. I do a fair number of cleanroom designs, and while I engineer them correctly [grin], you can easily mess that up.

 

[Jackfre]

Does this really happen? I hear it all the time...in fact, when I replaced my standard house unit a few years ago, a couple of contractors talked of peeling paint and such from the humidity gain if you oversize the heat pump. I oversized it (mostly because I know my house has some efficiency issues due to being constructed largely of concrete). They also told me that I was making a huge error by closing off unused rooms because I was bringing "outside temperature to lesser insulated interior walls"...which is not how thermodynamics work either (my specialty in my ME degree). There are other potential issues with closing off rooms like this - but it's not this goofball temperature idea.

It would seem to me that this incredible life-and-structure-threatening moisture build up would only happen in the MOST of extreme situations. To a large degree a heat pump / AC unit that is cooling more air in one pass is also removing more moisture on an almost equal basis. I think 90% of that whole thing is wives tale.

That said, however, it is important to properly size the unit to get the most efficiency out of it...but if you're saving 75% on the up front cost, that should entirely balance out.

Jake, I'm not an engineer and will not debate psychometrics and the science. I would loose miserably. I represented Fujitsu in the New England States and as the rep I only got the call to visit catastrophes. I think that what happens is you have a little problem with the structure here and a little problem with the occupant/owner there and a little infiltration here and little there and then an oversized unit and kaboom all hell breaks loose. Of course it is all the ac systems fault...or Pseudo's

These problems have greatly diminished with the inverter units. I always say that what is most important in sizing inverters is how low the unit will go as opposed to high it will go because they generally spend more time in the lower ranges than higher.

I guess that what I can say for sure is that every system has a personality. You just hope it is more Jekyl than Hyde.

 

[pseudorealityx]

Jake, I'm not an engineer and will not debate psychometrics and the science. I would loose miserably. I represented Fujitsu in the New England States and as the rep I only got the call to visit catastrophes. I think that what happens is you have a little problem with the structure here and a little problem with the occupant/owner there and a little infiltration here and little there and then an oversized unit and kaboom all hell breaks loose. Of course it is all the ac systems fault...or Pseudo's

These problems have greatly diminished with the inverter units. I always say that what is most important in sizing inverters is how low the unit will go as opposed to high it will go because they generally spend more time in the lower ranges than higher.

I guess that what I can say for sure is that every system has a personality. You just hope it is more Jekyl than Hyde.

We've had good luck where mini-splits were used. But we only use inverter mini-splits, and 99% from Mitsu for commercial applications. We've had some other stuff submitted, and most of the big names are acceptable. But yes, beware the cheap non-inverter type units that are basically using the same outside 'look' as the [big name] mini-splits while not delivering the performance advantage.

 

[Hroger]

Now that I have it in hand it is a 12K BTU. I was searching based on the max square foot rating thinking it was a 24K. I think it will fit in great.

 

[RKA]

Jake, think of it this way...removal of moisture is a function of how much air you push over the coils. If the unit is short cycling you're giving it less time to pull that moisture out. Now it's true that a larger unit can pull more pints per hour out of the air, but only if it's running the same amount of time that the smaller unit is running. It's not, it's running half the time.

Now imho, if you are marginally oversized, it probably won't matter, but if you're off by a factor of 2, you'll know. And then there is that big gray area in the middle. I know my upstairs unit short cycles a bit because I've got some rooms closed off (not the returns) and the thermostat resides in a room with a closed door. Still it manages to remove the humidity. I've been in other houses where you walk in and know right away something isn't right. Cold and clammy like psuedo said.

 

[pseudorealityx]

It's also of course location dependent. If you're in Denver or L.A., it's not likely an issue because they just don't have the humidity issues. But in the the SE, it is definitely a problem. You read through the trade journals and places in Colorado always having amazing cooling 'efficiency' numbers, because they can typically use outside air to cool everything, with no issue with humidity. Of course, they also deal with real winters, but you get the point.

Oh, another project closer to your location... a college in Columbia, SC had the metal stud walls in a central kitchen rusting away to the point they were quite easy to push around with your hand. The moisture issues were two fold having a minor water leak, but also due to the inability of the HVAC units to pull moisture out of the space. This caused all sorts of issues, including mold abatement that added 10's of thousands of dollars to a construction project and weeks to the time frame.

