On Sat, 12 Aug 2006 14:29:14 +0100, Andy Hall wrote:

Then unless the heater can be set to 75 degrees or more, the energy
storage will be more limited vs. running at boiler flow temperatures.

This is exacerbated in systems using external (plate) heat exchangers
because the pump circulating water round the PHE runs at full bore
regardless of the demand, so if you draw a gentle shower or run a hot tap
at part flow you're liable to lose most of your hot water capacity through
mixing within the cylinder long before you would have done if you were
drawing only enough to provide the amount of DHW demanded. It should be
easy enough to fix this either cleverly by modulating the pump or
brute-force by restricting the flow with a TRV type affair. Either way
would have the added benefit of doing away with the need for a TMV on the
output of the PHE.