"John Stumbles" wrote in message
news
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.

If the cylinder has German insulation standards they heat loss is minimal.
Levels in the UK are dire.

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

With adequate speading pipes in the cylinder this is not a big issue.

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

Yep, as Gledhill do.

or brute-force by restricting the flow with
a TRV type affair.

Yep, put a 2-port valve on the primaries from the cylinder that serve the
PHE and have a remote sensor on the DHW flow to the taps. Works well.
Danfoss do a good one. You may need a pressure by-pass valve around the
thermostat 2-port valve to cope when the valve is fully closed. There is
also the Grundfoss Alpha auto variable speed pump too. Setting up is a bit
tricky as it may fight with the DHW thermostat. I have seen a few work very
well and no pressure by-pass valve needed.

Either way would have the added
benefit of doing away with the need
for a TMV on the
output of the PHE.

Yep, and another valve eliminated that reduces flow. Another is the PRV
too.