mini-split vs window unit

[mastiff0]

For my 31x32 garage I'm planning on insulating the walls and adding A/C so I can still do work during the long houston summers. My plan is to section off a 12 x 10 section of the garage to house my dogs and to make little lounge area away from the dust of the rest of the garage. I want to keep this small room pretty cool during the day for the dogs, and keep the rest of the garage bearable, with an a/c system running to dehumidify things.

So I was going to add a small 5-6K BTU window unit to the small room (easy to do while the walls are still unfinished) and then add a 18000BTU unit for the rest of the garage. I originally thought that the the mini-splits would be the way to go because of the high SEER ratings, but after looking at it closer, I'd thought I'd ask you.

An 18000BTU window unit would cost ~$500 (with no installation cost) and you can find them with EER ratings of 10.7. I'm looking at a Sanyo 18000BTU mini-split system with a 20 SEER rating that costs $1600 before installation. Trying to compare apples to apples, a 20 SEER rating is probably equivalent to a 14-16 EER (EER estimate efficiency at 95F temps, which is more like Houston summers)

Using a couple of detailed A/C energy use estimators I found online, the energy cost between a 10.7 EER window unit and a 14-16EER mini-split system is only about $200/year. These are using calculators designed for a traditional home and not a garage, but I tried a lot of variables and the numbers were always low. At $200/year, it would take 7 years to break even if I went the mini-split route. And for Houston, your AC needs to be replaced every 7-10 years anyway.

So I'm not seeing the cost advantage of using a mini-split system, but maybe I'm missing some issue about the operation between these two systems. Can anybody give me any real experience about using a window unit or a mini-split? Will a window unit even last 5-7 years? Longevity of a window unit is one thing I haven't really thought about yet.

Thanks guys.

 

[redsky49]

There are a number of factors that go into such a consideration:

1. Annual hours of operation.
2. Requirements for enhanced operation, i.e., do you need humidity control, temperature off sets (occupied versus unoccupied for example), etc.
3. Are you financing or buying outright.
4. Are there aesthetic considerations that might make one system more desirable that the other.
5. Are there any incentives from the Utility Company for high efficiency units.
6. Is heating required, or enhanced filtration.

Just to name a few.

First off, you should confirm your load requirements by performing a heat load calculation. In the past I have described how to perform a simple calculation:

http://www.garagejournal.com/forum/showthread.php?t=47200

Thru wall/window units (PTAC's, or packaged terminal AC units) typically have a nominal service life of 10 years for quality units. Compare this number with 15 years for residential split systems. This statistical data however, is from a 1978 ASHRAE survey and my suspicion is that current compressor quality has dropped markedly with the influx of cheap imported devices from overseas. The 5-7 year expectancy is probably more accurate for current offerings.

For applications in the Houston area, there also may be a contributing factor (sodium chloride) due to the proximity to salt air and high humidity. In this case, better quality units will show a far better life expectancy due to improved component quality. In fact, for applications close to Galveston Bay, I would look for a phenolic or epoxy coated unit, stainless rather than galvanized, plastic condensate tray, etc. These extreme duty units will not likely be sold at the big box stores but are available from most manufacturers. This may also be a reason to select a split system from one of the better suppliers such as Mitsubishi or Sanyo.

As a general rule, unless a cost savings can be realized in less than seven years, the additional capital cost for improved equipment efficiencies are likely not a wise choice. This has to do with the opportunity cost of money, periodic costs, etc., etc.

If you decide to go with a PTAC unit you may wish to consider mounting the unit a bit higher than normal from the ground. I have seen that units located close to the ground seem to suffer faster deterioration than units mounted higher up. The presence of dog hair might also be a consideration for you with the smaller unit. Clogged filters do not make for an economical unit. In any case, follow the manufacturer's installation requirements for clearances, etc.

If the decision was up to me, I would select a quality corrosion-resistant split system, especially if I intended to occupy the dwelling for an extended period. Bear in mind however, that I am not aware of a single zone split system that is smaller than one ton, which would likely be way over-sized (please confirm through calculation) for the dog room.

Continue your research.

Here is a linky to the smallest Mitsubishi unit I could find. This is a 3/4 ton heat pump unit:

http://catalog.mitsubishipro.com/it...z-wall-mounted-inverter-heat-pumps/item-1386?

