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 38.5 c/kWh rate:
- Dehumidifier, 500W, 8 hours/day, every day: 4 kWh/day, about $47/month.
- Dehumidifier, 500W, run 24/7 in a wet living room: 12 kWh/day, about $141/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 $56/month. Plus you get the warming effect.
- Heat pump in heat mode, 700W effective, 8 hours/day: 5.6 kWh/day, about $66/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.
The same dehumidifier across the regions
The figures above are Christchurch. Because a dehumidifier is just a fixed wattage running for a fixed time, its monthly cost scales straight off your local rate - there is no COP to complicate it. Here is the same 500 W unit run 8 hours a day, every day, at a few regional rates:
| Region | Rate (c/kWh) | 8 hrs/day | Run 24/7 |
|---|---|---|---|
| Wellington City | 37.0 | ~$45 | ~$135 |
| Christchurch | 38.5 | ~$47 | ~$141 |
| Auckland Central | 41.2 | ~$50 | ~$150 |
| Balclutha | 52.5 | ~$64 | ~$192 |
The gap between the two right-hand columns is what matters here: an eight-hour run is a reasonable monthly cost everywhere, but forgetting to switch it off triples it. A unit with a built-in humidistat (it cycles off once the room hits a target humidity) or a plug-in timer turns the 24/7 column back into something closer to the 8-hour one. You can check the figure for your own rate on the dehumidifier cost page.
Sizing and the mistakes that cost most
Running cost aside, a few choices decide whether the appliance actually solves the problem or just adds to the bill:
- Buy a unit with a humidistat. Set it to 55-60% and let it cycle itself off. A unit that runs flat out until the tank is full is the one that lands you in the $140 column.
- Match the size to the room. A 10 L/day unit will run continuously and lose in a big, wet living room; a 20-25 L/day unit clears the moisture faster and then idles, which is often cheaper overall despite the higher wattage.
- Close the room off. A dehumidifier drying a single room with the door shut is working on a few cubic metres of air. Leave the door open to the hallway and it is trying to dry the whole house, and never gets there.
- Empty refrigerant-style units struggle in the cold. Standard compressor dehumidifiers lose capacity below about 10-12C. In an unheated, properly cold room a desiccant model holds up better, though it draws a little more power.
- Do not run a heat pump and a dehumidifier at once in the same heated room - the heat pump's warming already drops the relative humidity, so the dehumidifier is mostly just adding to your bill.
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 mean less condensation, so less mould and 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
- The most expensive appliances in NZ homes - where an always-on dehumidifier ranks