 

[BadgerBoilerMN]

It is not the "size" of the mini-split or the "size" of the room or garage it will be conditioning. It is the "size" of the load that the building presents and the output of the mini-split. One must fit the other or all manner of problems will arise.

If you don't want to waste time and money installing an improperly sized heating or cooling appliance have a knowledgeable contractor or HVAC designer perform a proper ACCA heat load analysis before you buy anything. Or...you could buy, try it, and put it on Ebay for half of what you paid for it. Still a good deal, right?

 

[Silverbullet404]

BUY IT! Ive used them for 4 years now. Keep on getting asked by friends who go and buy them as well. Cannot be beat! Also has a 'dryer' cycle on many of the models for drying clothes. Never use our clothes dryer any more. Hang clothes on a folding clothes thingie and walla! It will keep moisture and mold out of your shop.
One thing is worthy of note. Like, "How high is the sky?" It primarily depends on how or if your shop is insulated. Key to all energy requirement.

 

[Hroger]

I have a new question now. I see that the design is for the copper plumbing to run out of the back of the unit and through the wall to the outside, turn down then connect to the outside unit. Is there any reason why I could not run the copper plumbing down the inside wall to an existing hole near the bottom of the wall, then out and to the outside unit? Same distance, just turning at a different spot. I'd rather not have to make another hole in the wall if I could avoid it.

 

[pseudorealityx]

Running the lines inside a wall is fine.

 

[JakeKohl]

Yes, I've seen it. Some more extreme than others. It's typically not 'life and structure threatening', but it can be pretty nasty... here are some common examples I've seen.

1. Operating rooms. Doctors LOVE to push space temperatures down a LOT, but don't always want to pay for the insulation and HVAC to do it correctly. When combined with poor construction techniques without proper vapor barriers and whatnot, I've seen instances of condensation on/in walls, and relative humidity levels pushing up too high, which promotes microbial growth.

2. Schools. High ventilation requirements and oversized equipment leads to relative humidity in the 70+ range. That's far enough above code that legal action can be taken, again because of the possibility of mold growth.

3. A lot of restaurant kitchens. Many restaurants are NOT designed properly, and even less are actually operated correctly, due to the owners not understanding all the various components of a restaurant HVAC system. When properly designed and operated, the kitchen is typically one of the dryer spaces in the restaurant, but if you happen to frequent many restaurant kitchens, due to poor design, it's typically the most humid space, and when combined with slightly higher space temperatures, again.... mold, mildew, etc.

4. Hotels. Not much of a structural concern, but most cheap hotels have oversized PTAC units, which is why they always feel damp and clammy, especially when the temperatures are lowered.

5. Clean rooms. I do a fair number of cleanroom designs, and while I engineer them correctly [grin], you can easily mess that up.

Most of what you are talking about here is the result of condensation (and likely improper vapor barrier/insulation) where hot and cold surface contact each other - not necessarily the size of the unit. You also talk about undersized units with the same issue. The "issue" I'm talking about is where someone says that oversizing the HVAC unit results in removing too little moisture resulting in high humidity levels.

Jake, think of it this way...removal of moisture is a function of how much air you push over the coils. If the unit is short cycling you're giving it less time to pull that moisture out. Now it's true that a larger unit can pull more pints per hour out of the air, but only if it's running the same amount of time that the smaller unit is running. It's not, it's running half the time.

Now imho, if you are marginally oversized, it probably won't matter, but if you're off by a factor of 2, you'll know. And then there is that big gray area in the middle. I know my upstairs unit short cycles a bit because I've got some rooms closed off (not the returns) and the thermostat resides in a room with a closed door. Still it manages to remove the humidity. I've been in other houses where you walk in and know right away something isn't right. Cold and clammy like psuedo said.

I hilited a bit up there - I don't understand how that's the case. If you have one unit with coil size "A" and a second larger unit with coil size "A*2" which is moving twice the air, the larger unit will remove almost exactly twice the amount of moisture from the air passing through it.

