Now that they’re about to release the new law that prevents the installation of the so called “conventional boilers”, there’s every day more customers passing by our offices to ask the same questions: “What is a condensing boiler?” ”What’s the difference between this boilers and a normal boiler?”

For this reason, in this text we’ve tried to answer all this questions trying to make the explanation as simple as we can, so that in the end everyone who has read it can understand the advantages of the condensing boilers in front of the conventional ones, and people can also understand condensing boilers particularities.

Condensing boiler Junkers ZW BC25-2c

Inside of a Junkers ZWBC25-2C condensing boiler

The main difference that exists between a condensing and a conventional boiler is its energy efficiency. This can be seen on the boilers performance: When a condensing boiler is working on the best conditions, it can give up to a 111% performance. On the other hand, conventional boilers normally give a performance of around 80%.

What does having a high performance actually mean? In physics, performance can be expressed in the following equation:

Performance formula

This means that, the higher the performance is, the lower the energy that the boiler will consume in order to provide the user with the warmth he/she requires. This is shown in the user’s bill as less consume in gas.

The fact that a condensing boiler can offer such high performance is a big advantage in front of its competitors. It’s important though, to explain carefully which are the conditions needed to get close to this 111% performance. This performance is only obtained when working under optimal conditions, meaning installations with long heating pipes stretches or installations using an underfloor water heating system.

This happens for a reason: the secret hidden behind condensing boilers high performance is the way they use the heat generated by the fumes that are expelled to the outside after the heat exchange happens. In order to reuse this heat it’s necessary to have two conditions happening:  On the one hand the water has to be heated on a lower temperature than the one conventional boilers require (only 60 – 70Cº when it comes to condensing boilers). On the other hand the heating circuit has to have a long distance so that the water has time to cool down reaching the perfect temperature in order to get the best performance (35 – 40Cº).

As you can see in the majority of the cases this conditions do not happen, this is why, even though lots of companies promise reductions in gas consumption up to 35% right after installing a condensing boiler, in most of occasions changing from a conventional boiler to a condensing one doesn’t’ mean saving so much gas.

Another thing to take in account when talking about condensing boilers, and in which they are one step forward compared to their competitors, is in low CO2 emissions. Condensing boilers can emit 75% less CO2 into the atmosphere than conventional boilers. This is due to the fact that a large amount of the gases are reused to re-heat water, and therefore are not emitted into the atmosphere.

The reuse of these gases has also its negative sides, since in order to install a condensing boiler it’s always necessary to install drainage so that all the gases that were previously used to reheat water (and that now turned into water themselves) can be evacuated.

Junkers ZW BC25-2C condensing boiler

Lower connections ZW BC25-2C


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