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Energy saving, this type of boiler is destined to become the reference in the context of the 2012 thermal regulations. A few keys to make the right choice.

Notice and comparison: 8 condensing boilers

The operation of a condensing boiler is almost similar to that of a conventional or low temperature boiler. The difference: with a traditional boiler, the fumes from the combustion of natural gas and the water vapor they contain are released into the atmosphere without exploiting their calories. This is not the case with a condensing boiler that recovers this heat called "latent" and then reinjects this energy into the heating circuit. In the end, this technique generates 30 to 40% savings compared to a boiler over 15 years, and 15 to 20% compared to a recent standard model.

Condensing boiler: an installation under three conditions

To install this type of boiler, it is enough to have a central heating circuit.
But to guarantee a correct implementation and achieve the desired performance, it is necessary that the return water from the heating reaches the boiler at a temperature below 60° C, in order to take full advantage of the advantage that condensation provides.
For this, three factors are essential. First of all the installation of low temperature heat emitters (soft heat radiators or heated floors).
Then, any device that has the effect of heating this water to the boiler inlet should be avoided (4-way mixing valves and 3-way thermostatic valves on the radiators). Finally, it is necessary to provide a regulation by external probe and / or any system allowing to constantly adjust the flow temperature of the boiler to the heating needs.
A room sensor and programming of heating and domestic hot water (DHW) further improves performance.

Energy mix: the winning combination

Condensing boilers can be coupled with renewable energy. It can thus be combined with a heat pump and / or solar collectors. Comprehensive "system solutions" are now offered by manufacturers; the condensing boilers are thus equipped with all the necessary components for these new combinations: boiler, hot water tank, circulators and pipes are integrated and completely pre-assembled in a compact device.

Notice and comparison: 8 condensing boilers: boilers

If you have little space, it's better to opt for a wall-mounted boiler. Easy to integrate into the kitchen, some models are only 40 cm wide. A boiler on the ground requires a larger footprint (60 cm): it therefore installs rather in a garage or a basement.


A dual-service boiler is a device that provides both heating and domestic hot water (DHW) production. It therefore includes in a single element the heating body and an exchanger delivering hot water. Only problem: the lag time to get hot water to the tap. To solve this problem, some mixed boilers incorporate a storage tank. The same comfort and speed of response is obtained with an electric hot water tank. Note that there are also "simple service" boilers: the boiler is then coupled to a separate domestic hot water tank of up to 300 liters.

Notice and comparison: 8 condensing boilers: boilers

For the production of domestic hot water, this type of boiler can be ideally associated with solar energy. During cold or cloudy periods, the water from the individual solar hot water tank (Cesi) is preheated and the conventional energy (gas, fuel oil, etc.) takes over until the correct temperature is reached. A Cesi covers 50 to 80% of the annual hot water needs.

Notice and comparison: 8 condensing boilers: notice

For a boiler to actually condense, the water at the radiator outlet must be cold enough to allow a reduction of about 20% of the gas consumption.

The burner

Effective condensation requires operating the heating equipment with a small excess of air. This is the case with the modulating burner: it is used to vary the power continuously (between a minimum and maximum setting) while maintaining a continuous combustion. With the total premix burner, all of the primary air is premixed with the gas inside the burner, which ensures high efficiency and reduced pollutant emissions.

The regulation

The outdoor sensor automatically adjusts the outlet water temperature of the boiler to the outside temperature. His interest ????: decrease the average temperature of the device over the entire heating season and improve its seasonal performance. Taking into account an internal compensation temperature sensor makes it possible to obtain optimal control.


The condensates come from the condensation of the flue gases which, when in contact with the boiler exchanger, fed by the cold return of the heating circuit (temperature below the dew point), causes condensation.
The flue must therefore be made of a material resistant to condensate acidity: stainless steel, ceramic, synthetic material or glass. If your house has a chimney, you have to pipe it (double it) to evacuate the flue gases. Their evacuation must also be absolutely free (not in a closed or semifermed courtyard, for example) and away from neighboring homes.
• In the absence of a pipe, you can install a "sucker" composed of a double pipe: one brings the oxygen necessary for combustion, the other evacuates the gases outside the house.
• The flue pipe must have a slope of at least 3% (unlike a conventional boiler) to reduce the condensate to the boiler. The condensates are evacuated by a siphon connected to the sewage network.
• If you already have a suction cup, you can keep it, provided that a professional makes sure that it works and is compatible with the new boiler. He will also have to check that the new type of boiler is in compliance with the ventilation of the room in which it is installed.
• Condensate treatment is mandatory before disposal into wastewater. For this purpose, there are Polar purifiers ("Neutralizer" type), suitable for gas and oil-fired boilers, to be recharged during annual maintenance of the boiler, note that some devices are self-cleaning.

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