Heating systems using low-temperature or condensing technology are generating fewer and fewer exhaust gas losses. The reduction of exhaust gas losses is linked to the reduction of the exhaust gas temperature. However, low flue gas temperatures can mean that the flue gases no longer have sufficient energy to escape through the chimney or flue gas line to the outside. To ensure that the heating system nevertheless works properly, the exhaust gases must be transported outside with the aid of blowers or fans. This creates an overpressure within the exhaust system. Any leaks in the exhaust pipes would therefore lead to toxic flue gases escaping into the room air. It is therefore advisable to carry out a leak test of the exhaust system.