The flue gases are used to pre-heat more air than the combustion requires. The excess is diverted to pre-heat the fuel gas. Thus the fuel gas is kept well away from the flue gas. If corrosion of the heat exchanger leads to a fuel gas leak, the escape is easier to detect in a fuel-air mixture than in a fuel-smoke mixture. The air which pre-heats the fuel gas may be re-cycled. The combustion chamber (1) of an installation heated by weak gas, e.g. steam boiler or a gas and air heater is fed with the gas, pref. exit gas, through pipe (2) and combustion air through pipe (3). A heat-exchanger (5) is provided in the flue gas pipe (4).
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