The cheapest and most efficient way to warm a house

by Ruth Nichol / 02 June, 2017
Micathermic and oil column heaters have markedly different levels of thermal efficiency. Photo/Getty Images"

Micathermic and oil column heaters have markedly different levels of thermal efficiency. Photo/Getty Images.

Not all electric heaters are created equal.

Heat pumps are cheaper to run than any other form of heating. According to George Block, Consumer NZ’s heating and energy specialist, for every kilowatt of power that goes into a heat pump you get 3-4kW out.

Next cheapest are flued gas heaters and wood burners, which also offer a different kind of heating experience. “You can’t sit in front of a heat pump with a glass of brandy and read a book,” says Block.

However, all three types of heating are expensive to buy and install – more than $3000 in some cases – which makes them unaffordable for people on low incomes, and unrealistic for renters who can’t take them with them when they leave.

It’s possible to buy a 2.4kW electric heater capable of heating a medium-sized living room for less than $100, though it will cost about 60c an hour to run, compared with about 38c for a 4.5kW heat pump. It will also take much longer than more energy-efficient heating to warm the room to your desired temperature.

You might think that, apart from the way they look, plug-in electric heaters are all pretty much the same, but Block says that’s not the case. “There are differences in how quickly and how evenly they warm a room.”

Consumer tests have found that oscillating tower fan heaters heat rooms faster and most evenly. They have their downsides, though: they look slightly odd – the name is a bit of a giveaway – and tend to be noisy.

If noise bothers you, your next best bet is a micathermic heater. These have sheets of mica (a mineral similar to slate) encased in a metal housing, which heat up quickly. They also provide a combination of convection and radiant heat, which means that as well as heating the air in a room, they also heat the objects in it – including humans. That means that you will feel warmth coming from the heater. On the downside, the surface of micathermic heaters can get very hot, which may pose a danger for children.

Photo/Getty Images

Photo/Getty Images

The least effective plug-in heaters are oil column heaters. These are slow to heat up and take longer to heat a room, because they provide almost exclusively convection heat, which rises towards the ceiling and only gets to objects lower down as it cools and falls. However, Block says they are popular because they’re quiet and their surfaces stay relatively cool.

A small desk fan can help make both micathermic and oil column heaters more effective. Consumer found that putting a small fan on the floor facing an oil column heater raised the average room temperature by 5°C three times faster than when using the heater without the fan.

It’s important to get a heater that is powerful enough to do the job – at least 2kW for a medium-sized living room. A less-powerful heater is cheaper to run, but it will take longer to heat the room, effectively cancelling out any possible savings. When Consumer compared a 0.4kW panel heater with a 2.3kW oil column heater, it found the panel heater took 21 minutes to raise the temperature of a room from 8°C to 10°C, compared with just 8.2 minutes for the oil column heater.

You can use the heater’s thermostat and a timer to help control how long your heater is on and keep your power costs down. Most heaters have a thermostat, but not so many have timers. However, you can buy a cheap digital timer from a supermarket or hardware store.

Block also recommends checking Consumer’s Powerswitch website (powerswitch.org.nz) to find the cheapest electricity and gas plans in your area.

“There’s so much variation and so much competition in the retail space that you can save $500 to $600 a year by switching power companies.”

wood burner

A wood burner. Photo/Getty.

Double glazing holds the heat in better

Changes to the Building Code in 2008 made double glazing compulsory for most new houses. However, more and more homeowners are choosing to replace their existing windows with double glazing.

Double glazing creates an insulating layer of gas – either normal air or special heavy gases – between two panes of glass. It lets in just as much sunlight as single glazing, but is better at holding in heat.

Retrofitting double glazing may mean installing a new window, or alternatively what’s known as secondary glazing, which involves inserting a second pane of glass or acrylic into an existing window frame.

It’s also possible to fit temporary double glazing yourself, using a flexible clear film that’s attached with double-sided adhesive tape to the window frame. You then use a hair dryer to tighten the film.

According to the Energy Efficiency and Conservation Authority (EECA), double glazing can halve the amount of heat lost through windows, depending on the type: the most effective is made of low-emissivity (low-E) glass with a layer of argon gas between the two panes, fitted into either thermal aluminium or wooden frames.

Retrofitting permanent double glazing is expensive – as much as $1000 a square metre – and it provides less thermal benefit than ceiling and underfloor insulation. In some cases, good curtains may be just as effective.

“The first line of defence against the heat loss around windows is a good set of floor-length, double-lined curtains,” says David Pierce of the Sustainability Trust.

“Double glazing will improve heat retention, but it’s too expensive for some. Its value lies mostly in the improved overall comfort it delivers, including reduced condensation and noise from outside.”

The further south you live, the more financial sense retrofitting double glazing makes.

A study published in 2010 by Victoria University master of building science student Nick Smith found it would take 28 to 30 years to recoup the cost of secondary glazing in Auckland, compared with 13 to 15 years in Dunedin.

Smith’s study also gave the thumbs-up to temporary double glazing, which can be installed in a typical living room for less than $100 – though it lasts for only one winter.

He found using the thin film paid for itself in heating savings in Wellington, Christchurch and Dunedin, but not in Auckland.

This article was first published in the May 27, 2017 issue of the New Zealand Listener.


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