Mini split performance: interesting paper

ionized_gw

11 years ago

http://www.nrel.gov/docs/fy11osti/52175.pdf

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ionized_gw
Original Author

11 years ago

I have tried mightily, but I can't figure out what most of the comments mean in the previous post. Some complete sentences would probably help. I might understand the last paragraph so I will respond to it with practical observations. To put it in a nutshell, I have observed neither excessive run times to get to set temp, nor excessive humidity.

I have a 7-room house at 2000 sq feet, single level. I have 7 mini-split indoor units 4 x 6,000 and 3 x 9,000 BTU. It is a poorly insulated and leaky (air) structure including the raised floor. Average August conditions here are high 87, low 72 dew point in the mid 70s.

No one is home during the day. Typically I have three indoor units 6,000 + 9,000 + 6,000 come on in the kitchen, den and bedroom respectively at 5:30 PM. The den is joined to the kitchen by a standard doorway. The den connects to the hall to the bedrooms all within a few steps. By the time I get home at 7 or so, the kitchen and den temps are in the mid 70s (set point) and the RH is below 50%. The BR takes a little longer. Note that the BR is next to the hottest room in the house. (The utility room which has a flat roof on the West side of the home and a chest freezer that produces the waste heat that all refrigeration units do. It stays in the 90s in the utility room until very late.)

If we decide to occupy a different room, it does not seem to take too long to get comfortable if doors to cool rooms are then left open and the unit is turned on. (Sorry, I've not made any quantitative observations.) The unoccupied rooms do not get overly humid. (I do have hygrometers around.) We have not experienced any trouble cooling individual rooms or the whole house when we are home all day on hot weekends.

Typically in the warmest weather everything but the bedroom is off during the night and I set the kitchen to start cooling a half hour before I get up. We usually leave the BR, den and kitchen doors open but even with that open, it is uncomfortable in the adjacent parts of the house if I forget to set the kitchen to cool at bedtime.

Efficiency is inherently high in my system, ca. SEER 19. Of course the inherent efficiency alone is a poor comparison to ducted systems that have ducts external to the living space. That is a common characteristic in AC retrofit homes so duct losses can be tremendous.

One down side, I have to clean 7 filters on a regular basis compared to swapping out one disposable one.

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Comments (13)

Elmer J Fudd
11 years ago
bump
brickeyee
11 years ago
Tiny one ton units.
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energy_rater_la
11 years ago
this is a good article Ionized.
you bought mitsubishi??

is this part of your minisplit bible?
would you be willing to share more?

thanks for the minsplit ceu's.
you know on another forum they
got kinda upset that I didn't go
minisplit in my house. I'd like
to share some of your minisplit info
with them. is that ok?

tia!
and Cheers.
ionized_gw
Original Author
11 years ago
brickeye, Huh?

energy_rater, the "bible" is more like a rambling, uncompiled bible. I've posted much of it here, but it may be fragmented. I will pull something together for you.

That paper obviously describes a work in progress. They are solving some measurement problems that many would not anticipate. For me, the trump card in the Minisplit deck is this:

However, the potential zoning control offered by mini-split heat pumps could lead to an overall peak load reduction.

This was a key characteristic for my decision. Because of the way we use our house, we can seriously condition less than half of it most of the time. The half not directly conditioned almost always includes the part that is difficult to control. I suspect that we use half the electricity that we might otherwise due to this alone.
kashmi
11 years ago
Slightly off-topic question ....

We have both LG and Mitsubishi mini-split ACs in our house and love them. Two friends have been by to take a look, thinking of installing them, too. However, all they can find are mini-splits that provide both heating and cooling (as in the tests in the report above). But all they need is the cooling feature.

Have all the AC-only mini-split systems been discontinued?

Thanks in advance for the help.
Elmer J Fudd
11 years ago
The biggest problem I see with the ductless AC systems is that not only do they not have a positive cachet, they perhaps give a negative impression for resale purposes.

People demand and value central AC. Ductless systems are not yet familiar or well known. A ductless system's inside equipment will make people think of a big in-window unit, without the window. Or, like one of those garbage through-wall things you find in motel rooms, just higher up on the wall.

While it may be an efficient and flexible choice, it doesn't speak of quality or give a premium impression. In most average houses, that wouldn't be a problem. For anything of up-market caliber, I think installing such a system in anything other than an obvious addition could be a mistake.
brickeyee
11 years ago
"brickeye, Huh? "

The units cited are all 1 ton units (12,000 BTU/hr).

Those are not very large units, and likely incapable of more than a single not large room in a well insulated house.
kirkhall
11 years ago
Considering that a minisplit is only really meant to serve one room (there is no circulating mechanism other than the fan to move air), 12000BTU is plenty, imo. I have 2 12s (LG) and will be getting 3 9s or smaller for bedrooms after we complete our addition.

I do agree with the statement regarding resale. In our case though, we have A/C in an area that doesn't usually have A/C with the minisplit. And, we didn't use enough energy to qualify for any sort of program for a furnace when we transitioned from electric baseboard heating... So, we got the minis.

We've been happy with them.
fluffybunnysui
11 years ago
kashmi
Most all of the mini split manufactors make cool only unit. They are widely available.
JERRYKING58
11 years ago
If you check some of the Internet sales you will see that Mitsubishi does sale a few AC only units. I think the reason you do not see more is people like me who use Oil heat are looking for homes with Heat Pumps. Oil is now selling for over 3,59 a gallon with a .11 cent a gallon tax. Electric is only .88973 I hope to have my 3 zone Mitsubishi installed in the next month. I can shut off the area's I do not use only heating the main living room and kitchen area.
ionized_gw
Original Author
11 years ago
I think that you need to be careful with internet sales. The equipment could be grey market that was brought in from other countries. If that is the case, you could end up trying to get parts from Nigeria, Saudi Arabia,.....
brickeyee
11 years ago
"Considering that a minisplit is only really meant to serve one room (there is no circulating mechanism other than the fan to move air), 12000BTU is plenty, imo."

For a not large room in a well insulated house.

Do not have a 'wall of windows' facing south.

The claim to efficiency comes from 'spot' cooling a limited area.

Wait till you have high humidity and saturate the items in uncooled areas, then decide to occupy those areas.
The latent load (humidity) is going to result in long run times, or poor humidity control.

Mini split performance: interesting paper

ionized_gw
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Mini split performance: interesting paper

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