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Discuss Thermal store combi boiler, hot water flow rate. in the Plumbing Forum | Plumbing Advice area at

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Gas Engineer
Can anyone tell me the hot water flow rate on a thermal store combi boiler? As opposed. To a 40kw combi that will deliver nearly 16litres per minute. And any recommendations for a good thermal store boiler thanks.


Advent Win
Unless I am misunderstanding your point, I assume you mean a combi boiler with an integrated hot water storage facility behind it all mounted on the wall?

If so, a combi like a Valliant 938 with a shift load cylinder (Valliant’s term not mine) will only deliver hot water at the incoming flow rate. The shift cylinder, if so configured, holds around 1 minutes worth of hot water at a delta T of 35 degrees. The pump in the shift cylinder is to circulate stored hot water not to deliver it.

I am not recommending the Valliant, just using it as an example ( I don’t really like large Combis’).

However, from my experience, again using the Valliant as the example, unless you have a consistent cold water flow at the boiler inlet of at least 23 lpm the boiler won’t perform to specification.

A common issue I come across large properties, where a Combi should never have been installed, is :

The toilet being flushed, hot tap at the basin turned on, then the shower - cold flow to the boiler dropping well below that required for maximum output. Unhappy customer who really wants a “hotel quality” shower.


Gas Engineer
Thanks I do mean a wall mounted boiler with internal thermal store. Are you saying they only offer about a minutes worth of stored hot water?


Advent Win
The only boiler with a thermal store that I know the idiosyncrasies of well is the Vaillant 938. The store volume (shift load cylinder) is 15 litres. With the right inlet flow rates the boiler will deliver 20.5 litres/minute at delta 35 degrees or 13.2 lites/minute at delta 42 degrees.

We use them in externally installed, heated modular units in London coupled to a pumped accumulator and water softener. Very well suited to three storey terraced houses where internal space is at a premium in areas of low pressure and flow.

Others with know better, but I am unconvinced of the benefit of the storage module if you can get the flow to the boiler to allow it to deliver hot water at full capacity for 20 minutes.


Gas Engineer
Advent Win
The viessmann111 has a 60 litre store the 222 think has 100 litre both give amazing flow rates never fitted the 222 but fitted a couple of 111 and have been very impressed by them


Staff member
S. Mod
Gas Engineer
viessmann 111 will give around 18lpm

viessmann 222 with give what you put in eg 25 lpm you get out 25lpm its a good piece of kit

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