A number of combi boilers on the market in the UK have small (approx. 20 litres) internal hot-water tanks. These boilers are commonly marketed as being suitable for households with high hot-water demand. Sales literature typically claims reduced delivery time and increased initial flow rate of hot-water. I understand these benefits but would like to know how these boilers behave during low-demand periods.

Specifically, when a hot water tap is opened for less than 5 seconds or an appliance repeatedly draws small amounts of hot water.

My current boiler immediately fires-up in these situations and shuts down again before any meaningful amount of hot water can be generated, let alone reach the tap. I realise that there is scope for more efficient user behaviour but do combi boilers with in-built hot-water storage help to overcome this particular user inefficiency, by delaying fire-up until a given amount of stored hot-water is discharged, for example? Or do they fire-up immediately, just like their none-storage counterparts, in order to replenish their internal storage?

Many thanks.

1 Answer 1


There's no one-size-fits-all answer. In my particular unit, made by Westinghouse (whitelabeled from HTP, I'm pretty sure), there's a configurable delay. Some units will have a minimum number of gallons that need to flow prior to starting. Others may start immediately.

  • Thanks David, a configurable delay seems like an ideal solution. I'll leave the question open for a little while to encourage people to comment about their experience. Sep 7, 2017 at 13:00

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