How to Set Your Boiler Temperatures for Maximum Efficiency

If your boiler is running constantly but your home still feels cold, or your gas bills are unnecessarily high, the issue is often in the settings. Heating systems use multiple controls to operate efficiently, including a room thermostat, radiator controls, a boiler thermostat, and potentially a hot water cylinder thermostat. In this post we’re going to focus on condensing boiler temperature settings.

Setting these correctly isn’t just about comfort; it is about forcing your boiler to operate in its most efficient technical state.

The Combi Boiler Sweet Spot

Modern combi boilers feature digital dials to set the output temperatures for both your heating and your hot water. Getting this balance right is critical.

• Radiator Output: For combi boilers, the recommended radiator output temperature is 75 °C. The maximum temperature recommended for radiator output is 80 °C.
• Hot Water: The domestic hot water (DHW) temperature for a combi boiler should be set between 50 °C and 60 °C. The temperature of the tap water should always be lower than the radiator output temperature.
• The Efficiency Trap: Setting the boiler output temperature higher will heat the home more quickly, but it can increase heating bills and reduce boiler efficiency by 10 to 20 per cent.
• Weather Compensation: For the best efficiency with combi boilers, weather compensation is recommended. This technology automatically adjusts the heating boiler flow temperature to the optimal level based on external weather conditions.

System & Conventional Boilers: The Safety Factor

If you have a conventional or system boiler with a separate hot water tank, you will have a cylinder thermostat, which is typically fitted to the side of the tank.

• Optimal Setting: The cylinder thermostat temperature should be set between 60 °C and 65 °C. For maximum efficiency, it should be set to around 60 °C.
• The Legionella Risk: The hot water cylinder temperature must not be set lower than 60 °C. This is a critical safety measure to prevent the risk of Legionnaires’ disease. This illness is a form of pneumonia caused by Legionella pneumophila bacteria, which can grow in water systems if the temperature is not hot enough.

Room Thermostats & TRVs

Your boiler is only half the equation. Localised control stops you from heating empty rooms.

• Main Thermostat: The room thermostat is typically set to a comfortable and healthy temperature of around 18 °C to 21 °C. Bedrooms should be kept slightly cooler, at approximately 16 °C to 18 °C, for an optimum night’s sleep.
• Radiator Valves: Thermostatic radiator valves (TRVs) provide local control by managing the flow of water passing through a specific radiator, which in turn changes the amount of heat it gives off. The TRV on each radiator should typically be set to number ‘3’, which will provide a room temperature of approximately 20 °C.
• Energy Waste: Turning up TRVs, the room thermostat, or the boiler thermostat excessively can waste a significant amount of energy and increase heating bills.

The Science of Condensing Boilers

A condensing boiler is one of the most efficient types available because it is designed to recover virtually all the heat from the flue gases and use this to heat the home. However, this only happens if the system is balanced correctly.

For the boiler to work at its maximum efficiency, the heat exchanger needs to be equal to or below the dew point temperature. The dew point is the temperature at which water droplets (condensate) form in the heat exchanger. For natural gas boilers, the dew point is around 55 °C. This means the water returning to the boiler must be 55 °C or lower for the boiler to maintain its high efficiency (i.e., operate in condensing mode).

If you’d like to speak to us about upgrading to a new efficient condensing boiler, we’d be happy to help. iWarm are your local experts in boiler installations, repairs and gas safety certification.