Multi-zone or several wall-mounted heat pumps: what's best to heat the whole house?

In short: one wall-mounted heat pump heats an open area well, but the heat doesn't travel well into closed bedrooms or up to another floor. To go further, there are two ways to do it: several wall units, each with its own outdoor unit, or a multi-zone, meaning one outdoor unit connected to several indoor units. Both work; the right choice depends on your floor plan, the room you have outside and what you want to heat.

"We have a wall unit in the living room, but the bedrooms stay cold. What should we add?" We hear this one a lot, especially in homes heated with baseboards and no ductwork. Here are the options, what Hydro-Québec and Natural Resources Canada (NRCan) say, and the questions to ask before you choose.

Single-zone, multi-zone: what's the difference?

NRCan sorts ductless heat pumps into two families:

• Single-zone (mini-split), the classic wall-mounted heat pump: one indoor unit served by one outdoor unit.
• Multi-zone (multi-split): several indoor units around the house, served by a single outdoor unit.

With a multi-zone, the indoor units don't all have to be wall-mounted: depending on the manufacturer, there can also be ducted units (hidden, with short ducts), ceiling units or floor consoles. If your home already has ductwork, it's a different question: see our page on central units and heat storage.

Why one wall unit isn't always enough

A wall-mounted heat pump heats the room it's in and the open spaces around it. Hydro-Québec actually recommends aiming the airflow toward open spaces so the heat spreads to other rooms. But a closed door, a long hallway or a staircase gets in the way of the air. The result: the baseboards in the bedrooms and on the other floor still do much of the work.

That isn't a flaw: backup heat is what they're there for. But if you want the heat pump to heat (and cool in summer) more rooms, you need more than one indoor unit.

Option 1: several wall units, one per zone

For a two-storey home without a forced-air system, Hydro-Québec says two mini-split units, one per floor, would guarantee uniform heat distribution while optimizing energy savings.

The pros:

• Each unit works on its own. If one heat pump needs a repair, the other keeps heating.
• Each unit is chosen for its zone. You can go bigger on the main floor and smaller upstairs, and pick models built for very cold weather.
• You can do it in steps: one wall unit this year, a second one later.

Plan for: one outdoor unit per heat pump. You need room for each one, you have to keep each one clear of snow in winter, and you should think about noise for the neighbours.

Option 2: a multi-zone, one outdoor unit for several rooms

The pros:

• A single outdoor unit for several rooms: handy when space is tight on the lot or along the house, and there's only one to clear of snow.
• A setting for each room: each indoor unit has its own temperature, and you can keep a room you rarely use cooler.
• More choice of indoor units (wall, ducted, ceiling, console) to suit each room.

The limits:

• The model limits how many indoor units you can have. For example, according to Gree's brochure, its Multi R32 multi-zone takes 2 to 3 indoor units in the 24,000 BTU size and 2 to 4 in the 30,000 BTU size.
• Shared capacity. The outdoor unit provides a total capacity that's split between the rooms. In very cold weather, it also depends on the type of indoor units: Gree's brochure lists different values for wall units, ducted units or a mix of both.
• One compressor for everyone. If the outdoor unit has a problem, every room it serves is affected at the same time.
• Distances to respect. The manufacturer sets a maximum piping length between the outdoor unit and each indoor unit, which affects where each unit can go.

Which option for which home?

These are starting points for the conversation, not a rule: every home is different.

Either way, each unit's capacity has to be calculated for your needs. NRCan explains that an undersized heat pump leaves more of the work to your backup heat, and an oversized one turns on and off often in mild weather, which hurts comfort and wears out the unit.

Questions to ask before you sign

• How much heat in the cold? Ask for the spec sheet and look at the heating capacity at -8°C and -15°C, not just the number on the label.
• How many indoor units, and which ones? For a multi-zone, check how many the model takes and whether the proposed combination really covers each room.
• Where will the outdoor unit go? Hydro-Québec recommends checking your municipality's rules for its location and choosing a model that suits your surroundings noise-wise. Also think about snow sliding off the roof.
• What changes for maintenance? Each indoor unit has its own filters to clean, and NRCan recommends yearly maintenance by a qualified contractor. See our article on cleaning the filters.

Sources

• Hydro-Québec, How do you match heat pump capacity with your home's needs? (condo and two-storey house sections).
• Hydro-Québec, Conseils d'utilisation des thermopompes (in French; wall-mounted heat pump section).
• Hydro-Québec, Sept points à considérer avant d'acheter une thermopompe (in French; noise and location).
• Natural Resources Canada, Heating and Cooling With a Heat Pump (Air-source heat pumps, Sizing and Maintenance sections).
• Gree, Multi R32 brochure (number of zones and capacities by indoor unit type).