Yes, a heat pump can pull some moisture out of the air. Just not as much as you’d hope. Most units have a dry mode, and it does take water out of a room. What it won’t do is fix a damp house on its own, especially in the middle of a Southland or Waikato winter. Damp is a heating, ventilation and moisture-source problem. The heat pump is one part of the answer.
What does dry mode on a heat pump actually do?
It’s the setting with the water droplet symbol on most remotes. It removes water much the same way cooling does: moisture in the room air hits the cold indoor coil, condenses, and drains away through the drain pipe, usually to the outside.
Two things follow from that:
- Dry mode doesn’t heat. While it runs, the room isn’t being warmed, and it can get cooler.
- The drying is limited. It’s a side effect of cooling, so heat pumps aren’t built to pull out big volumes of water the way a dedicated dehumidifier is.
So the sensible use of dry mode is short spells when you don’t need heating. A mild, muggy day in Auckland or Northland. A sticky summer evening. An hour after a steamy shower on a warm day. On a cold winter’s night, it’s the wrong setting.
How do you use dry mode properly?
- Close the doors and windows. Otherwise the unit is drying outside air that keeps flowing in.
- Run it in short spells. An hour or two is plenty, then switch back to heat if the room needs warming.
- Keep the filters clean. Regular cleaning keeps the unit efficient and limits the build-up of condensation inside it.
- Check the drain outlet. The water has to get away. If the drain pipe outside is blocked or kinked, water can back up into the indoor unit.
Is dry mode or cooling better on a muggy summer day?
On a humid summer day, damp air feels warmer and stickier than the thermometer suggests. You have two choices. On a mild day, dry mode takes the edge off without chilling the room. On a hot day, cooling mode set to about 25 to 27°C removes moisture as it cools, and setting it a little lower helps on especially muggy days. Heat pumps vs air conditioners covers cooling in more detail.
→ Related: Heat pump winter settings that keep rooms dry
Does heating with the heat pump dry out a damp house?
Not by removing water. It helps in another way. Warmer rooms have warmer walls, ceilings and windows, and moisture is less likely to condense on warm surfaces. That’s why heating your living areas to a healthy temperature is one of the main ways to control damp. Aim for 18 to 22°C in the rooms you use. At 18°C or above, you’re helping to fight damp and mould.
The catch is that heating alone doesn’t get rid of the water. If moisture keeps going into the air from cooking, showers, washing and the ground under the house, it has to go somewhere. That’s where ventilation and source control come in.
Where is the damp in your house coming from?
Before buying anything, find the source. The big ones in most Kiwi homes are:
- The ground under the house. A surprising amount of moisture can rise from under a suspended floor, up to 40 litres a day, even when the soil looks dry. A sheet of thick polythene laid over the ground, run several centimetres up the foundation walls and taped at the joins and around the piles, is one of the most effective fixes.
- Kitchens, bathrooms and laundries. Cooking, showers and baths put a lot of water into the air. Extractor fans that vent outside, not into the roof space, remove it at the source.
- Clothes dryers. A dryer that isn’t vented outside pumps moisture straight into the house. Vent it outside, or use a condensing or heat pump dryer.
- Leaks. Check for plumbing leaks under the house and around the bathroom.
Sort those out, heat to a healthy temperature and improve ventilation, and you usually won’t need a dehumidifier at all.
→ Related: The Healthy Homes heating standard explained
Heat pump dry mode or a dehumidifier: which do you need?
| Heat pump dry mode | Dehumidifier | |
|---|---|---|
| Main job | Heating and cooling, with some drying | Removing water from the air |
| Heats the room | No, not in dry mode | No, it isn’t a heater |
| Where it works | The room the indoor unit is in | Any room you carry it to |
| Best for | Short spells on mild, humid days | Problem rooms while you fix the cause |
If you do use a dehumidifier, empty the water tank often and keep the windows and doors closed while it runs, or it ends up trying to dry the whole neighbourhood.
Do you need ventilation as well?
Good airflow helps stop moisture building up, but for real damp problems a ventilation system or a dedicated dehumidifier may be needed alongside the heat pump. A heat pump moves the air that’s already in the room. It doesn’t bring fresh air in, so it can’t replace ventilation.
Simple habits help too: open windows for a while on fine days, use the extractor fan every time you cook or shower, and dry washing outside whenever the weather lets you.
What if the house is still damp?
If the house is damp despite heating, it may be time to look at ventilation as well as heating. Find a home ventilation installer near you, browse heat pump installers, or choose your town and filter by the job.