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Gas boilers

Gas Boilers

For the past few years it has been written into the Building Regulations that new and replacement gas boilers have to be of the condensing type because they are potentially more efficient. This is because they can extract the latent heat of evaporation out of the steam before it goes up the chimney or out of the flue.

SEDBUK ratings

SEDBUK ratings

The actual efficiency is measured by the SEDBUK rating. The government database lists all UK boilers, past and present. However, how efficient they are in practice is a different matter. More on that later.

Boilers can be of two basic types –

  • conventional , where the boiler heats the central heating system and also heats up a cylinder full of water ready for baths and washing. After the hot water in the cylinder has been used you have to wait at least half an hour before it has heated up again.
  • combination , where the boiler is split into two separate parts – one part heats the central heating and the other part heats up domestic hot water instantaneously and continuously so in theory you could have a never ending shower.

It gets a little more complicated when solar collectors are involved because they are normally used to pre-heat domestic hot water. The boiler then tops up the heat if it is not warm enough. With the conventional boiler setup there is an extra large hot water cylinder with two coils in it instead of one. The second coil is at the very bottom of the tabtank and is connected to the solar panels.

Ariston Primo Twin Coil (Solar)

Ariston Primo Twin Coil (Solar)

With the combination boiler the mains cold water feed needs to go through a coil in a tank which is heated from the solar collector so one of the advantages of combination systems (not needing a hot water cylinder) is lost.

It is also important to make sure that the boiler you choose will work properly in this mode. Probably the time to get advice from competent installers.


The actual efficiency of a condensing boiler depends on whether it is working in condensing mode or not. They only do that when the return water temperature (from the radiators) is 55ºC or lower. Although condensing boilers are very efficient by their nature it can still make a difference of 10% or so if they are not in condensing mode. This is significant and it is quite difficult to guarantee that they are going to be in condensing mode for a number of reasons –

  • the boiler may be oversized and not be able to dissipate all its heat so the return water is too hot to allow condensation
  • there may be only a few radiators on at any given time so the boilers heat is not dissipated (especially in summer)
  • the boiler temperature setting may be so high that the return water is too hot
  • the pump speed may be too great, causing water to get back to the boiler before it has lost enough of its heat

Enter the modulating boiler which can turn itself down when needed rather than constantly cycling on and off. These boilers tend to modulate from their maximum output down to about 10kW or even 7. This may still not be low enough if your house is very well insulated. A superinsulated house may only be using a couple of kilowatts for heating even in very cold weather (and maybe 3 kW for DHW heating when needed) so the boiler will still be cycling. This is approaching the territory of the Passivhaus where a central heating system is not needed. The situation can be partially remedied by a good plumber who can balance the system well by doing a careful job of adjusting the check valves on radiators in conjunction with adjusting the pump to its lowest setting to function properly (saving electricity). This can take some achieving because most plumbers are quite dismissive about doing this, partly out of a kind of ignorance of the importance of balancing low energy systems and partly because it can be a very lengthy job to cover all the combinations of radiators being on or off and the plumber feels he is perceived to be wasting time.

Positioning of boilers

When possible it is best to situate a boiler next to a service duct . This will work well for the pipe runs, the condensate outlet and the flue, particularly if the duct is next to an external wall.

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