In the past decade, New Zealanders have fallen in love with heatpumps.
In 2005, just 4% of New Zealand homes had heatpumps, whereas 28% had them in 2012 (Branz Build 128, 2012).
The speed of this transition has surprised many.
Heatpumps are an application of refrigeration technology.
Refrigeration has been used on a large scale since the 1880s when the refrigerated meat industry was launched by the first voyage of the refrigerated vessel, Dunedin, from Port Chalmers to London.
But frozen meat is a different business from heating a home.
If you want to ship meat around the world, refrigeration is essential, whereas for home heating there are other options, from wood and pellet burners to electric radiators and gas fires.
Uniquely, heatpumps are the most energy efficient, providing more energy as heat than they use in the form of electricity.
The efficiency of a heatpump is measured by its coefficient of performance (Cop), the ratio of heat delivered to the electrical energy consumed.
Once I helped to measure the practical working Cop of two heatpumps in Dunedin houses.
The cost of the units was about $2300 at that time, roughly the same as current heatpumps.
But the cost of domestic electricity was much less then, 3.5c/kWh.
Based on the measured working Cop (about 2.3) it would take more than 10 years for the electricity savings to pay for the cost of the unit.
Not that attractive, really.
After this trial I installed one of these units in my own home and I was surprised that our electricity bills stayed much the same.
So was it all a waste of time and effort?
The fact is we kept the house better heated than before and the benefit came in the form of a warmer, drier house.
Children could leave doors open if they liked.
As shown later by Prof Howden-Chapman, of the University of Otago, and others, people living in warm, dry homes are healthier than those who don't.
This provides real benefits personally and for society because of fewer illnesses.
Unfortunately, the unit I tested was a bit basic: the fan noise and vibration were intrusive, when it defrosted we froze and the lights dipped briefly each time it started.
This was not a product that everyone would go for.
Since that time there have been major advances in fans, compressors, power electronics and computers.
Most heatpumps now are inverters, which turn their heating capacity up or down as needed.
This enables them to heat faster on a cold morning and keep the room temperature steadier once it is warm. You hardly notice when they defrost, there is little noise and they are very efficient.
One manufacturer now advertises a domestic heatpump with a rated Cop of 5.8.
This is two and a-half times the Cop of the first one I tested and a lot more than most other products on the market.
So the technology is continuing to improve.
• Emeritus Prof Gerry Carrington is a fellow of the Royal Society of New Zealand and the Institution of Professional Engineers of New Zealand. Each week in this column, one of a panel of writers addresses issues of sustainability.
