[Derek Monckton]

Combi storage is very much for higher demand households. Vaillant produce the ecoTEC plus 937 with 15 litres in built storage.
Generally storage comes with high output, the Vaillant 937 delivers up to 15 litres/min with a 37kW rating. I have recently specified one of these for a client – a large 4/5 bed house so it fits the bill. For a smaller property the combi should not give cold blasts, it can only reduce the flow and heat exchange to all open outlets if the demand exceeds heat exchange. Typically a 28/30kW DHW rated boiler should work well enough in a smaller property.
I have a combi in my house and that's a 1940s 2 bed semi with rads, I don't get cold draw off of water with bath and kitchen going together.
So it is on demand hot water, where as storage needs to be kept warm and the boiler will be on/off a lot more to satisfy that.
Vaillant are at the top end of efficiency, but they do have this keep warm function which cycles all day just to keep a small amount ready for that tap draw off, apparently very popular in Germany, but I can't work out how to turn that off.
Best thing is do the calculations for x litres of stored water working between a 5 or 10 deg temp variation between the longest interval for drawing off -overnight or when you are out all day for example. Are those extra kWh worth it?

Derek Monckton

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