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 the central heating to a temperature of around 70C and your hot water to 60C should give you the highest efficiency levels for your boiler, although this doesn’t necessary mean the lowest running costs. To cut running costs, adjusting the temperature of your central heating and hot water is required, although getting the right level is a little more fiddly. We’ll explain all.
How condensing boilers work
First, it’s important to understand how a condensing boiler works. 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 fuel burning. 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.
What about cost savings?
Running your boiler at its maximum efficiency doesn’t necessarily mean that you’re getting the best cost savings. It’s actually more cost-effective to run your boiler at the lowest central heating temperature that still gets your home to a comfortable temperature.
That may seem like the wrong way round, but whacking your boiler up to maximum temperature may heat your house faster, but you’ll get worse efficiency and burn a lot more gas. At today’s prices, in particular, that’s not a good thing.
So, how low should you go? Well, a heat pump, a more eco-friendly way of heating your water, will only hit temperatures of around 40C to 45C. It makes sense to aim for 40C to start with, as you’ll cut gas consumption.
You’ll need to allow more time for your rooms to heat up at this temperature. Depending on how well insulated your home is and how big your radiators are, you may find that that this low temperature isn’t enough for your house. In that case, try increasing the temperature by 5C to see if that works. Repeat until you find the right temperature for your home, with the one limit that you need to have your boiler’s temperature set low enough that it runs in condensing mode.
There’s also a myth that it’s best to leave your heating on permanently to keep your house up to temperature, as this is cheaper than cooling your home down and then reheating when you’re in. This is not true and you’ll save far more money by efficiently heating your home when you need it and having the heating off when you don’t.
What about hot water to the tap?
The temperature your hot water should be set at depends on the type of system that you have. However, it’s important to note that your boiler can’t operate in condensing mode for hot water, as there’s no return flow to create the condensing process. With that in mind, if you have a hot water tank, then your hot water flow should be at least 60C and your hot water tank should be set to store water at this temperature. This is because this is the temperature that kills legionella bacteria. As a result, you may want to go a few degrees warmer.
For a combi boiler, water is heated on demand and there’s a very small water tank. As such, there’s very little likelihood of legionella bacteria developing. While it’s recommended that combi boilers are set to 60C for safety, it is possible to go cooler. We set our hot water to a temperature of 45C: hot enough for washing dishes and hands without the need to add in cold water.