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

Both columns below come from one publisher’s zone table (HomeGuide; historical cost estimate, archived source dated December 9, 2025): the multi-zone figure is its row for that zone count, and the separate-systems figure is its single-zone row multiplied out. HomeGuide prints no 4-zone row, so the 4-zone figure is derived by interpolating between its 3- and 5-zone rows. 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. One outdoor unit means one compressor and one defrost cycle serving every head, so one failure can take every room offline at once. Separate systems mean more outdoor units on the wall or pad, but a dead compressor is one cold room rather than a cold house. If the midpoints are within 10 percent of each other, this site declines to pick a winner on price. Our own cut-off: two layouts whose midpoints are within this fraction of each other are called too close to call.

One structural note the table hides: at eight zones^[4] HomeGuide prints its row open-ended, as $18,000 – $20,000+. We carry the stated top 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 HomeGuide’s two- and three-zone midpoints ($6,250 and $8,750, derived) 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; some open-plan houses may want fewer still.

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

The smallest size in our standard head-size list is 6,000 BTU/h. A ten-by-eleven bedroom in a reasonably tight house works out to less than that on the sizing table. An oversized head may run at the bottom of its output range or cycle on and off; ask the installer how the specific model behaves at low load. The practical response is often to ask whether two small rooms can share one head, which turns two zones into one and takes a cost step out of the quote.