Smarter Heating & Hot Water

Boiler Optimiser

Lowering your flow temperature is the easiest way to cut your gas use and lower energy bills. It can also help reduce boiler wear and tear over time. Select your system type below, then use the slider to estimate your savings.

Which boiler or system type do you have?

[1.]

Annual gas usage

[3.]
Boiler Annual Savings £0.00
Partially condensing
75°C flow ≈ 55°C return
75°C

Flow temperature

Efficiency and energy label

Boiler efficiency against return water temperature for condensing boilers, with dew point at 54°C.

Heating programmer timings

You may need to add more time on the heating programmer. Reducing flow temperature means the house takes longer to warm up, but the system is more stable, so add more time if necessary.

Single channel heating programmer, such as a Honeywell Home ST9100C
Single channelHoneywell Home ST9100C
Dual channel heating and hot water programmer, such as a Honeywell Home ST9400C
Dual channelHoneywell Home ST9400C

Cylinder thermostat

Warning: Condensate pipe must be correctly installed

That white plastic pipe under the boiler is the drain for the water a condensing boiler makes. If it freezes, the boiler usually stops.

Fitted properly

It should slope downhill all the way, with no dips. Better if it stays indoors and goes into a drain. If it has to go outside, keep that bit short. Plastic pipe, not copper.

Size

A thin pipe on an outside wall is the one that freezes. Outside, or in a cold loft or garage, it should be a fatter 32 mm pipe.

Insulation

Any outdoor length needs waterproof lagging, including the bends and the open end. Bare pipe on a cold wall will freeze.

If it freezes

Warm it from the ground with a hot-water bottle or warm water — not boiling, and not a flame. Then press reset on the boiler. Turning the flow back up can get you through a cold night, but you lose the saving. Ask a Gas Safe engineer to lag it properly, or run it indoors.

Advanced System Controls

Custom annual gas usage and unit rates

Do you use gas for cooking?
Estimated annual space heating usage
[5.]
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Bill kWh minus typical gas cooking (3% of annual use).

Did you adjust the slider to see what your savings are?

Change starting flow temperature

[.]

Leave blank for 75°C, or type the flow it is already set to.
Annual Condensing-Mode Saving 0% Saving Versus 75°C Initial Flow Temp 75°C Flow Temp And 55°C Return Temp
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Your Annual Space Heating Cost (Not Including Standing Charge)
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Have you successfully turned the heating flow down? Come back here and let us know.

Choose the boiler brand and type, then what the flow was and what you set it to. Add annual kWh and unit rate if you know them. We log this anonymously.

Notes, Keys and Information
    Boiler efficiency against return water temperature for condensing boilers, with dew point at 54°C.
    Single channel heating programmer, such as a Honeywell Home ST9100C
    Single channelHoneywell Home ST9100C
    Dual channel heating and hot water programmer, such as a Honeywell Home ST9400C
    Dual channelHoneywell Home ST9400C

    Load compensation (OpenTherm)

    A load-compensating thermostat, sometimes called a modulating thermostat, talks to the boiler instead of just switching it on and off. As rooms get close to the temperature you have set, it turns the flow temperature down. OpenTherm is one common way of doing this; some boilers use the maker’s own protocol.

    This is usually the easier extra option to set up: often just a compatible thermostat swap, with no outdoor sensor or heating curve. Many boiler makers sell their own load-compensating controls that talk to their boilers over the maker’s own connection; these usually only work fully with that maker’s boilers, so check compatibility before you buy. If your boiler does not support OpenTherm, the maker’s own control is often the simplest match; some also offer OpenTherm. It can also be used with weather compensation.

    A sensible fixed, lower flow temperature is still a good, simple start. That is what the Boiler Optimiser helps you find. If you rent, check with your landlord before you change any controls. See OpenTherm boilers and controls and Vaillant controls.

    Weather compensation

    Weather compensation goes a step further and is a little more advanced. It uses a sensor fitted on an outside wall to turn the boiler’s flow temperature down as the weather gets milder. It works well when it is fitted and set up properly, but for some homes it can complicate things.

    That only works well if the heating curve is set for your home. Many combi boilers never had that sensor fitted, or it is there but not set up.

    For most homes a sensible fixed, lower flow temperature is the simpler route. That is what the Boiler Optimiser helps you find. If a compensation sensor is already fitted and working, keep using it. If you rent, check with your landlord before you change any controls or have a sensor fitted.

    This tool is in development. We accept no responsibility for issues that arise from these adjustments. Some systems are not installed correctly and may cause problems, especially where the condensate pipe is incorrectly installed or not insulated.

    Last updated 3 Nov 2026 · Version 43