Half of your gas bill is wasted heat

State and federal governments and local councils have long been repeating the mantra, “replace your inefficient electric space or water heater with a gas one”.

But when put to the test, does the rhetoric stand up to scrutiny?

In order to answer that question Zero Emissions Australia has teamed with Efficiency Matrix to produce the following diagram showing gas energy use in a typical Victorian household. (We've chosen to use Victoria for our examples as it is the state with the biggest gas consumption due to its colder winters and legacy of high penetration of gas appliances).

In dollar terms, a household pays $1400 for their gas and loses $720 directly to the atmosphere – as seen in the diagram:

A Victorian household uses about 60,000MJ of gas, however to simplify things we've converted everything into kilowatt-hours (kWh) because that's the unit that the public at large is more comfortable with.   

So, 16,600kWh of energy is supplied at the gas meter of a typical house that is consuming gas for three primary applications; gas space heating (the largest consumer), gas water heating and cooking.

Space heating

4200kWh of heat is lost here:

The average ducted gas heating unit sold in Australia has a 3-star rating, however the average unit installed in the field – due to their 20-25 year operational life – has a 2.5 star rating. This means that 35% of the energy contained in the fuel for the heater (gas) is lost up the exhaust and only 65% flows into the heating ducts.   

1560kWh of heat is lost here:

A number of studies have shown that newly installed ducting is (at best) 80% efficient, losing 20% of the energy it is transporting due to air gaps (failing pressurisation) and poor performing insulation which is often at or below the pathetically low R1.0 standard. So, for our diagram we've assumed 20% losses. 

624kWh of heat is lost here:

Then there's the problem of heating ducts installed in bathrooms, laundries and drying cupboards along with bedrooms with their doors shut. This creates pressure equalisation mismatch meaning air from the duct outlets can't make its way back to the centralised air intake. This is highly likely to cause losses far greater than the conservative 10% figure that we've assumed. Those who are conscientious about their bills and deliberately zone their heater by shutting off vents and closing doors make this problem considerably worse.

Final equation:

– Total amount of heat fed into your typical central heater 12,000kWh

– Total amount of heat that is useless and lost to the atmosphere 6384kWh - 53%

– Total amount of energy that is useful for heating the space 5616kWh - 47%

Water heating

1786kWh of heat is lost here:

Gas hot water heaters are pretty inefficient, especially storage heaters and older instantaneous units. Even the newest gas hot water units lose 35% of their heat energy to atmosphere. To illustrate this we took a typical 3-star unit; which experiences 45% energy loss to the atmosphere and 55% useful heat as hot water available in the tank, to be delivered to hot water services such as showers as well as bathroom and kitchen taps. 

Figure 2: Star ratings for gas hot water heaters; table shows how much energy is required to deliver 13761MJ (3823kWh) of water heated from 15C – 65C at each star level per year  

Source: Elgas

Cooking

Of all the applications for gas in the home, the most inefficient is gas cooking however (fortunately) gas cooking does not make up a huge proportion of household energy consumption. Most houses have moved to electric ovens and that leaves just the stovetop still running on gas. Gas stoves generally achieve (if you can call it an achievement) 30% efficiency, meaning 30% of the energy in the gas heats up the food in the saucepan, frypan or pot. This is particularly annoying in summertime when the wasted heat also heats up your kitchen, meaning you have to crank up your air conditioner to compensate. By contrast an induction cooktop is three times as efficient with an efficiency of 90%, has better heat distribution, causes much less additional heating load in summer, lowers burns and fire risk and leaves you with better air quality.

Additional savings

Considerable improvements to the net energy requirements of a house can be made through, lagging (insulating) pipes, installing a 4.5 litre/minute venturi showerhead, air infiltration sealing, better insulation in ceilings, walls and floors. The biggest improvement that can be made is by changing to all-electric reverse-cycle air conditioners, taking advantage of the renewable energy stored in the air as ambient heat and eliminating the Swiss cheese effect through the covering up of leaky evaporating cooling and central heating ducts. The Swiss cheese effect causes thermal chimney updraft (winter) and downdrafts (summer).

The inefficiency of gas heaters is extraordinary and inherent in this outdated technology. Government agencies and the public at large generally overlook the poor performance of gas as a domestic energy source. In a typical house, as outlined above, move than half (52%) of heat energy is wasted, being lost directly to the atmosphere outside of the home. Even when considering a new house, purchasing the average gas appliances currently supplied we get; a 3-star space heater instead of a 2.5 star unit, a 5-star hot water heater instead of a 3-star, the house still loses just under half (47%) of its input gas energy to the atmosphere.

The answer to this inefficient and outdated gas technology is to replace it with heat-pumps or reverse-cycle air conditioners. Instead of losing heat to the atmosphere, these units actually gain heat from the atmosphere. Heat-pump heaters trounce gas heaters, in terms of efficiency, because they can gain up to 600% (when tested at 7C outside) additional energy from the outside air; as is the case with Daikin's industry leading Ururu Sarara reverse cycle air-conditioner. 

For water heating Sanden has the answer with their CO2 heat pump water heater, which achieves an annual efficiency of about 400% (C.O.P of 4.0). While gas heaters are losing almost half of their energy to the outside air, a Sanden heat pump is drawing in 400% of the energy used to heat your hot water from the outside air.

Induction cooktops are safer and do a better job and they're 300% as efficient as “cooking with gas”. Preferred by Michelin chefs and making up half of sales in France and Germany, they are the answer to a technically advanced renewable energy future.

The future is electric and it's cheaper for homeowners, so take the foot off the gas.

Matthew Wright is executive director of Zero Emissions Australia, technical director at Efficiency Matrix and resident columnist at Climate Spectator.