Your condensing boiler is probably
not condensing

There is a dial on the front of most combi boilers that almost nobody touches, partly because it is not obvious what it does and partly because nobody has ever explained it. It sits separately from the room thermostat and from the hot water dial, and it sets the flow temperature, which is the temperature of the water the boiler sends out to the radiators.

On a lot of the boilers we see it is sitting somewhere near 80, and at 80 the boiler cannot do the one thing it was designed to do, which is condense.

What "condensing" actually means

Every gas boiler sold in the UK for around the last twenty years is a condensing boiler, and the name describes something the boiler physically does.

When gas burns it produces hot flue gas, and that flue gas contains water vapour. A lot of the energy from the gas is held in that vapour as latent heat. An older non-condensing boiler sent all of it out of the flue, which is why you used to see a plume. A condensing boiler runs the returning heating water past that flue gas first, and if the returning water is cool enough, the vapour condenses back into liquid on the heat exchanger and gives up its latent heat into the system instead of going outside.

That is how it recovers the heat, and it depends entirely on one thing, which is that the water coming back from the radiators has to be cool enough for the vapour to condense on it.

If the boiler is sending water out at 80, the water coming back is nowhere near cool enough and the boiler simply cannot condense. It still works and it still heats the house, it is just running as a non-condensing boiler with a condensing boiler's badge on the front, and burning more gas than it needs to.

Send the water out at around 60 and it comes back cool enough, and the boiler starts doing the thing you paid for.

Hotter is
not faster

This is the objection everyone has, and it deserves a proper answer because it sounds like it should be true.

Turning the flow temperature down does not lower the temperature of your house. The room thermostat is what decides how warm the room gets, and it is still set to whatever you set it to. The flow temperature decides how hot the radiators run on the way to getting there.

What changes is the shape of the heat-up. At 80 the radiators get very hot quickly, the room warms fast, the thermostat is satisfied and the boiler shuts off. Then the room cools, and it fires again. At 60 the radiators run cooler for longer, the heat comes into the room more steadily, and the boiler cycles less. The room arrives at the same temperature, it takes a bit longer to get there in the morning, and it holds more evenly once it does.

The trade-off is real and it is worth naming, because a house that is genuinely under-radiatored, or somebody who is out all day and wants the place warm within twenty minutes of walking in, will feel that longer heat-up. In that case the answer is usually to bring the timer forward a little, and the flow temperature can stay at 60.

The 60 degree check

Two minutes, no tools, no cost.

You can try it for a week or a few days. If you need to you can turn the temperature up slightly. Or if you need to you can turn it up higher and that is okay too, you tried and it wasn't suitable for you. Not all properties are the same and not everyone's requirements are the same.

Nesta ran a national campaign on exactly this and published the evidence behind it. Their figure is that households could save about 9% on their total gas bill by doing it, assuming the flow temperature is currently set at 80. Their advice, like ours, is specifically about condensing combi boilers.

Two things not to do

Do not turn the hot water down. The hot water setting is a different thing entirely, and hot water is stored and distributed at particular temperatures for reasons that have nothing to do with efficiency, so leave that one alone.

This advice is for combi boilers. If you have a hot water cylinder, the picture is different, because the cylinder has its own requirements and the flow temperature has to be able to satisfy them. If you are not sure which you have, that is worth asking about before changing anything.

The short version

Most condensing boilers in London are running too hot to condense, and the fix is a dial that takes two minutes and costs nothing to change. You can do it tonight, and an installer makes nothing from telling you about it, which is probably part of why nobody mentions it.

The bigger version of the same point is that how a system is set up matters as much as what is in it. A well-chosen boiler badly commissioned will cost you money quietly for fifteen years, and nothing on the invoice will ever show it.

Quick answers