What Temperature Should My Boiler Be Set To?

Recommended Flow Temperature Settings

The right setting depends on your type of system and whether you are prioritising maximum efficiency or maximum output.

Traditional radiators (standard systems)

Many UK homes have radiators sized for a 70–80°C flow temperature. If your system was designed for this range, the boiler is often set to 70–75°C for heating. This delivers plenty of heat but may not allow the boiler to condense fully (see below). It is a safe starting point if you have not changed the settings before.

Modern condensing boilers for efficiency

Condensing boilers recover extra heat when the return water is cool enough. Running at 60–70°C flow temperature (or lower if your radiators can cope) lets the boiler condense more and use less gas. Try reducing from 70°C to 65°C, then to 60°C if the house still heats up comfortably. Not all systems will perform well at 60°C—it depends on radiator size and insulation.

Heat pump compatible or low-temperature systems

Systems designed for low-temperature operation—such as well-insulated homes with large radiators or underfloor heating—often run at 45–55°C flow temperature. This is the most efficient range for condensing boilers and is similar to heat pump flow temperatures. Only reduce to this level if your installer or heat loss design allows it; otherwise you may not get enough heat.

Hot water cylinder temperature

If you have a hot water cylinder (system or regular boiler), the cylinder thermostat should be set to a minimum of 60°C to prevent legionella bacteria from multiplying in stored water. This is a health and safety requirement. Do not lower the hot water temperature below 60°C for stored water; combi boilers heat water on demand so the same rule does not apply in the same way, but 55–60°C at the tap is typically recommended.

Flow Temperature vs Room Temperature — What's the Difference?

Flow temperature is the temperature of the water leaving the boiler and going into your radiators or cylinder. You set this on the boiler itself (often via a dial or digital control labelled "central heating flow" or "radiator temperature"). It is typically between 50°C and 80°C.

Room temperature is the air temperature in your home. You set the desired room temperature on your room thermostat (e.g. 18–21°C). The thermostat tells the boiler when to turn on or off so that the room reaches and maintains that temperature. So: flow temperature is how hot the water is; room temperature is how warm you want the room. Lowering the flow temperature does not necessarily mean a colder home—if the heating runs a bit longer, the room can still reach the same thermostat setting, often with less gas used.

How to Set Your Boiler Temperature

Controls vary by brand and model. On most boilers you will find either a dial or a digital menu for central heating flow temperature (and a separate one for hot water on combis).

• Worcester Bosch: Many models have a central heating dial on the front (often marked 1–6 or with a temperature scale). Some use a digital display—consult the manual for the menu path to flow temperature.
• Vaillant: Flow temperature is usually adjusted via the control panel or a separate programmer; some use a dial on the boiler. Check the user manual for your model.
• Ideal: Similar mix of dials and digital menus depending on the range. The heating flow temperature may be under a "Settings" or "Installer" menu—refer to the manual.

If you cannot find the flow temperature setting, your boiler manual (often available online) will show the exact location. Make small changes (e.g. 5°C at a time) and allow a few days to see if the house still heats comfortably before going lower.

Why Lower Flow Temperatures Save Money

Condensing boilers are most efficient when the return water from the radiators is cool enough for the flue gases to "condense"—releasing extra heat that would otherwise go up the flue. The lower the flow temperature, the cooler the return water tends to be, so the boiler spends more time in condensing mode. At 80°C flow, many boilers condense little; at 60°C or below, they can recover significantly more energy.

Reducing flow temperature from 75°C to 60°C (where your system can still heat the home) can typically save around 6–8% on gas bills, according to industry guidance. The exact saving depends on your insulation, radiator sizes and how long the heating runs. It is one of the easiest changes you can make without replacing any equipment.

Weather Compensation

Weather compensation is a feature on some boilers that automatically adjusts the flow temperature based on the outside temperature. When it is mild outside, the boiler runs at a lower flow temperature; when it is very cold, it increases the flow temperature so the house can still reach the desired room temperature. This avoids running at full temperature all the time and improves efficiency. If your boiler and thermostat support weather compensation, an installer can set it up. Not all systems have it—check your boiler manual or ask your engineer.

Common Mistakes

Setting the flow temperature too high: Many people leave the boiler at 75–80°C "to be safe." If your radiators are adequately sized, this wastes gas. Try turning the flow temperature down in steps; if the house still reaches the thermostat setpoint, you will save money with no loss of comfort.

Setting hot water too low: For stored hot water in a cylinder, keeping the temperature below 60°C to save energy is a health risk. Legionella can multiply in tepid stored water. Always keep the cylinder thermostat at 60°C or above. For combi hot water, 55–60°C at the tap is usually sufficient and safe for normal use.

Frequently Asked Questions

What temperature kills legionella?

Legionella bacteria are killed at 60°C and above when water is held at that temperature. UK guidance recommends storing hot water in cylinders at 60°C or higher to prevent growth. Water at the tap can be slightly lower (e.g. 55°C) for comfort, as long as the stored water in the cylinder is kept at 60°C. Do not set the cylinder thermostat below 60°C.

Should I turn my boiler down in summer?

You can leave the central heating flow temperature as it is when the heating is off—it only affects the system when the heating is on. For hot water, keep the same safe temperature (60°C for cylinders, or your usual combi setting) year-round. There is no need to change boiler settings seasonally unless you have weather compensation, which does it automatically.

What is the most efficient boiler temperature?

The most efficient flow temperature is the lowest one that still heats your home to the temperature you want. For many condensing boilers that is in the 55–65°C range, depending on your radiators and insulation. Start at 65°C and try reducing by 5°C every few days; if the house still reaches the thermostat setpoint, you are saving gas. Stop reducing if rooms start to feel cold.

Does flow temperature affect how hot my radiators get?

Yes. Lower flow temperature means the water entering the radiators is cooler, so the radiators themselves will be less hot to the touch. They can still heat the room—they may need to run for longer or you may need larger radiators for the same output. If you lower the flow temperature and the room no longer reaches the thermostat temperature, the flow is too low for your current radiator size; turn it back up slightly or consider larger radiators if you want to run at lower temperatures long term.