Natural gas contains a large amount of hydrogen. When hydrogen burns it becomes water. In a hot gas boiler when the hydrogen burns it will change into steam. In a condensing boiler this steam is made to condense on a set of metal plates (a heat exchanger) and the heat is recovered to pre-heat incoming water. This makes the boiler more efficient with efficiencies of 89% being typical of modern household boilers and efficiencies of well over 90% being achieved by some industrial boilers.

Are they a good or a bad thing? Condensing boilers may be more efficient but they are also more complicated. In the past they developed a reputation for breaking down. In the winter of 2010 there were many stories in the papers about the boilers ceasing to work because the pipe that takes the condensation into the drain can freeze in cold weather. This does not happen very often in the UK – only in an exceptionally cold winter – but it’s really annoying if your gas boiler suddenly goes off when you need it most. Some manufacturers responded to this with the development of a ‘drain free solution’ that doesn’t require a separate pipe going into the drain for the condensed water but sends it through the ordinary drain. Others looked at the possibility of a heated electric element around the pipe that can be turned on in very cold weather. The other problem is that the condensed water is acidic. It has a PH of between 3 and 5 which means that it is something like lemon juice. What happens when you put metal in acid? It corrodes. Stainless steel or aluminium need to be used as these are resistant to acids or some sort of modern metal alloy or specialist plastic. You also need to neutralise the acidic fluid by passing it through limestone before allowing it to drain – damage and discolouration can occur if it is allowed to fall on to the ground or the building untreated, to say nothing of damage to the environment by putting untreated condensate into the drains.

So are they reliable? They have been around for 30 years and most of the problems inherent in the design have now been overcome. The metal shouldn’t corrode and some modern types won’t fail at subzero temperatures. As long as they’re cleaned and serviced at proper intervals they’re reliable and last a reasonable number of years.

Condensing Boilers – Right or Wrong?
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