When does a multi-room heat pump make sense?
It suits a house where three or four rooms need heating, there's no roof space for ducts, and you'd rather not line the outside wall with boxes.
Outdoor space and rules often decide it. In one Tenancy Services example, a body corporate rule bans external heating units on common property without its consent. One outdoor unit means one location to agree on rather than several.
Next to a ducted heat pump, there's far less work in the ceiling, but you'll see an indoor unit in every room. Next to separate single-split units, you give up some independence: if the shared outdoor unit stops, every room on it goes cold.
Multi-room, separate units or ducted?
| Multi-room heat pump | Separate single-split units | Ducted heat pump | |
|---|---|---|---|
| Units on the outside wall | One | One for each room | One |
| What you see in each room | An indoor unit | An indoor unit | A small vent |
| One room heating while another cools | Not possible, Consumer NZ says | Possible, each runs on its own | Not possible |
| If the outdoor unit fails | Every room on it stops | Only that room stops | The whole system stops |
| Needs a roof space or subfloor | No | No | Yes, for the unit and ducts |
Getting the design right
Each indoor unit is chosen for its own room, and the outdoor unit for all of them together. Mitsubishi Electric NZ warns that a heat pump's output drops as the temperature falls. For places with sub-zero winters, which it says includes most of the South Island, it recommends a larger capacity heat pump or one of its upgraded models.
Pipe length matters more than with a single unit, because a bedroom can be a long way from the outdoor unit. BRANZ says the pipe maker's recommended run lengths shouldn't be exceeded, and Mitsubishi Electric NZ says long runs and extra bends cut output. A central spot for the outdoor unit keeps every run shorter.
Think about noise as well. Consumer NZ says the outdoor unit can be quite noisy, so consider the neighbours if it has to sit near your boundary. BRANZ suggests keeping it away from bedrooms, which is easier with one unit to place than three.
Check the warranty for your exact combination
Multi-split terms don't always match a brand's single-room terms, so ask which one applies to your system. Black Diamond Technologies (BDT) lists Mitsubishi Electric's MXZ and PUMY multi-split systems, indoor units included, at 5 years parts and labour. Temperzone gives Hitachi Multi-Split (RAM) systems 6 years parts and labour. Samsung NZ covers the compressor on its multi-split (FJM) types for 5 years parts and labour, against 10 years for its wall-mounted single splits.
Commissioning can matter too. MHIAA, the distributor for Mitsubishi Heavy Industries, lists some KX Multi systems at 1 year, extended to 5 once it receives supervised commissioning data from its own engineer or an approved agent. That alone is a good reason to use an installer who sets these systems up often.
Who does which part, and the paperwork
The electrical connection is prescribed electrical work for a registered and licensed electrical worker. They must hand you a Certificate of Compliance (CoC) no later than 20 days after finishing, and you should also get an Electrical Safety Certificate (ESC) for the connection.
Refrigerant work across the whole circuit is for a heat pump technician. No law licenses it here. Under a 2011 industry deal the Commerce Commission allowed, wholesalers may supply refrigerant only to holders of an RLTB Refrigerant Licence, or an EPA Approved Handler or Approved Filler certificate.
Christchurch City Council says installing a heat pump needs no building consent when registered electricians do the work and certify it. Wherever you live, the Building Code, the district plan's noise limits and any body corporate rules still apply.
Before and after the install
You can
- List the rooms you'd heat, and when each one gets used.
- Look for an outdoor spot near the middle of the house, away from bedrooms on both sides of the fence.
- Clean each indoor unit's filters the way its manual describes.
- Keep the outdoor unit clear of plants and clutter.
Leave to a licensed installer
- Matching the indoor units to the outdoor unit's capacity.
- Keeping every pipe run inside the maker's limits.
- Charging and commissioning the refrigerant circuit.
- The electrical connection and the Certificate of Compliance.
Questions to ask before you say yes
- What heating capacity does each indoor unit have, and how did you size them?
- How long is the longest pipe run, and is it within the maker's limit?
- Would separate single-split units suit this house better, and why or why not?
- Which warranty term applies to this exact combination, and does it depend on commissioning records?
- Where will the outdoor unit go, and how near is it to the neighbours' bedrooms?
- Who connects the power, and when will I get the CoC and the ESC?
What we'd do
Our take
We'd go multi-room when the rooms you heat sit fairly close together and outside wall space, or a body corporate, makes more than one outdoor unit awkward. If one bedroom is at the far end of a long house, we'd ask for separate single-split units to be quoted alongside it. And we'd want an installer who designs multi-split systems often, because the pipe runs and the commissioning decide how well it heats and how long the warranty lasts.