As always, offered only as opinion

 

[mastiff0]

Thanks for the good info Redsky49. To answer some of your questions:

1. Annual hours of operation.- I found these values for Houston homes online
2. Requirements for enhanced operation, i.e., do you need humidity control, temperature off sets (occupied versus unoccupied for example), etc.- humidity control would be a nice feature
3. Are you financing or buying outright. Buying outright
4. Are there aesthetic considerations that might make one system more desirable that the other. This makes little difference to me
5. Are there any incentives from the Utility Company for high efficiency units. I haven't looked into this, but I think high SEER units get a tax rebate still.
6. Is heating required, or enhanced filtration.- no heating needed

I liked your post on how to calculate heat loss. After being unable to quickly get all the R values I needed, I went back to this calculator I found.
http://www.cooloff.org/sub_cool.html

Its pretty detailed an even goes as far as to calculate heat gain from electrical appliances and human bodies in the room. If these values are off, they are most likely conservative, since one usually keeps the garage hotter than you could keep a home. This means that if anything, the electric bill for cooling would be less than I think, and the difference in EER ratings is even less important.

I am on the southside of Houston nearer to Galveston, but from my experience we do not have to purchase more corrosion resistant A/C systems because of the salt. Galveston itself might be a different story.

As for the small room for the dogs, I've pretty much decided to use a window unit for that room. Like you said, you cannot find mini-split units that small. And I'm fine with using a window unit for that small room. Its the overall garage area that is trickier.

The two big unknowns that could be the deciding factors might be
1)incentives/tax rebates for getting a high efficiency mini-split, and
2)longevity/durability of the various units.

I don't have a lot of experience with window units, but I get the impression that they are usually cheaply built. Can you expect them to last 5-8 years seeing moderate duty?

 

[BOONEY7750]

I was facing a similar debate for my second floor in my 1876 house. I decided against the window units and I am glad I did. I went back and fourth on a traditional PTAC or a mini split. I think the mini-splits with heat pump are the way to go for fit and finish. There is something trashy feeling about window units in my opinion. I also like the features and programability and some servicability of the mini splits. All that being said I bought a stand alone A/C unit to get me through the summer until I can make a final decision and I am quite pleased with it. I can't heat with it, but it works well for cooling and is offsetting central air cost. So no real answers just my experiences.

 

[Ironcrow]

The window unit basically hangs the noisy unit compressor unit in your ear. With the mini-split its behind the building on a pad.

 

[skeletonizer]

The window unit basically hangs the noisy unit compressor unit in your ear. With the mini-split its behind the building on a pad.

What?










Good point. I had been thinking of using a big through wall/window unit...

I might just save my pennies a little longer and put an air handler in my attic with a split system.

Quieter, longer lasting, probly cheaper to run.

 

[Car54]

The window unit basically hangs the noisy unit compressor unit in your ear. With the mini-split its behind the building on a pad.

This.

I just had a split system installed that replaced a 12k BTU window shaker. Unbelievable sound difference, granted this is in an 13'x25' office in a 39'x25 building. I had a multi-zone installed with 9k in the office and 12k in the shop. Haven't realized any power difference yet, but the split operates at much less of a "furious" sounding pace. The outdoor unit is nearly silent as well.

Check dB levels of the devices.

 

[mastiff0]

Well, I did some more research to compare window units vs mini-splits and here is my conclusion:

Mini-splits are cleaner looking and quieter, but if those are unimportant, there extra efficiency cannot justify their extra cost. for my application they cost 3-4 times as much as a window unit, depending on how much installation you do yourself. My calculations for operation cost in Texas showed that the mini-splits wouldn't break even until at least 5 years down the road.

The 30% tax rebate sounds appealing, but only a limited number of mini-splits actually qualify. To qualify, the SEER and EER ratings must be above certain values, and I was stunned to see some of the 20 SEER units didn't qualify because their EER rating was only 10-12. The units that do qualify for the tax credit were more expensive, so this basically offsets the tax rebate.

So the window unit seems the better deal for me. This could be different if you already have finished walls and don't want to bust a hole in your wall, or if the window unit would be unsightly. The Mini-split might also be practical to those who plan to use their garage like a man cave and plan to spend a lot of time in there just chilling. But I couldn't ever get the the numbers to work out for my situation.

