Hastings summers, and the winters around them
Hot afternoons
A Wikipedia table citing NIWA puts Hastings' average January high at 25.5 degrees, and NIWA records 34 days a year above 25 at Whakatu. We don't have a Hastings degree-day figure; the nearest is Napier Nelson Park, with 218 cooling degree-days a year.
Cold nights
The same Hastings table gives a July average low of 3.6 degrees, though it doesn't state its normals period. That's the other half of the sizing job.
Plenty of existing heat pumps
70.02% of Hastings District homes used a heat pump as a main heater in the 2023 Census. Mitsubishi Electric NZ points out heat pumps are sized to heat a room, not the whole house, so an older unit that struggles in summer may simply be covering more space than it was chosen for.
Sizing a heat pump that has to do both
EECA's rule is to size on the 35 degree cooling capacity if a unit will mainly cool, and on the 7 degree heating capacity if it'll mainly heat. In Hastings, a lot of living rooms do serious amounts of both, so read all three figures on the label: 7 and 2 degrees for heating, 35 for cooling.
BRANZ says cooling capacity generally runs 10 to 20% under heating capacity. A unit chosen only for winter can come up short in a hot west-facing room, and one chosen only for summer can come up short on a frosty morning.
For mostly summer use, BRANZ suggests a high EER, the cooling version of a COP; for mostly winter use, a high COP. Ask the installer to show you both numbers.
Glass matters as much as floor area. EECA says sizing should take in the size and orientation of windows, the insulation and the shading. In a high-ceilinged villa, mention the height, because Consumer NZ's sizing estimates assume 2.4 m.
The outdoor unit in a Hastings summer
BRANZ says heat pumps work most efficiently when the gap between indoor and outdoor temperatures is smallest, and suggests a warm north or west wall for heating. In Hastings, that same wall bakes on a January afternoon, so the spot is a genuine trade-off to talk through with the installer.
Cooling runs into warm evenings, when windows are open on both sides of the fence. Consumer NZ says outdoor units can be quite noisy, and noise limits sit in the district plan, so keep the unit away from bedrooms.
On cooling, a heat pump drains water steadily from the indoor unit. BRANZ says to run that hose to a drain or the garden, not onto a path.
Questions for the installer
- What are the heating figures at 7 and 2 degrees, and the cooling figure at 35?
- Which room did you size it for, and did you allow for its windows and sun?
- What are the COP and EER, and which matters more for how I'll use it?
- Will the outdoor unit sit in the afternoon sun, and is there a better spot?
- How far is that spot from the neighbours' bedrooms?
Before you add cooling
You can
- Note which rooms overheat and at what time of day.
- Draw the curtains on the west side before the afternoon heat.
- Choose Cool rather than Auto in summer, as Mitsubishi Electric NZ advises.
- Clean the filters and clear around the outdoor unit before summer.
Leave to a licensed installer
- Sizing for both seasons.
- Refrigerant pipework and commissioning.
- The electrical connection, with a CoC and an ESC.
- Routing the condensate drain.
What we'd do
What we'd choose in Hastings
We'd pick a unit whose 2 degree heating figure holds up, then check that its 35 degree cooling figure covers the room with the afternoon sun. If either falls short, we'd look at the next model up, keeping in mind BRANZ's warning that an oversized unit cycles on and off. And we'd ask someone who installs heat pumps for both seasons to look at that room's windows before quoting.