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I have to change gas boiler (heating + hot water). House of 120m2 dating from the 1920s (50 cm walls on the ground floor and 30 cm on the first floor, 2 people, roof insulation, glass wool and double glazed PVC windows). The boiler is in the cellar. I am offered a condensing boiler but the chimney pipe is not cased, or a low temperature boiler. Which is the best suited? What are the best brands?

A condensing boiler can often be connected with a suction cup, which avoids having to tuber the chimney, but this type of installation should be condemned by the new European Ecodesign or Eco-design directive applicable in September 2015.
For that, it is necessary that the cellar is semi-buried, which makes it possible to make leave the suction cup above the natural ground, provided to benefit from a mechanical protection if it opens at less than 1.80 m of the soil. It is also necessary that the urban planning bylaw authorizes it, and that the exit of the sucker is located in a zone protected from shocks. Finally, the distance between the boiler and the outlet must be as small as possible.
The alternative of low temperature boiler solves the problem of the conduit, if it is in good condition. A low temperature boiler is also less expensive, but does not benefit from the 30% tax credit on equipment and risk, too, to be condemned by the European Directive.
Assuming that the low-consumption boiler remains authorized, to compare it to a condensing boiler, it is necessary to estimate the economy of consumption obtained by it. For these savings to be a reality, the boiler must be condensed regularly. This is the case in new construction, with low temperature heating systems. This is less obvious in the old, or the heating water temperature often needs to be higher. Concretely, if the water temperature of the heating circuit reaches and regularly exceeds 50/55° C, the condensing boiler is not of real interest.
As for mentioning brands... All the famous brands are valid. Just as important as the brand, the installer. An installer close to your home, known and installed in the region for a long time, is an insurance "tranquility" in terms of maintenance and troubleshooting.

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