It's also of course location dependent. If you're in Denver or L.A., it's not likely an issue because they just don't have the humidity issues. But in the the SE, it is definitely a problem. You read through the trade journals and places in Colorado always having amazing cooling 'efficiency' numbers, because they can typically use outside air to cool everything, with no issue with humidity. Of course, they also deal with real winters, but you get the point.

Oh, another project closer to your location... a college in Columbia, SC had the metal stud walls in a central kitchen rusting away to the point they were quite easy to push around with your hand. The moisture issues were two fold having a minor water leak, but also due to the inability of the HVAC units to pull moisture out of the space. This caused all sorts of issues, including mold abatement that added 10's of thousands of dollars to a construction project and weeks to the time frame.

This issue in Columbia could just as easily be an issue with improper insulation / vapor barrier and you have warm air meeting cold inside that wall without a barrier and it's leading to condensation on the cold side....it is probably not due to the size of the HVAC unit.

 

[pseudorealityx]

Most of what you are talking about here is the result of condensation (and likely improper vapor barrier/insulation) where hot and cold surface contact each other - not necessarily the size of the unit. You also talk about undersized units with the same issue. The "issue" I'm talking about is where someone says that oversizing the HVAC unit results in removing too little moisture resulting in high humidity levels.

The school example was directly an oversized HVAC issue. The water source heat pumps were single speed units, and we sat there and when the units would run, RH would quickly drop to ~50-55%. The units would satisfy the t-stat, and the compressor would shut off. Before the dry bulb climbed high enough to energize the compressor, the RH would climb to almost 70%. Rinse and repeat every ~20-30 minutes.

We had an extremely similar school with the same type of systems, except some of the HVAC was 2 speed units, so they would run longer, as if they were smaller units. ZERO issues.

I hilited a bit up there - I don't understand how that's the case. If you have one unit with coil size "A" and a second larger unit with coil size "A*2" which is moving twice the air, the larger unit will remove almost exactly twice the amount of moisture from the air passing through it.

It removed the same amount of moisture total. But in the 15 minutes between when A*2 shut off and A kept running, RH started climbing again.

Think about heating... what's more comfortable? Maintaining 50 degrees, or waiting till it's 30, heating it up to 60, and waiting for it to fall to 30 again before kicking the heating back on?

This issue in Columbia could just as easily be an issue with improper insulation / vapor barrier and you have warm air meeting cold inside that wall without a barrier and it's leading to condensation on the cold side....it is probably not due to the size of the HVAC unit.

There were TONS of issues with that job in Columbia. I'm not saying the HVAC units were sized wrong, but over time, a number of 'fixes' had been performed on the building without anyone looking at the big picture. That said, for enough money, you CAN design an HVAC system that will pull enough moisture to deal with lots of latent load, but you need to know that's your environment during the design phase.

 

[Jinks]

I have a new question now. I see that the design is for the copper plumbing to run out of the back of the unit and through the wall to the outside, turn down then connect to the outside unit. Is there any reason why I could not run the copper plumbing down the inside wall to an existing hole near the bottom of the wall, then out and to the outside unit? Same distance, just turning at a different spot. I'd rather not have to make another hole in the wall if I could avoid it.

This is so late that you've probably already figured it out, but you can run the copper out the side of the unit, along a wall, & out whatever opening you have. The only consideration is the drain line, it really needs to be down hill all the way......but you knew that...

How did the installation go? Did you DIY?

 

[SiGmA_X]

I have a semi related question. In the event of a hot/cold wall, what is the recommended vapor barrier installation? I'm curious and it seems we have a few very very well versed people in here!

Thanks guys.

 

[Jackfre]

This is so late that you've probably already figured it out, but you can run the copper out the side of the unit, along a wall, & out whatever opening you have. The only consideration is the drain line, it really needs to be down hill all the way......but you knew that...

How did the installation go? Did you DIY?

The lines must be well insulated, including the drain lines or they will sweat. Leave no portion of it bare.

 

[SiGmA_X]

The lines must be well insulated, including the drain lines or they will sweat. Leave no portion of it bare.

What type of insulation level counts as 'well insulated'?

 

[Jackfre]

1/2" Armaflex is fine for line set and condensate line. Insulate every bit of it. Seal it well. Don't overlook the Flare nuts! Speaking from experience, you do not want sweatie nuts