Modern, efficient condensing boilers use the heat contained in the exhaust gas for heating or for hot water preparation. As a result, the exhaust gas enters the exhaust system in a relatively "cold" state, hence condensate usually forms in the chimney. The material of the exhaust system used must therefore be condensate-proof and insensitive to moisture.
Due to the low exhaust gas temperature, the exhaust gas in gas condensing appliances cannot be dissipated solely by the natural chimney draft, hence an overpressure is necessary to "push" the exhaust gases through the line. This overpressure is generated by the blower of the boiler or the condensing boiler and is usually max. 200 Pa. The exhaust pipe must therefore be suitable for overpressure.