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How to heat your house efficiently

Keeping the heat in the places you want it is the most important part of any heating plan. Wunkai/Flickr

Winter is coming, and all across the southern states eyes turn to energy bills and minds towards how to make them smaller. What is the most efficient way to heat your house?

As with anything to do with thermodynamics the answer is complicated, but there are some solid rules to help shape your thinking.

Insulate, insulate, insulate; then add more insulation

Both heating and cooling come down to one fundamental problem; how to put heat where you want it and keep it there. That means outside in summer and inside in winter.

The rate that heat enters or leaves a building is governed by the materials that make up your “building envelope”: the surface which surrounds your living spaces. You have to stop heat crossing the boundary of your walls, ceiling, floor, windows and doors without your permission.

We say “without your permission” because in winter your first priority must be getting sunlight into the house and stopping it leaving again.

Direct sunlight is almost entirely ultraviolet (UV) radiation, which passes easily through glass and into your living room. Once it strikes an object it heats up and the sunlight becomes infrared (IR) radiation, which is best thought of as radiant heat. IR radiation doesn’t pass through glass as readily as UV radiation, so the room heats up. Ever left your car in the sun for a day where it is a lovely 25 degrees outside but 60 degrees inside your car? It is exactly this mechanism at work.

Insulation will stop heat crossing boundaries. If you own your house, the path is well trodden and fairly simple. Start with the ceiling, adding the thickest insulation you can find.

Next consider how the wall insulation can be improved. There are some new materials and processes available for this task now and you can pay someone to come and pump insulation into your wall cavity for $2000 and upwards. Doing this can make your house about 2 degrees warmer.

Window treatments are just about on-par with walls for making a difference; even more so if you include doors.

If you’re on a budget, or renting, there’s a lot you can do with windows and doors. First, seal all the gaps around doors in the house, even the internal ones, to control the size of your heated space; there are rubber seals that can do the job for doors and windows.

Then seal all the ancillaries that let heat escape, like bathroom vents and the rangehood, which vents outside. This is where renters can get creative and reap rewards; rolled towels under doors are a good start, but why not go further? Tape a garbage bag over the window that opens onto the neighbour’s bathroom. Block the old chimney with anything you can find. Do anything you can to stop heat going where you want it to go.

Curtains and pelmets are virtually mandatory in cold climates. Curtains must be as heavy as possible, touch the ground and the walls next to the window. Windows are the last great frontier in home insulation; and honestly, there’s no cost-effective way to retrofit them. Energy payback on double-glazing your windows could be close to 70 years. But as with all energy efficiency upgrades there is more to it than the energy savings. What is niceness worth to you? How much nicer is it being in a house which is 14 degrees instead of 10? What is that worth to you?

Once your windows are done think about what you can do to the floor. Slabs are hard to change, but there are products for suspended boards which will make a big difference.

Which fuel?

You have three choices for heating: electricity, gas and wood.

The cost of wood is variable and will likely increase in coming years. The environmental impacts are strongly dependent on your fuel source. Fallen timber is not fair game: it is vital habitat for some animals, so should not be burnt. Particulate emissions are a concern as well.

Gas is great in that it can deliver astonishing amounts of heat quickly, with lower greenhouse gas emissions than grid electricity, but not as low as renewable energy, and the future costs are quite uncertain. Gas heating is traditionally considered to be pretty cheap, but gas prices are tipped by many analysts to climb steeply in the next few years , much as electricity has in the last few years.

That leaves electricity. Unintuitively, its the only option which can have zero emissions. Switching your electricity supply to GreenPower, an accredited and audited scheme which supplies 100% renewable power, means zero emissions electricity any time you want it. But how you use it is very important, as all electric heaters are not created equal.

There are two distinct classes of electric heater; plug-in heaters, and heat-pumps (or reverse cycle air conditioners).

All of the plug-in heaters end up at the same efficiency; they convert electricity into heat, the most basic form of energy, and will only ever produce a maximum of 2.4kW. It doesn’t matter how it does it - radiant bars, a hot wire and a fan or a clicking oil-heater - 2.4kW of electricity becomes 2.4kW of heat and not a drop more. Buy the cheapest one you can.

Heat-pumps have been common in Australia for years - reverse-cycle air-conditioners are a form of heat-pump - but the technology has advanced markedly recently. Heat-pumps have an advantage over the plug in units as they use electricity to move heat around, not create it.

Heat-pumps have access to the ambient heat outside your house, even in small amounts, and can concentrate it and put it in your house. Even well below freezing there is enough heat available for this to be worth doing.

Heat-pumps’ performance is reported as a “co-efficient of performance” or COP. This describes the amount of heat transported per unit of energy used to move it. A good heat pump will approach a COP of 5 - five units of heat produced for every unit expended - dependent on the ambient temperature. Newer designs are achieving COPs greater than 3 even when the ambient temperature is as low as -10 degrees.

If you’re thinking of getting a heat-pump, look for one with an EC (electronically commutated) motor; it will be described as “Inverter driven”. A good heat-pump will have a high energy efficiency rating (that is, lots of stars on its label).

To summarise then: insulate your house, as much as you can, in every way available. Then think about how you want to heat it. If emissions are important to you, go for GreenPower and a heat pump. If you just want heat, then gas is probably your best bet, but be warned the price is on the way up and likely to outstrip electricity price rises in coming years.

This article was substantially based on research by Evan Beaver. Evan is a mechanical engineer and a senior consultant at Energetics, where he advises government on energy policy and industry on energy efficiency.

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