Damien McGill, an engineer by trade and member of the Superhome Movement, says it’s no more expensive to run heat pumps full-time, and that doing so has health benefits, for humans, and for the building.
McGill has been monitoring three adjacent units he owns in central Christchurch. He lives in one himself and has not found any significant difference in his power bill from leaving the heat pump running 24/7 set at a constant 18 degrees Celsius.
“If you’ve already got it at 18, it doesn’t use much energy to heat to 20 or 21 degrees. If it gets down to 12 or 14 degrees then the heat pump has to work hard out to get it back up,” he says.
“It hasn’t cost me any more.”
However, McGill recognises that this advice may not work for everyone.
So Stuff called in the experts: Scientist Andrew Pollard, from building research organisation BRANZ, and Gareth Gretton, from government agency EECA, the Energy Efficiency and Conservation Authority.
Unfortunately, the answer is not simple.
Gretton says it is context-dependent.
If a house is extremely well-insulated and air-tight, it will not cost much to maintain at about 18 degrees, leaving a heat pump running even when no-one is home or overnight.
But most New Zealand homes are single-glazed and not optimally-insulated, and therefore leak heat.
“For your average New Zealand house with quite a lot of heat loss, leaving it on all the time is going to use a lot of energy.”
Instead, EECA’s advice is to run the heat pump when people are home, and to use a timer.
”You can set it to turn on just before you get home, or before you wake up in the morning. Doing that also means you’ll be less tempted to crank the heat pump at a high temperature when you get home to a cold house, or wake up in one.”
He said, of course if people were home during the day with children, working from home, or for any reason, to keep the heat pump running for comfort.
Gretton said he strongly recommended people leaving for the day should ventilate their home well beforehand.
”That’s 10 minutes of windows wide open – let all that air out of the house before you leave for the day.
You’re filling your house with nice dry morning air, and since you’re about to leave, it doesn’t matter that you’ve lost the heat.” He recommended buying a temperature and humidity sensor, so that people could “learn to operate their house”.
”A famous architect once said a house is a machine for living in. We need an instruction manual for the machine. By buying a sensor, you can learn how to operate your house.”
Pollard agreed that relative humidity was also important to control: People should aim for humidity between 40 and 60 per cent.
“A lot of homes will average about 70 per cent: That’s where the mould starts to grow,” he says. “It’s invisible before it becomes visible. It can hide behind surfaces, so often behind wallpaper rather than on the surface.”
To control humidity, people needed to heat their homes, ventilate, and also reduce the amount of moisture being released into the home’s atmosphere – for instance by using a rangehood when cooking, by not drying laundry inside, by closing bathroom doors and using extraction fans in the bathroom.
Pollard said children’s rooms should be heated overnight to the 18 degrees Celsius recommended by the World Health Organisation, but a small electric heater was a better way to do this than a heat pump, unless a ducted central heating-type system was installed.
“If it’s very cold outside, they’re working harder and are less efficient. Sometimes the outdoor unit has to be defrosted, actually blowing cold air inside. A 2015 study showed they actually produced twice as much heat energy as the energy that went into them.”
BRANZ studied New Zealanders’ use of energy in the home in its 1995 to 2005 Household Energy End-Use project, finding most houses were under-heated, too cold and “creating unhealthy living environments”.
Another study is underway.