How to heat your house efficiently

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The Conversation

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.

Will J Grant is a Researcher/Lecturer at the Australian National Centre for the Public Awareness of Science at Australian National University. He does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.The Conversation

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  • Bruce Easton

    Nice article, although there is no reason why you can’t offset your gas use just the same way as you can with electricity to make it effectively zero emissions.

    Also, if you live in Victoria there are generous rebates through the VEET scheme for changing to 5 star gas ducted heating. Chimney plugs are also available free through the scheme.

    One last thing – ugg boots and a nice blanket for telly time!

  • Giles

    from a reader who can’t access disqus for the moment ….

    “For the past twenty years I have heated our home with an off-peak
    electric storage heater….
    It is a 6kW unit on Off Peak 1 and ‘charges’ overnight. We then ‘draw
    down’ that heat with a fan, that can be thermostatically or manually
    Even without running the fan, convective air flow warms the house,
    and we run the fan for a ‘boost’.
    Our unit is English, and no longer available, but there is an
    Australian company (SA based) producing similar units……
    So, heat your home with clean, green off-peak electricity…..”
    Chris Dalitz – EV Owner

  • as a renting uni student in Canberra I found bubble wrapping windows to make a huge difference, no more mornings with the kitchen at 7 degrees. Ghetto double-glazing.

  • Ronald Brak

    I used blu-tac to seal up a dafty window a few days ago. It seems to be a lot more effective than tape. Just one little nitpick, at ground level sunlight is about 53% infrared, 44% visible light, and about a third of a percent ultraviolet. Normal glass blocks ultraviolet, lets in visible light which wil be absorbed by non transparent things causing them to warm up and emit infrared, and very roughly blocks about two thirds of infrared.

    • Ronald Brak

      If anyone is wondering why my percentages don’t add up to 100 it’s because sunlight consists of 2.7% Solarian mind control rays.

  • tsport100

    No mention of geothermal heat pumps? A DX ground sourced heat pump is almost identical to a conventional A/C unit but with a ground loop as the source.

    As the ground below 4 feet deep is approx 22c all year round simply moving that temp inside during winter provides as much as 70% energy saving compared to conventional systems. It also improves cooling during summer as the evaporator doesn’t have to struggle against raised ambient temps.