High Efficiency and Standard Efficiency Boilers

Efficiency in boilers isn't about how quickly the boiler heats your home; it's about how much of the heat energy from gas you pay for is used to heat your home and how much is wasted in the flue gases. And it's not just about energy costs. It takes a certain amount of heat to keep our homes warm. All of that heat is eventually lost to the environment and contributes to global warming. However, because high efficiency boilers lose less of their heat directly to outside air through the flue, they use less energy overall to warm our homes and are significantly better for the environment.
Changing water from the liquid state to the steam state takes a lot of heat energy; if steam is condensed back to liquid water, that heat energy can be recovered.
Old fashioned, standard efficiency boilers weren't able to condense the high energy steam, produced by combustion, back to liquid water. The steam simply passed out of the house via the boiler flue, taking the extra heat (known as latent heat) with it.

High Efficiency Boilers

High Efficiency boilers (also known as condensing boilers or HE boilers) cool the flue gases further, as they pass across the heat exchanger, and cause the steam to condense back to liquid water. When this happens, the extra heat (latent heat) is recovered. The latent heat is about 10% of the heat produced in combustion and recovering it makes these boilers more efficient and cheaper to run. High efficiency boilers also take a little more heat from all of the gases passing out of the flue, and transfer it into the central heating water via the boiler heat exchanger. The flue gases from high efficiency boilers are therefore much cooler than those from standard efficiency boilers.
Because high efficiency boilers condense the steam back to water in the heat exchanger, they have to have a condensate trap fitted to the boiler and a pipe to take the condensed water away to a drain. The design of the condensate trap and size and route of the pipe is important, to prevent nuisance shut-down of the boiler caused by condensate freezing in winter.
Many early installations of condensing boilers suffered from this problem, and got condensing boilers a bad name, but it is now largely a thing of the past.
Standard efficiency boilers lose about 20% of the heat produced from the fuel gas, via the flue. High efficiency boilers are a big improvement but they still lose about 10%.

Savings, changing to a high efficiency boiler?

Will changing from a standard efficiency boiler to a high efficiency boiler save you money? In energy costs it will but is that the whole picture? If your existing standard efficiency boiler is room-sealed and in good working order, it probably wastes about 20% of the energy. A new high efficiency boiler will only waste about 10% of the energy. For most homes the savings may be less than £200 per year.
If your existing boiler is open-flued, or constantly needs repair, the overall savings might be much higher.
All boilers sold in the UK now are high efficiency (HE) boilers. For the difference between gross and net efficiency, we have another article.

© BSL 2017-2018. Feel free to make individual copies of this article but please don't copy it onto your own website; link to the article instead.