One Zone or Multi Zone? Choosing a Mini Split Layout
Multi zone systems look like the obvious choice on paper. Here is where they work well and where separate single zone units are the better buy.
By Chris Wheeler, Co-owner ·
When people picture a ductless system for a whole house, they usually picture one outdoor unit feeding four or five heads. That is a multi zone, and it is a good product. It is just not automatically the right one.
How the two differ
A single zone system is one outdoor unit matched to one indoor head. If you want three rooms done, you install three complete systems.
A multi zone system is one larger outdoor unit with refrigerant lines running to several indoor heads. One box outside, several heads inside.
What multi zone is good at
One outdoor unit. If you only have one sensible place to put equipment outside, or you do not want three units lined up along the side of the house, this settles it. It matters more than people expect on a nice house.
Less line set penetration. One set of lines leaving the building instead of several.
Lower cost per head, sometimes. Once you are past three or four heads, the multi zone often comes out ahead on equipment.
Where multi zone gets you in trouble
The turndown problem. This is the big one and it does not show up until you own it.
A multi zone outdoor unit is sized for all the heads running. In the shoulder seasons you might have one bedroom head calling for a little heat and nothing else. The outdoor unit still has to run, and it has a minimum output. If that minimum is well above what the one room actually needs, it overshoots, shuts down, and cycles.
The result is a system that works great in January when everything is calling, and feels twitchy in April when only one room is. Single zone units do not have this problem because each one only ever serves its own load.
One failure takes out everything. If the outdoor unit goes down, every room goes down. With separate systems, a failure is one room and the rest of the house is fine. On a house that relies on these for primary heat in a New Hampshire winter, that is worth thinking about.
Less flexibility in what you install. With single zone systems you can put the right size and style in each space. A multi zone limits you to heads the outdoor unit supports, and the capacity available to each one depends on what the others are doing.
Harder diagnostics. More refrigerant circuits and more shared controls. Troubleshooting takes longer, which shows up in labor.
How we usually land
Two or three heads, and the rooms have very different schedules? Often single zone units, especially if one is a bedroom that only needs conditioning at night.
Four or more heads, whole house, running most of the time? Multi zone usually makes sense, and the turndown problem is smaller because something is generally calling.
A primary living space plus occasional rooms? Frequently a mix. Single zone on the main space that runs constantly, multi zone for the bedrooms that share a schedule.
Aesthetics rule the decision? Multi zone, and we design around the turndown by being careful about head sizing.
The question to ask any contractor
Ask what the minimum output of the proposed outdoor unit is, and compare it to the load of your smallest room. If the minimum output is well above what that room needs on a mild day, you will feel it.
A contractor who has thought about your house can answer that. One who is reading off a package price will not know what you are asking.
On cost
Equipment pricing moves, and what a job costs depends on line set runs, electrical work, wall construction, and how many stories we are working on. We are not going to publish a number that will be wrong by the time you read it.
What we will do is quote both options when both are reasonable, and explain the tradeoff. If a customer wants the single outdoor unit for how it looks, that is a legitimate reason and we will design it properly. We just want you choosing it knowing what you are choosing.