to use either natural gas or propane will eliminate exposure to the dangerous toxins wood burning generates including dioxin, arsenic and formaldehyde. "Renewable" should not be confused with "greenhouse neutral". A recent peer-reviewed paper found that, even if burning firewood from a sustainable supply, methane emissions from a typical Australian wood heater satisfying the current standard cause more global warming than heating the same house with gas. However, because a large proportion of firewood sold in Australia is not from sustainable supplies, Australian households that use wood heating often cause more global warming than heating three similar homes with gas. High efficiency stoves should meet the following design criteria: Well sealed and precisely calibrated to draw a low yet sufficient volume of air. Air-flow restriction is critical; a lower inflow of cold air cools the furnace less (a higher temperature is thus achieved). It also allows greater time for extraction of heat from the exhaust gas, and draws less heat from the building. The furnace must be well insulated to increase combustion temperature, and thus completeness. A well insulated furnace radiates little heat. Thus heat must be extracted instead from the exhaust gas duct. Heat absorption efficiencies are higher when the heat-exchange duct is longer, and when the flow of exhaust gas is slower. In many designs, the heat-exchange duct is built of a very large mass of heat-absorbing brick or stone. This design causes the absorbed heat to be emitted over a longer period - typically a day. Renewable natural gas Renewable natural gas is defined as gas obtained from biomass which is upgraded to a quality similar to natural gas. By upgrading the quality to that of natural gas, it becomes possible to distribute the gas to customers via the existing gas grid. According to the Energy research Centre of the Netherlands, renewable natural gas is 'cheaper than alternatives where biomass is used in a combined heat and power plant or local combustion plant'. Energy unit costs are lowered through 'favourable scale and operating hours', and end-user capital costs eliminated through distribution via the existing gas grid. Energy efficiency Renewable heat goes hand in hand with energy efficiency. Indeed, renewable heating projects depend heavily for their success on energy efficiency; in the case of solar heating to cut reliance on the requirement supplementary heating, in the case of wood fuel heating to cut the cost of wood purchased