In Boilers Pt 1 – Regular Vs Combi, we looked at the two main types of boiler which you find in the home. This post takes a look at replacement boilers, in particular condensing boilers.
Since 2005, the Building Regulations have required that if you replace your boiler, you have to replace it with a condensing boiler. There are a few exceptions to this (see Appendix A here), but essentially all new boilers condense. But what does this mean?
A condensing boiler is not all that different to your old boiler, but it is more efficient. To achieve this, condensing boilers are fitted with an extra heat exchanger. The heat exchanger transfers heat from the burning gas in your boiler to the water in your heating or hot water system.
A heat exchanger is a very simple metal component through which water is passed. It has a large surface area which makes it easier for the heat to pass from the hot burning gases to the cooler water in the heating or hot water system. A radiator is an example of a heat exchanger, passing heat from the hot water to the air in your home.
By adding an extra heat exchanger to the boiler, more heat can be extracted from the hot gases that would otherwise escape up the flue and into the atmosphere. This energy goes into preheating the water returning from the heating or hot water system, before it passes to the main heat exchanger to be heated up the rest of the way.
The “condensing” part refers to the moisture (or “condensate”) which condenses out of the gases and is flushed down the drain. The action of condensation transfers additional heat to the heat exchanger (see here for the Wikipedia explanation).
Condensing boilers reduce the temperature of the flue gases from about 150°C to about 55°C so there is a substantial amount of heat there to be recovered but they won’t always be in condensing mode. In order for this to happen, the temperature of the return pipework (which carries the water back to the boiler after passing through the radiators) has to be below 55 degrees.
For this reason you need to make sure that a regular boiler is correctly sized for your home. This is one of the reasons why you should reduce the heat demand of your home as much as possible through insulation and draught proofing before replacing your boiler. If you do it the other way round, your boiler is likely to be too big and it won’t work as efficiently.
Your boiler thermostat also needs to be adjusted so that the flow temperature (the temperature of the water leaving the boiler) isn’t so high that the return temperature is too high for the boiler to enter condensing mode. I’ll cover this in more detail soon.