One of the big benefits of condensing boilers is their ability to reclaim heat that would otherwise be lost out of the flue, improving efficiency. In fact, condensing boilers have efficiency levels of greater than 90%; that is, little energy is wasted heating water. Yet, if your boiler is set at the wrong temperature you won’t get the full efficiency, so what’s the best temperature for a condensing boiler?
The simple answer is setting your central heating to a temperature of around 70C and your hot water to 60C is the right answer, but why, you might ask? It all comes down to the way that condensing boilers work.
How condensing boilers work
With a traditional boiler, fuel is burned to heat your water but a large portion of this is lost out of the flue. With a condensing boiler, there’s a secondary heat exchanger, which is designed to recycle some of this heat, boosting efficiency.
Using a boiler for your central heating, you have two pipes, the flow and the return. The flow is the pipe heading out of the boiler, which is used to heat your radiators. The flow pipe has passed through the boiler’s main heat exchanger and is heated via the burning of fuel. This is effectively the same for all boiler types.
The return pipe is the pipe that comes back in from your radiators. This water will be warm but not as hot as the flow, as some heat is released into your rooms, via radiators, to heat them. In a condensing boiler, the return pipe enters a secondary heat exchanger, which has the job of capturing heat from the waste gasses in the flue.
The process of doing this condenses water vapour in the exhaust gasses, recovering some of the heat that would have otherwise have been wasted out the top of the flue. This condensate, which is slightly acidic, is taken from the boiler via a condensate pipe. Ideally, this should be internally led to a drain; where condensate pipes are led outside, there’s a danger of freezing in cold weather.
The return pipe is then heated by this process, before flowing back into the main heat generator. Through this process, waste energy is recycled and the return pipe requires less energy to heat it as it has been pre-warmed.
Is your condensing boiler in condensing mode?
For the condensing boiler to operate at maximum efficiency, the secondary heat exchanger’s surface needs to be equal to or below the dew point temperature of the fuel used. This is the temperature at which water droplets form. For natural gas boilers, the dew point is around 55C. In other words, the water in the return pipe needs to be 55C or lower or your boiler will not operate at maximum efficiency and potentially will not even condense.
Turning down the temperature on your boiler can help increase its efficiency. Most modern boilers will have a mark on the screen that shows you the point at which condensing mode will be turned on. On my Ideal Logic+ C30, there’s an ‘E’ icon on the central heating dial, which is 68C.
There are factors that will affect this. For example, if you have smart heating with individual radiator valves, such as with the Honeywell Evohome, the return temperature can change. For example, if you’re heating the entire house, with all valves open, you’ll get a cooler return. If you’ve only got one radiator valve open, the return will be warmer.
However, the benefit of only heating a single room will mean that your boiler will shut down sooner, so the overall energy savings will be higher. For the best energy savings, a thermostat and boiler that can use OpenTherm will work best, as this lets the thermostat adjust (modulate) the water temperature for the best results. You can also follow our guide on saving money with smart heating.
Related: Best smart thermostat
What about hot water to the tap?
Your condensing boiler should be set to generate water at a flow temperature of at least 60C (hot water tanks should store water at this temperature). This is because this is the temperature that kills legionella bacteria.