How many heads, and should they share one outdoor unit?

Both columns below come from one publisher’s zone table: the multi-zone figure is its row for that zone count, and the separate-systems figure is its single-zone row multiplied out. Using one table on both sides is the only way this comparison means anything — mixing publishers would let the answer flip on a sampling difference rather than on the thing being compared.

The price answer is not the whole answer, and at three or four zones it is not even the interesting part. One outdoor unit means one compressor, one defrost cycle, one set of refrigerant lines to leak, and one failure that takes every room offline at once. Independent systems cost more line-set labour and eat more wall space outside, but a dead compressor is one cold room rather than a cold house. If the difference at the midpoints is within about ten percent, this site says so and declines to pick a winner on price.

One structural note the table hides: at eight zones^[4] the published row is open-ended, printed with a plus sign. We carry the stated number and say here that the real top is unbounded, rather than silently treating an open interval as a closed one.

Paying for a head that cannot do anything

A head holds one temperature across whatever volume of air it can reach. If your kitchen opens into the dining room through an opening wide enough to walk through two abreast, the two rooms are one thermal volume no matter what the floor plan calls them. Putting a head in each means the two units spend their lives fighting each other across an open doorway, and you paid roughly the difference between $6,250 and $8,750 for the privilege. The worksheet drops one head when you tell it the main space is open, which is the conservative version of this rule — real open-plan houses often want two fewer.

Buying a head that is too big for the room it is in

The smallest head on the market is 6,000 BTU/h. A ten-by-eleven bedroom in a reasonably tight house does not need that much, and there is nothing smaller to buy. On a fixed-speed unit that would be a real problem; on the inverter units almost all mini-splits now use, it means the compressor spends its time at the bottom of its modulation range, which is inefficient rather than harmful. The practical response is not to hunt for a smaller unit that does not exist — it is to ask whether two small rooms can share one head, which turns two zones into one and takes a real cost step out of the quote.