Combi Boilers – It’s All About Flow Rate

For anyone considering combi boilers when choosing a new heating device, it all comes down to the words, flow rate.

Or, to be more precise, with combi boilers, it’s all about hot water flow rate.

And these few words are actually as important as that other key phrase, output to hot water.

Firstly, bear in mind that a combination boiler has two functions in life: to heat the water to keep the radiators warm and also to heat the hot water that it going out of the hot tap.

Now, we’re considering here the hot water issue. When a householder turns on the hot tap, they expect hot water to be available for as long as they need it; to wash their hands say, or to fill a bath. The problem for combi boilers, was that they could only provide hot water at the rate at which they can heat it. The hot water (unlike a conventional system), is not stored in a tank for later retrieval. When the hot tap is opened, the water needs to be available, and in the right sort of quantities. The problem with early combination models is that they struggled to do both roles effectively, meaning that they were usually installed in smaller properties, ones that did not theoretically need large amounts of flowing hot water.

But that meant that many people were missing out on the benefits of combi technology. And that mainly comes down to the inefficiency, with an old traditional system, of having to store water in a hot water tank for later use. A combi only needs to heat the water when its exactly required, either by the radiators, or the hot water taps. That’s more efficient and saves money. What’s more, by not having a hot water tank and associated piping, means that you avoid that cost altogether. So a combi boiler is less expensive than a traditional boiler.

But, let’s get back to the hot water flow rate. This figures basically tells you how much hot water (measured in litres) the boiler will generate in a given minute. So, let’s take an average combi output of 24kW. This will, on average, provide nine litres of hot water at a temperature of 35 degrees every minute. The higher the output of the boiler, the higher the water flow rate. Thus, with a boiler rated at 54 kW, it can provide a hot water flow rate of 25 litres per minute. But, with that kind of flow rate, you’re talking a lot more energy and consequently, it is far more expensive.

So, remember, you’re thinking about combi boilers, think hot water flow rate, as you need to ensure that this equates to the amount of water you’ll need for your property.