Cutting Carbon in Stoke Newington's Homes

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.

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Existing boilers in UK homes fall into two main types – regular boilers (which might also be called “heat only” or in some cases “system” boilers) and combination or “combi” boilers.

A regular boiler delivers heat to both the heating system (usually radiators) and a separate hot water system which takes heat to the hot water cylinder. A hot water cylinder consists of a metal tank, hopefully well insulated, inside of which is a coil of tubing. The boiler passes hot water through this tube and this indirectly heats the water in the cylinder.

So instead of taking the water to the boiler you take the heat to the cylinder. When you turn on the hot tap, hot water is taken from the top of the tank and replaced with cold water at the bottom.

A combi boiler on the other hand, supplies heat directly to both the radiators and to the cold mains water so it directly heats the water itself and delivers it on demand to the hot taps. Combi boilers store no water and so they avoid the problem of sometimes heating up water that  just sits there cooling down (known as standing losses).

Now you may be thinking “why bother with a regular boiler and hot water cylinder if you can avoid those losses?”, but there’s a catch with combis: they heat water less efficiently than regular boilers. So there’s a trade off between the benefit from avoiding standing losses and the penalty from reduced hot water efficiency.

The upshot of all this is that a combi is most suitable for smaller homes with a low hot water demand while a regular boiler is most suitable for a home with high hot water demand. Combis are also more suitable in smaller homes because they don’t need the space for a cylinder.

In my flat I unfortunately have a cylinder which means that I don’t use anywhere near the full amount of hot water. But it is very well insulated with factory fitted foam which means that I just have it come on once before I get up and there’ll still be hot, well warm, water when I get back in the evening.

There is a trend at the moment towards the inappropriate replacement of regular boilers with combis but it should be resisted. If anything, removing the cylinder makes it that little bit harder and more expensive to install a solar hot water system, which needs as much storage capacity as possible, so if you’re considering going down this route you should try and keep hold of your cylinder.

Next up we’ll cover high efficiency condensing boilers. If you want to read in more detail about gas heating systems then your best bet is to check out the Energy Saving Trust publication Domestic heating by gas: boiler systems (CE30)

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