About a quarter of New Zealand homes are using heat pumps to provide warmth this winter, but experts warn that if buildings have inadequate insulation, or the heat pumps are too small, the power bills may be unnecessarily high.
A BRANZ building research levy-funded study shows about a quarter of houses now have heat pumps, up from just 4 per cent in 2005. Up to 70 per cent of new houses are built with heat pumps already installed.
The pumps are more effective than other electric heaters because their heat output is about three to four times the energy they use.
A 2kW fan heater will put out 2kW of heat and cost about 50c an hour, or twice the electricity company's per-kWh tariff. But a 2kW heat pump will put out up to three times that much for the same price.
Energy Efficiency and Conservation Authority (EECA) technical adviser Christian Hoerning said heat pumps were good for large rooms where the ongoing reduction in running costs paid off the initial cost.
But, he said, in small rooms, electric heaters could be equally efficient.
Installing a heat pump costs between $2,000 for a small room and $12,000 for a whole-house system.
However, Consumer NZ adviser Maggie Edwards said there could be problems with heat pumps in houses with poor insulation.
"If you have an uninsulated villa, it might not be worth getting one," she said. "A few years ago, we had reports of lots of people having heat pumps where the thermostat never went off because it never got warm enough. All the heat was going straight out the top of the house."
She said in those cases people who bought heat pumps would not find their power bills decreased much.
"If you install a heat pump and keep your home about as warm as you do now, you could save a considerable amount in heating costs. But some of our subscribers with heat pumps tell us they use their units to keep their homes warmer than before, so their heating bills haven't dropped by much."
Heat pumps are cheaper to run when they can heat a room easily to a thermostat-controlled temperature and keep it there, rather than constantly working to add heat.
Edwards said people sometimes bought heat pumps that were too small for the room. "You're better off buying one that's larger than you think you need because then it doesn't have to work as hard."
Hoerning said several online calculators could help people work out what size heater they needed.
"I don't want people to think 'I'll get a smaller heater because it will cost less to run' because it will struggle to heat a home."
Some models of heat pump do not work when the outside temperature is between -2°C and 2°C. Edwards said consumers should buy Energy Star models if they lived in areas where temperatures reached that level.
Lynda Armitrano, of BRANZ, said a third of the heat pumps put in were designed as an additional heating source, a third were to replace a different form of electric heating and another third were to replace a different kind of heating, such as a fireplace.
A study into household renovations found while 30 per cent of householders preferred an enclosed wood burner in their living room, a quarter reported a heat pump preference. Out of 162 households undertaking renovations, 15.2 per cent installed heat pumps but only 4.8 per cent installed woodburners.
Just over half of all houses still have fireplaces, only a small drop from the 59 per cent reported 30 years ago.
Fireplaces are most common in rural areas and smaller towns. Ministry for the Environment statistics suggest about 15 per cent of homes have fireplaces that were installed before 1994. Another 6 per cent have open fire places.
Only about 5 per cent of Kiwi homes have central heating, compared to 91 per cent in Britain. Armitrano said a Government's subsidy - now no longer available - for people installing heat pumps in some parts of the country had raised awareness of their benefits. "That probably will continue although heat pumps have a life of about 15 years so sales may level out."
Between 2009 and 2012, under the EECA scheme, 40,000 efficient heaters were installed, including heat pumps, pellet burners and flued gas heaters.
About 80 per cent were heat pumps but BRANZ research showed lower-income areas had a preference for woodburners.
Consumer NZ found 92 per cent of customers were satisfied with their heat pumps and there was no difference in reliability between Daikin, Fujitsu, Mitsubishi, Panasonic and Toshiba.
Ministry for the Environment data shows that 57 per cent of households say they use electric heating, 34 per cent use gas and 52 per cent use wood, whether in an open fire or woodburner.
Some households use more than one fuel type, hence a total above 100 per cent.
Who is using what?
Energy sources households are using, according to energy and consumer officials:
• 57 per cent - Electricity
• 9 per cent - Flued gas
• 24 per cent - Unflued gas
• 52 per cent - Wood, including wood burners, open fires and pellet burners
*Some people use more than one source.
Number of days a week, in June, energy sources are being used:
• 6 - Gas
• 4 - Open fires
• 6 - Wood burners
Next week: How cheap are eco-heaters? Consumer Watch looks at the options.