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The Viessmann one (111) is 46L and as far as I can tell is blended with the combi output to give higher flow equivalent of a 150L unvented tank, whatever that means.
The Vaillant one (938) is more opaque as to how big it is and what it actually does - flow seems the same as the equivalent model without a tank but I think the idea is that a) you’ve got instant hot water with no startup time and b) if someone runs a tap during a shower, there’s additional capacity and flow is maintained at higher temps.
Dramatic_Hammer
Howard
We don’t but I wonder how much storage you’d need for it to be useful? It’s not like you could pump from a 5 litre tank for example, right?