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One of the most common questions homeowners ask is, “What temperature should my boiler set at?” In this blog, I’ll guide you through the optimal settings for your heating system, whether you have a combi, system, or heat-only boiler. By adjusting the flow temperature and understanding how condensing boilers work, you can save both energy and money without sacrificing comfort.

Optimal Boiler Temperature Settings for Combi Boilers

For condensing boilers, such as combi boilers, efficiency is highest when the return water temperature is low enough to trigger the condensing process—typically below 55°C. Setting your boiler’s flow temperature to around 55°C keeps it in condensing mode, ensuring it operates at maximum efficiency. At this temperature, the boiler extracts more heat from the flue gases, which would otherwise be wasted.

![Image Suggestion: Diagram showing the flow temperature and condensing process in a combi boiler.]

However, older systems, particularly those designed for higher flow temperatures (60-70°C), may not perform optimally if the temperature is set too low. If your home’s heating system was designed for higher temperatures, it’s important to strike a balance. You can often lower the temperature during milder weather, but in colder months, you may need to increase it to maintain comfort.

Flow Temperature and Heat Curve Adjustments

In most homes, lowering the boiler’s flow temperature while keeping your house comfortably warm is the key to saving energy. For example, instead of setting your boiler to a high temperature and running it in short bursts, try setting it to a lower temperature and running it for longer periods. This way, the boiler works less intensively and remains in condensing mode longer, using less gas.

Let’s break it down with numbers: If you set your boiler to run at full output, say 10 kW, it may heat your home quickly in just an hour, but then it will turn off. You’ll feel the warmth immediately as the radiators heat up fast, but when they cool down, the air temperature will drop, and the room may start to feel cold again. In my personal experience, when you heat your home at a lower temperature, say at 4-5 kW for a longer period (e.g., 2 hours), the warmth lasts longer and feels more consistent. Your boiler doesn’t have to work as hard, and the slower operation helps the system stay in condensing mode longer, making it more efficient.

![Image Suggestion: Graph showing temperature output over time with different flow temperatures.]

If your thermostat has a heat curve or weather compensation feature, you can automate this process. This function adjusts your boiler’s output based on the outside temperature, ensuring your heating system runs at the most efficient setting for the current weather. For example, in Leeds, we design systems for outside temperatures as low as -3°C, but during milder days, you can reduce the boiler temperature manually or through the thermostat’s compensation feature.

System Boilers and Hot Water Storage

For system and heat-only boilers that use hot water cylinders, keeping the water temperature at a minimum of 60°C is essential to prevent the risk of Legionella bacteria. Although there’s some debate within the industry suggesting that 55°C might be safe, the general recommendation is still to keep the water at 60°C. This higher temperature reduces the boiler’s efficiency slightly because it can’t stay in condensing mode as long, but safety is the priority here.

![Image Suggestion: Infographic on Legionella bacteria and safe hot water temperatures.]

In my experience as a heating engineer, you don’t need to run your boiler at this higher temperature all the time. You can set your hot water demand to come on in the evening when you’re more likely to need it and manually adjust the system’s temperature higher just for that time. During the day, when you’re not using hot water as much, you can lower the system temperature to improve efficiency. If you miss a day or two of setting it manually, it’s not a big issue. A “Legionella cycle” should be run every 5-7 days, where the water is heated to 60°C to kill any bacteria, but it doesn’t need to happen daily.

Using an Immersion Heater for the Legionella Cycle

If adjusting the system temperature manually feels like too much effort, there’s another option: using an immersion heater on a timer. In my opinion, this is a simple and effective solution, particularly in retrofit situations. You can set the immersion heater to come on once every 5-7 days, just after the boiler heats the water, to top the temperature up from, say, 50°C to 60°C. This way, the immersion heater handles the Legionella cycle, allowing you to keep the boiler running at lower temperatures the rest of the time, improving overall efficiency.

For more modern systems, an even better solution is to use a “hot water priority” system, or W plan. With this setup, the boiler automatically ramps up the temperature when heating hot water, ensuring quick heating of the cylinder. Once the hot water is ready, the boiler drops back down to a lower temperature for space heating. This gives you a great balance of efficiency and performance, especially if you’re looking to optimize energy use.

Reusing Flue Gases for Better Efficiency

One of the key benefits of condensing boilers is their ability to reuse flue gases to extract more heat. The return temperature of the water plays a big role in this. When the return temperature is lower, the flue gases cool down more slowly and spend more time in the heat exchanger, allowing more heat to be transferred to the water. If the return temperature is too high, the flue gases pass through the system too quickly, and much of the heat is wasted.

For instance, if your boiler’s return temperature is 10-15°C, which is typical in well-designed systems, your boiler will condense effectively, capturing more heat from the flue gases. However, if the return temperature is closer to 55°C, your boiler will only start to condense, meaning it won’t be operating at its highest efficiency. In systems where the flow temperature is set too high (e.g., 75°C), the boiler won’t condense at all, leading to a significant loss in efficiency.

Additional Energy-Saving Tips

Here are some practical tips to maximize your energy savings:

  • Set your hot water temperature to 40°C for tasks like showering and washing up. This saves both gas and water.
  • Lower your boiler temperature during the day when you’re not home or at night while sleeping. You can always increase it later when needed.
  • Make small adjustments based on the outside temperature: On milder days, lower your boiler temperature manually or use a smart thermostat with weather compensation to make automatic adjustments.
  • Use lower flow temperatures to keep your boiler in condensing mode as long as possible.

For those not ready to invest in a heat pump, these small adjustments can significantly reduce your gas consumption and bills.

Conclusion

Adjusting your boiler’s temperature settings is one of the easiest ways to save energy without sacrificing comfort. For combi boilers, aim to keep the flow temperature at around 55°C to maintain condensing mode, while for system and heat-only boilers, ensure hot water is periodically heated to 60°C for safety. Whether you manually adjust your settings or invest in a smart thermostat with weather compensation, these changes will help you see a noticeable reduction in your energy bills.