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For 2 bedrooms and 1 living room, and assuming you have the flexibility for both options and the total output is the same, is it better to install a Multi-Split 3x1 Air Con System (3 interior units / 1 exterior unit), or to install 3 separate Split Air Con Systems (3 interior units / 3 exterior units)?

Also, which setup is generally more energy-efficient?

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3 Answers

There are three issues here, which system is better, which system is more energy-efficient and which system is more reliable.

The first issue encompasses all aspects and involves a priority ranking which only you can do.

An issue not brought up by anyone else is the installation. Do you have room for 3 outside units? Does it matter if more space is used or would you prefer to use that space for landscaping or something else?

What about electrical power? Will the installer need to run 3 circuits, one for each compressor? Does your existing breaker panel have the room for 3 circuits?

Another installation consideration: which system is easier to run the linesets (tubes which carry the liquid and gas freon) for?

I expect 3 compressors to cost more than one big compressor. Have you gotten any price quotes?

The one big compressor, assuming it is capable of variable speed, will certainly be more significantly more efficient under anything less than full load because the condenser of a single outdoor unit system has three times the cooling area of a single condenser in a 3 unit system which means the compressor motor doesn't have to work as hard.

The major factor in power consumption is turning the freon back into a liquid and that depends on how fast/easily the condenser can cool it off. All else being equal, a bigger condenser is better (removes the heat faster).

The faster you condense the freon, the less back pressure on the compressor and the less work for the motor.

Most of the time you will not be running full load or all indoor units at once. Even during full load, the big unit will never be less efficient.

The last question is reliability and available. One answer correctly claims that a 3 unit system will be more available but neglects to mention that it will also be more prone to failure because you have more units to fail and each unit is more likely to be run at full capacity while the big unit will usually be running well below its full capacity. Imagine the lifetime of two cars, one run at 90 MPH and one run at 30 MPH.

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Very good answer. One correction, it's the condensing refrigerant temperature that determines the work needed by the compressor. As the system unloads, there is less mass flow rate of refrigerant through the condenser and the refrigerant temperature leaving the condenser gets closer to the air temperature entering the condenser. The condensing temperature also determines the refrigerant pressure. The lower the refrierant temperature, the lower the head pressure at the compressor. System performance determines if the larger unit is less, greater, or equal efficiency to the smaller unit. –  Richard Raustad Sep 6 '13 at 14:18
    
Assuming an equal load at the evaporator, the larger condenser will allow cooler refrigerant. If two condensers are made of the same material and construction, then the surface area determines the thermal resistance and a larger area means less resistance. –  Philip Ngai Sep 6 '13 at 18:16
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These days this is a great question. The mini-splits have exceptional SEER ratings (as high as 28), almost too good to be true. The multi-split (a.k.a. VRF or VSHP), not as good. There have been tests of a mini-split for efficiency (at the Florida Solar Energy Center) and the claims were found to be accurate. For now I tend to believe that the more indoor units are connected to a single outdoor unit, the more difficult it is for the control algorithm to "efficiently" control the entire system. If it's not too much imposition to install multiple outdoor units, I don't think you would ever regret it, especially after you saw your first electric bill. Good Luck!

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Besides efficiency, something to consider is that with the major shared component in the multi-split system, you run the risk of a failure taking out your entire air conditioning capacity.

A way to mitigate this would be to have a dedicated mini-split for the master bedroom and multi-split the rest of the rooms. However, with only three rooms to condition, there is probably not much advantage to this arrangement, so just go for three mini-split systems.

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