Why the upper valley is its own climate
Frost most winter weeks
NIWA's frost table gives Wallaceville 54.0 ground frosts and 24.1 air frosts a year, 7.1 of them in July. It drops below 0 degrees on 24 days in an average year, and the region's lowest recorded temperature, -7.2 degrees, was measured there in June 1944.
A longer heating season
Wallaceville has 2,081 heating degree-days a year, 312 of them in July, against 1,625 at Wellington Aero. MBIE's H1/AS1 puts Upper Hutt City in climate zone 4, one zone colder than Lower Hutt City and Wellington City.
Fog, humidity and very little wind
Fog settles in the sheltered valley on 43 days a year at Wallaceville. Mean 9 am humidity there is 84.2% across the year and 89.9% in June, and the mean wind speed is just 9.1 km/h, the lowest of NIWA's selected Wellington stations.
Burners and postwar suburbs
In the 2023 Census, 68.59% of Upper Hutt City homes used a heat pump and 29.81% a wood burner. The city grew fast from the late 1940s, with subdivisions such as Clouston Park, Maoribank and Tōtara Park into the 1980s. Postcodes are 5018 and 5019.
What that weather does to a heat pump
It spends time defrosting. EECA says that below 5.5 degrees outside, a heat pump diverts some heating capacity to defrost its outdoor coil, and Mitsubishi Electric NZ says the more moisture in the air, the more freezes on it. Foggy valley mornings are exactly that, so a short pause in heating is normal, not a fault.
It loses output in the cold. The label EECA regulates shows heating capacity at 7 and at 2 degrees outside. Where nights get to 2 degrees or less, EECA says the 2 degree figure should match or beat the 7 degree one. That's the number to compare here.
It needs a good spot outside. Mitsubishi Electric NZ says units under houses, decks or anywhere airflow is blocked can create their own microclimate. On a still night, a boxed-in unit sits in its own cold, damp air.
Questions to ask an installer quoting in Upper Hutt
- What's this unit's heating output at 2 degrees, against the room you measured?
- Where will the outdoor unit go, and how much clear air will the coil have?
- Where will the defrost water drain, so it doesn't freeze across a path?
What we'd do
What we'd do in Upper Hutt
We'd size for the 2 degree figure, not the headline one, and lean bigger for a lounge that loses the sun early. We'd put the outdoor unit somewhere open and raised, away from the deck and from bedroom windows, because sound carries on a calm night. If the windows still run with water once it's warm, we'd look at home ventilation next.