Nelson summers, by the numbers
Warm, not scorching
On NIWA's 1991 to 2020 normals, Nelson's February days average a high of 22.7 degrees. That's comfortable for most of the house, so cooling earns its keep in the rooms that catch the afternoon sun rather than everywhere.
A small cooling load next to heating
Nelson's 56 cooling degree-days a year are a sliver of its 2,108 heating degree-days (NIWA). Size for winter first, then check the cooling figure, because BRANZ says cooling capacity generally runs about 10 to 20% below heating capacity.
Humid air and the drain
Nelson's mean 9 am humidity is 81% across the year (NIWA). On cooling, a heat pump pulls water out of the air, so the condensate drain needs a proper route: BRANZ says into a drain or garden, not onto a path.
Salt near Tasman Bay
Homes within 500 m of Tasman Bay or Nelson Haven, or 100 m of a tidal estuary, fall in NZS 3604's exposure zone D, and breaking-surf beachfronts can be zone E. BRANZ wants extra anti-corrosive coatings on outdoor units there, and Samsung's NZ warranty doesn't cover damage from salt spray.
Sizing a heat pump for a Nelson summer
EECA's rule is to size on the 35 degree cooling capacity if the unit will mainly cool, and on the 7 degree heating capacity if it will mainly heat. In Nelson, most homes fall in the second group.
A sunny room with lots of west-facing glass can need more cooling than a bigger, shaded one. That's why EECA's sizing advice counts the size and orientation of windows, the insulation and the shading, not floor area alone. Someone who installs heat pumps for summer cooling regularly will look at the sun on that room before quoting.
Efficiency differs by season too. BRANZ suggests a high EER (energy efficiency ratio) for a unit that mostly cools and a high COP for one that mostly heats. A Nelson unit does both, so ask for both figures. Our national air conditioning page explains the label in detail.
What makes the job bigger or smaller
One high-wall unit in a living room is the simplest job. Cooling several bedrooms points to a ducted or multi-room system, and a long pipe run between indoor and outdoor units adds work. Near the coast, corrosion protection is an extra step, so make sure it's on the quote.
Noise matters more in summer, when cooling runs on warm evenings with windows open. Noise limits sit in the council's district plan, and a building sign-off doesn't protect a noisy outdoor unit from enforcement, so BRANZ's advice to keep it away from quiet areas like bedrooms should include the house next door.
Ask before you buy for cooling
- What's the cooling capacity at 35 degrees, and the heating capacity at 2?
- What's its blue star rating for the Cold zone on the label?
- Where will the outdoor unit sit, and how near is it to the neighbours' bedrooms?
- Where will the condensate drain end up?
Your jobs and the installer's
You can
- Note which rooms overheat, and at what time of day.
- Clean the filters before summer, the way the manual shows.
- Keep plants trimmed back from the outdoor unit.
Leave to a licensed installer
- Sizing for the room's glass, sun and insulation.
- Refrigerant pipework, commissioning and the drain.
- The electrical connection, with a Certificate of Compliance.
What we'd do
Our view for Nelson
We'd buy a heat pump for Nelson's winters and treat cooling as the summer bonus, checking that the cooling figure covers the hottest room. If one west-facing room is the real problem, we'd size that unit on its 35 degree cooling capacity and put the outdoor unit where it won't hum outside a neighbour's open window. Near the bay, we'd add corrosion protection to the quote.