 

[sirswank]

ratings and calculations and cost per minute to operate, oh my!

from my "in practice" experience, i can tell you that a 10K BTU window unit i picked up at a yard sale for $50 cooled my garage to a comfortable level during New England summer weather. we're talking 80's-90's and high humidity. on full blast, it took about 30-45 minutes to cool the space, and my bargain unit lasted 5 years. my garage is 700sq and insulated, and yours is effectively the same size once you count out the dog room.

 

[JamieK]

Couldn't the mini splits handle heating the garage too? That would be a factor of not needing a seperate heating source.

 

[Car54]

Mine does heat, it has a heat pump but not a heating element. It's "ok", not what you'd want in the north. I'm in FL.

 

[heavyfoot86]

Look into a mini-split heat pump with 2 indoor units sharing the outdoor. It's what I plan to do on my house whenever we close the screen porch into a sun-room. One unit in the sun-room, one unit in the garage!

Here's the possible combinations for Mitsubishi: http://catalog.mitsubishipro.com/Asset/MXZ-System-Combination-Options.pdf

You could do a 9,000 and a 15,000 BTUH with the smallest unit or a 9 and an 18 with one model larger.

 

[Car54]

One thing to keep in mind. You may buy a 12k + 9k indoor units...but your outdoor unit might be a 18k. Meaning at any one point they'll only be able to produce 9k and 9k or 12k and 7k, for example. I would go larger with the outdoor unit.

 

[heavyfoot86]

One thing to keep in mind. You may buy a 12k + 9k indoor units...but your outdoor unit might be a 18k. Meaning at any one point they'll only be able to produce 9k and 9k or 12k and 7k, for example. I would go larger with the outdoor unit.

It's called Diversity, rarely will both units need full blast, especially with how small his "dog room" is. All your doing by over-sizing the outdoor unit is lowering your efficiencies. Mitsubishi wouldn't say a 20k outdoor unit can run 9 and 15k indoor units at the same time if it couldn't.

...but good lookin' out.

 

[mastiff0]

Couldn't the mini splits handle heating the garage too? That would be a factor of not needing a seperate heating source.

Yes, some mini-splits also have heaters, but I've found some window units that have heaters as well. Being in Texas, heat isn't that big of an issue for me.

Look into a mini-split heat pump with 2 indoor units sharing the outdoor. It's what I plan to do on my house whenever we close the screen porch into a sun-room. One unit in the sun-room, one unit in the garage!

You could do a 9,000 and a 15,000 BTUH with the smallest unit or a 9 and an 18 with one model larger.

This was my original idea, and the cleanest solution. Unfortunately, the smallest mini-splits made are 9,000btu, and that's way too big for my small room. Also, a multiple unit mini-split still cost a lot more than 2 window units.

 

[Falcon67]

I have a similar issue coming up in the next 18 months or so and already decided that I'll go window unit. I have a 12K unit now high up on the wall in the 20x24. It doesn't make much noise and works as well now as when I put it in 11 years ago. I think it was a 10 SEER back then - paid decent $ for it. I'm planning a 24x7' work area addition and I'll put a nice 5 or 7K unit in there in a similar configuration. There will not be many times that I'll need to have both running and I would expect to be in the smaller shop area 70% or better of the time, so I quit worrying about the extra power cost of running two units.

 

[Car54]

It's called Diversity, rarely will both units need full blast, especially with how small his "dog room" is. All your doing by over-sizing the outdoor unit is lowering your efficiencies. Mitsubishi wouldn't say a 20k outdoor unit can run 9 and 15k indoor units at the same time if it couldn't.

...but good lookin' out.

Well last weekend I had both running and it was still plenty humid and 80+ in the shop. At that point I wished I had a little more oomph.

 

[krooser]

I have a 1966 Gibson window unit (9K?) that will cool my main space (1100 sq. ft.) to 50F... and keep my lift area (400 sq. ft. without ceiling insulation) to 55F in 90F plus temps. And it was free!

I see a lot of a/c units at the local pawn shop... good prices is you buy out of season... a few months ago they had a 12K btu on sale for $50.00!

 

[Car54]

It gets to 90° in Wisconsin???

 

[BirdRacer]

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