Where does summer cooling earn its keep?
Heating still does most of the work almost everywhere. NIWA's degree-day tables measure the energy needed to heat or cool a building to 18 degrees, and even Napier Nelson Park logs 1,482 heating degree-days a year against 218 for cooling.
Some places do get proper summers, though. NIWA records an average of 44 days a year above 25 degrees in Napier, and gives Whangārei Airport 228 cooling degree-days a year. Auckland's warmth comes with damp air: NIWA describes its summers as warm and humid, and puts the year-round mean 9 am relative humidity at Auckland Airport at 82%.
Further south, cooling is mostly about a few hot spells. Dunedin's Musselburgh station averages only 22 cooling degree-days a year, though NIWA says warm north-westerly days often push Canterbury's summer highs past 30 degrees.
The room matters as much as the region. EECA says sizing should allow for the size and orientation of the windows, the insulation and the shading, so a west-facing room full of glass needs more cooling than its floor area suggests.
Sizing a heat pump for cooling
EECA's rule is clear. If the unit will mainly cool, size it on its cooling capacity at 35 degrees outside. If it will mainly heat, size it on the heating capacity at 7 degrees.
Don't assume a unit sized for winter will cool just as hard. BRANZ says cooling capacity is generally about 10 to 20% lower than heating capacity, so check both numbers for the rooms you care about.
For a unit that will mostly cool, BRANZ suggests choosing one with a high energy efficiency ratio (EER), the cooling version of a COP. It puts typical domestic heat pumps at an EER of about 2.5 to 4.0. Our sizing guide covers the heating side.
Reading the label for summer
The Zoned Energy Rating Label, which EECA regulates here, shows cooling capacity at 35 degrees outside. Blue stars rate cooling efficiency and red stars rate heating, and all of New Zealand sits in the Cold zone, so read the stars for that zone.
The kWh per year figures give an idea of how much power the unit may use in each zone, and lower is better. EECA says they're only an indication, because every household uses its heat pump differently. Our running cost tool turns them into an estimate using your own power rate.
The house icon shows noise. EECA says those figures are measured at full capacity on cooling, with one number for near the indoor unit and one for outside, though showing noise is optional here. For scale, EECA puts a quiet home at 30dBA and a typical conversation at 60dBA.
Keeping summer nights quiet
Cooling often runs late on hot nights, when the neighbours have their windows open. Noise limits are set in each council's district plan. Auckland's Unitary Plan, for example, measures noise in residential zones within the neighbouring site. The limit there is 50dB LAeq from 7am to 10pm Monday to Saturday and 9am to 6pm on Sunday, and 40dB LAeq at all other times.
BRANZ suggests placing the outdoor unit away from quiet areas like bedrooms, and that should include the house next door. A council sign-off on the install doesn't protect an outdoor unit from noise enforcement later, so get the spot right first. Our consents and noise guide has the detail.
One more thing on placement. BRANZ's advice to use a warm north or west wall is about heating, and it says heat pumps run most efficiently when the gap between indoor and outdoor temperatures is smallest. If summer is your main use, ask the installer whether the outdoor unit will sit in full afternoon sun.
Running it well when it's hot
Pick Cool rather than Auto for summer: Mitsubishi Electric NZ recommends choosing the mode by season. Rinnai NZ suggests running a heat pump at around 18 or 19 degrees on cooling. Check your own manual too.
Position matters for comfort. Consumer NZ says to keep a high-wall unit's airflow away from where you usually sit, so you're not stuck in a draught with fan noise in your ear.
On cooling, the indoor unit pulls water out of the air, so the condensate drain needs somewhere sensible to go. BRANZ says hoses should run into drains or gardens, not onto paths.
Jobs for you, jobs for the installer
You can
- Pick the rooms that actually overheat, and when.
- Check the cooling capacity at 35 degrees and the blue stars on the label.
- Look at where the outdoor unit would sit compared with bedrooms, yours and the neighbours'.
- Clean the filters before summer and keep the outdoor unit clear.
Leave to a licensed installer
- Sizing for the room's glass, sun and insulation.
- Refrigerant pipework and commissioning.
- The electrical connection, with a Certificate of Compliance and an Electrical Safety Certificate.
- Routing the condensate drain to a drain or garden.
Questions to ask if cooling is the main job
- What's the cooling capacity at 35 degrees, and the heating capacity at 7 and 2?
- How many blue stars does it get in the Cold zone?
- What noise figures does the label give for the indoor and outdoor units?
- Where will the outdoor unit go, and will it bake in the afternoon sun?
- Where will the condensate drain end up?
What we'd do
What we'd look for
In most of the country we'd still buy for winter, then make sure the cooling figure covers the hottest room. Where summer really is the problem, say a humid Northland home or a glassy room in Hawke's Bay, we'd size on the 35 degree cooling capacity and take night-time noise seriously. And we'd ask someone who installs heat pumps for summer cooling regularly to look at the sun on that room before quoting.