Where the extra efficiency comes from
Burning gas produces hot water vapour, and an old boiler sent that heat straight out of the flue. A condensing boiler passes the flue gases over a second heat exchanger, cools them until the vapour condenses back to water, and recovers the heat that would have been lost. That recovery is the difference between roughly 70% efficiency on an old unit and around 90% on a new one.
The catch is in the word condenses. The vapour only turns back to water if the return pipe from your radiators is cool enough, roughly 55C or lower. Run the boiler hotter than that and the vapour leaves as vapour, taking its heat with it, and your 90% boiler quietly behaves like an 80% one.
The setting that gets you the efficiency you paid for
Most combis leave the factory with the heating flow set at 70C or more, which is higher than most homes need. Turning the flow temperature down to around 60C keeps the return below the condensing threshold for much more of the time, costs nothing, and in most homes the radiators still get fully warm, just slightly more slowly.
If the house struggles to stay warm at a lower flow temperature, that usually says something about radiator sizing rather than the boiler, and it is worth raising on the next service or when pricing a replacement, because an installer can size the system for low-temperature running.
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Compare boiler quotesThe condensate pipe, and the January callout
All that recovered vapour becomes slightly acidic water, around a litre or two an hour when running, which drains away through a plastic condensate pipe. Where that pipe runs outside to a drain, a hard freeze can block it, and the boiler shuts itself down with a fault code rather than back up.
The fix is usually pouring warm water over the frozen section and resetting the boiler, and the prevention is insulating the outside run or having it rerouted internally. It is the single most common winter boiler callout, and one of the questions worth asking during installation: where does my condensate go, and is the outside run insulated?
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