Palmerston North summers, in context
Warm spells
February days average a high of 23.2 degrees on 1991 to 2020 normals, via Wikipedia, and NIWA's AWS records an average of 25 days a year above 25 degrees.
A small cooling need
NIWA's degree-day table shows 88 cooling degree-days a year at the AWS. The same station averages 10.1 air frosts a year, which is why winter should drive the choice of unit.
The humid season is winter
Mean 9 am humidity at the AWS peaks at 86.8% in June and July, NIWA says, so a heat pump's dry mode is more of a winter tool here than a summer one.
Wind
Gusts over 63 km/h blow on 50 days a year at the AWS. On an exposed wall, the outdoor unit's bracket and fixings need to suit that.
Getting good cooling from a winter heat pump
Size on heating first. EECA says to size on the 7 degree heating figure for a unit that mainly heats, and to check the 2 degree figure where nights get that cold, as they do on frosty Palmerston North mornings.
Then check the cooling. BRANZ says cooling capacity is generally about 10 to 20% lower than heating capacity, so read the cooling figure at 35 degrees for your hottest room, often the one with the most west or north glass.
One unit in the main living area covers the space most people use on a hot evening, and Consumer NZ says that's where split systems work best in an open-plan home. If upstairs bedrooms overheat, a multi-room heat pump runs several indoor units from one outdoor unit, but they all have to be on cooling or all on heating at once.
What makes the job bigger or smaller
Smaller: you already have a heat pump in the living room and it only needs checking. Try Cool mode rather than Auto, as Mitsubishi Electric NZ suggests by season, and clean the filters after a long heating season.
Bigger: adding a unit for a hot bedroom, going multi-room, or long pipe runs to an upstairs room, since Mitsubishi Electric NZ says excessive lengths and bends reduce output. Switchboard work may also need a licensed electrical inspector, and each new connection needs a Certificate of Compliance from a registered electrician.
What to ask before adding cooling
- Is my existing heat pump's cooling capacity enough for this room?
- What's the new unit's heating capacity at 2 degrees, and cooling at 35?
- Where will the outdoor unit go, and how close is that to a neighbour's bedroom?
- Will the switchboard need work, and who arranges any inspection?
Try before you buy
You can
- Run your existing heat pump on Cool for a hot spell before deciding.
- Clean its filters after the winter.
- Shade west-facing windows on hot afternoons.
- Note which rooms overheat, and when.
Leave to a licensed installer
- Checking an existing unit's cooling capacity against the room.
- Adding indoor units or a second system.
- Refrigerant work and commissioning.
- Electrical work and the Certificate of Compliance.
What we'd do
What we'd do before adding cooling
We'd give the heat pump we already had a summer on Cool, with clean filters and the west blinds drawn, before buying anything. If one room still cooks, we'd add cooling for it with a unit that's also strong at 2 degrees, because here it'll spend far more hours heating than cooling. And we'd want an installer who knows frosty-winter sizing to look at it.