Damp NZ homes in winter: dehumidifier or heat pump?

New Zealand homes are famously damp in winter. The combination of high outdoor humidity, old housing stock with limited ventilation, and a national habit of drying washing indoors means most homes spend July sitting at 70-85% relative humidity, well into the range where mould thrives and laundry refuses to dry.

Two appliances tackle this. They do quite different jobs and the right one depends on what you actually need.

What each one does

A dehumidifier pulls moisture out of the air and dumps it into a tank or down a drain. A typical household unit removes 10-20 litres of water a day, drawing 400-600 watts continuously. It does not meaningfully heat the room, although it will add a degree or two from the latent heat of condensation.

A heat pump in heat or dry mode warms the air, which lowers the relative humidity even though no moisture is removed. In dry mode specifically, it also condenses some water onto the indoor coil and drains it outside. A heat pump uses more total electricity (electrical input ~600 W to 1 kW depending on size) but does two jobs at once.

The cost numbers

At Christchurch's current 36.6 c/kWh rate:

• Dehumidifier, 500W, 8 hours/day, every day: 4 kWh/day, about $45/month.
• Dehumidifier, 500W, run 24/7 in a wet living room: 12 kWh/day, about $135/month. This is a worst-case number, but a lot of people end up here in July without realising.
• Heat pump in dry mode, 800W effective draw, 6 hours/day: 4.8 kWh/day, about $53/month. Plus you get the warming effect.
• Heat pump in heat mode, 700W effective, 8 hours/day: 5.6 kWh/day, about $62/month. The dryness effect is a free side-effect of warming the room.

So they are in the same ballpark per month. The real question is what you want from the hour you spend running them.

When to use which

• Use a dehumidifier when: the room is unheated and you need to dry it without spending money to warm it (a garage with stored items, a sleepout you only use sometimes, a bathroom after showers). Or when you are drying a specific load of laundry in a small room.
• Use the heat pump in dry mode when: the room is already going to be heated, you want lower humidity but not aggressive drying, and you want to save the cost of running two appliances at once. Most modern heat pumps have a dedicated dry mode for this.
• Use the heat pump in heat mode when: the room is in active use. The heating itself drops humidity from 80% to 55-60% by warming the air, even without dry mode.

The real fix is upstream

Both appliances are treating symptoms. The water in the air comes from somewhere:

• Drying washing indoors: every kg of clothes releases about 0.5 kg of water into the air. A full load is 4-5 kg of water dumped into your house. Either dry outside, in a heat-pump dryer, or in a small enclosed room with the dehumidifier targeted at it.
• Cooking and showers: rangehoods that vent outside (not recirculating filters) and bathroom extract fans on timers move moisture out before it condenses on cold walls.
• Unflued LPG heaters: a 9 kg cylinder produces roughly 13 litres of water vapour as it burns through. They are explicitly prohibited as the main heater in rentals under the Healthy Homes Standards (alongside open fires). If you still use one in your own home, this is by far your biggest indoor moisture source.
• Subfloor moisture: a polythene ground vapour barrier under the floor in older houses can dramatically cut moisture rising into the building.
• Insulation and double glazing: condensation forms on cold surfaces. Warmer interior surfaces means less condensation, less mould, less need for either appliance.

Most NZ homes that solve their winter dampness problem do it with a combination of one heat pump + ventilation discipline + drying washing somewhere it can vent. The dehumidifier becomes optional rather than essential.

To check what your specific dehumidifier or heat pump usage costs, plug both into the NZ Power Bill Calculator and see them side by side.

Related guides

• How much does a heat pump cost to run in NZ? - monthly cost figures by region and COP
• Insulation or a heat pump: where to spend first - fixing moisture at source via the building shell