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I'm about to get the A/C installed in my new apartment, and was wondering which of the following two situations would be more efficient:

(a) Cooling 2 rooms on a single A/C compressor (same system)

or

(b) Cooling 2 rooms on their own separate A/C compressors (different systems)

*both of these rooms are likely to have the A/C on at the same time, and are likely to run through most of the same times of the day (say, 11am - 8pm)

Some background:

My new apartment has 3 bedrooms, a living room and a dining room that I would like to have air conditioning in. Each of these rooms will be fitted with an air-conditioning blower, and I can have up to a maximum of 2 A/C compressors on the outdoor ledge meant for that.

Two of the bedrooms will be converted into our 'offices' - a computer workspace for me, and a piano room for my wife. Both of these rooms will likely need to be air-conditioned throughout the daytime, but will likely be turned off at night.

In the other areas, ie. our master bedroom, living and dining areas, I think we are more likely to use the fan since we won't be there much while it's hot in the daytime. However, we'd still want to have air-conditioning available since it can get insanely hot in some parts of the year (even at night) here.

Our current plan is to have two A/C compressors, one for 3 blowers, and one for 2 blowers. Should we have both 'offices' on one compressor so that in the daytime only that compressor is used? Or should we go for having them on separate compressors so that both compressors are used in the daytime, but are each at a lower load.

I think our main concern is the energy consumption of either option, since electricity costs are pretty high here. I understand that from a reliability standpoint, having our offices on separate systems might be good, but I think we can deal with that, especially since air-con servicing here is pretty easily available.

  • I think it depends on the temperature difference between the two rooms. If your wife likes it much warmer then you, or you condition one room more than the other, you may benefit by a separate unit, otherwise I think a single would suit you better. Just make sure the unit(s) is/are properly sized for the space it is conditioning. A unit that is too large will cool too quickly and not dehumidify as well. – Gary Bak Oct 31 '18 at 11:37
  • I assume you are installing "ductless" mini-split units, right? – Jim Stewart Oct 31 '18 at 11:39
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I would install a split system with a single compressor. A mini split system is very efficient and is designed to supply cooling seperatly where needed the compressor condenser units only run at the speed needed to supply the load on the units I have installed. Electricity is expensive but a 2nd compressor will end up costing significanty more compared to 1 compressor with 2 evaporators (or more) because with 2 you now need 2 disconnects and the extra wiring since they will be in the same location. The only time I would suggest to have 2 compressors is when the home is large and the pressure and suction lines to the evaporators get long in this case it can be cheaper to have units located on opposite sides of the home reducing the line set length. But since the compressors will be located in the same location I would go with a single sized for the evaporators you need.

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    Some mini splits can handle up to 8 zones. I have a 5 zone system at my home it just depends on the model chosen – Ed Beal Oct 31 '18 at 14:59
  • I am guessing that this installation is in a market dominated by ductless mini-splits, at least for retrofits. AFIK the normal multi-zone mini-split condensing unit can supply four air handlers or less if the total capacity is 42 kBTU/h or less.. Since they have five rooms to cool, they need two condensing units. There are 48 kBTU/h condensing units that can feed five air handlers, so if they could use one that size then they could have only one condensing unit. – Jim Stewart Oct 31 '18 at 15:00
  • ah yes, i forgot to mention it's a ductless mini-split type of system. that's the predominant type available for residential units here. Based on what's available I think we will need to have 2 compressors to handle all 5 zones, the question is just how to split up the zones for the compressors to handle. Have the two rooms with high usage on the same compressor, or separate compressors. – Jimmy Hii Oct 31 '18 at 17:46
  • My system is a 5 zone on a single compressor it provides more cooling than we really need but in winter the heat is just on the edge when the temps get down in the teens. I have installed a few systems with 2 compressors and when doing this I try to balance the rooms based on the demand for example the master suite, and a bedroom and living room the family is usually in the living room or the bed rooms this allows the system to cool more in the living room during the day and at night more for the bedrooms. The second unit controlled the kitchen and dining area + 2 other bedrooms it worked well – Ed Beal Oct 31 '18 at 19:02
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If you don't need the added reliability of two systems, I think you'll want a single compressor outside rather than two small separate compressors.

It will work the same either way - you can cool two areas either way.

The inside part of the installation will be about the same cost either way.

The outside part of the installation will probably be cheaper with one unit - just one pad or hanger to prepare, one big electrical disconnect rather than two, etc. Less space used up by one big compressor than two small ones.

For maintenance, generally parts for one big unit will cost less than parts for two smaller units, and labor to maintain two small units will be double one large unit. So I'd expect lower maintenance costs with a single larger unit. (Again just speaking in generalities.)

You may be able to oversize the compressor a little bit so you have room for expansion, so the single large compressor system may be more flexible.

  • Thanks for the input! The thing is, based on what's available at stores here, we will probably need to go with two compressors to handle all 5 rooms I think. I'm more concerned with how the room evaporators are distributed between the two compressors, is there a more efficient layout since two of these rooms will see heavy use? – Jimmy Hii Oct 31 '18 at 17:49
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I assume that you are installing ductless mini-splits so that each room will have its own evaporator/air handler. This means that each room has fully independent temperature control and the air handler can be turned on and off at will. In the past, central a/c condensing units ("compressors") were more efficient operating at full load and continuously. I think that this is still the case with modern units, just to a lesser extent than formerly. This suggests that you should have both offices on the same condensing unit.

On the other hand, if you have the two offices on different condensing units, then it might be possible to have smaller max capacity condensing units because during day the evaporator/air handlers other than the offices would presumably be at a reduced cooling level, and after the office work is over, at night, only the master bedroom would be cooled. Of course, it is risky to try for the minimum total max cooling capacity because you could end up with an apartment which is not quite cool enough and which is too dry because the evaporators run continuously.

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