Should I lower my boiler's flow temperature?

Why a condensing boiler is more efficient at lower flow temperatures, how far to turn it down, and the houses where it does not work.

Difficulty: beginner
Time: 20 min
Read: 3 min
Safety: caution

Short answer

Yes, for most condensing combi boilers. Dropping the central heating flow temperature to around 55 to 60 degrees lets the boiler condense properly, which is where its claimed efficiency comes from. Radiators run cooler and take longer, so it suits continuous heating better than short bursts.

A condensing boiler recovers extra heat by condensing water vapour out of its own flue gases, and it can only do that when the water returning to it is cool enough. Run it at 80 degrees and it barely condenses at all, which is why so many boilers never reach their rated efficiency.

Safety

Changing the flow temperature dial on the boiler's own control panel is a user setting and is safe to adjust. Do not open the boiler casing, alter internal settings, or change the hot water temperature on a system with an unvented cylinder without understanding it — those are engineer tasks. If lowering the flow temperature leaves the house cold in a cold snap, put it back up rather than leaving rooms unheated.

What you'll need

Access to the boiler's user controls
A few weeks to judge the result

Step by step

Find the central heating temperature control.Usually a dial or a display setting marked with a radiator symbol, separate from the hot water setting marked with a tap symbol. It is on the boiler's front panel, not inside.
Note the current setting before changing it.Most are set to 70 to 80 degrees from installation. Write it down so you can go back.
Understand what you are trading.Lower flow temperature means cooler radiators, so rooms warm more slowly but the boiler runs more efficiently and cycles less. It suits longer heating periods rather than short blasts.
Drop it in stages.Try 65 first for a week, then 60, then 55. Going straight to 50 in January is how people conclude it does not work.
Check the house still reaches temperature.If rooms get to the temperature you want within the programmed period, the setting is fine. If they never get there, go back up a step.
Leave the hot water setting alone.On a combi, the hot water temperature is a separate control and should stay high enough for comfortable washing. Lowering it does not gain the same efficiency.
Extend the heating hours if needed.Lower flow temperature works best with the heating on for longer at a steadier output rather than short intense periods. Adjust the programmer accordingly.
Expect it to work better in a well insulated house.A house with modern insulation and reasonably sized radiators copes easily. An older, draughty house with small radiators may need the higher setting in cold weather.
Consider it seasonal.Many people run 55 for most of the year and put it up during a hard frost. There is nothing wrong with adjusting it.
Do not confuse it with the room thermostat.The flow temperature sets how hot the water in the radiators is. The room thermostat sets how warm the room gets. They are different controls doing different jobs.

Tips

Change one thing at a time and give it a week. Judging a heating change over a single evening tells you nothing.
Feel the flue in cold weather: a condensing boiler working properly produces a visible plume and a condensate drip.
If the house never reaches temperature at 55, that is useful information about the radiators, not a reason to abandon the idea.

Common mistakes

Lowering the hot water temperature at the same time — The efficiency gain comes from the central heating circuit. Turning the hot water down makes showers unpleasant and, on a cylinder system, raises legionella concerns.
Combining a low flow temperature with short heating bursts — Cooler radiators need longer to warm a house. Two short periods a day at 55 degrees will underheat where 80 degrees just about coped.

If it doesn't work

House takes much longer to warm up

Cause: Expected at lower flow temperature — Fix: Start the heating earlier, or run it for longer periods.

House never reaches temperature

Cause: Radiators undersized for the lower flow temperature — Fix: Put the setting back up a step; consider larger radiators in the worst rooms.

Boiler cycles on and off frequently

Cause: Flow temperature too high for the demand — Fix: Lowering the flow temperature usually reduces cycling, which is one of the benefits.

Hot water gone lukewarm

Cause: The wrong dial adjusted — Fix: Return the hot water setting to its original value; only the heating dial should change.

Questions people ask

What flow temperature should I use?

Around 55 to 60 degrees suits most condensing combis in a reasonably insulated house. Some go lower successfully. The test is whether the house reaches the temperature you want within the hours you heat it.

Will it save much?

Independent trials have found meaningful savings for many households, typically in the region of several per cent of gas use, with no loss of comfort where the radiators are adequate. It costs nothing to try.