"Christian McArdle" wrote in message
. net...
It will still cycle the boiler much more than a storage based system,

On what do you base this article of faith?

It is based on the fact that a storage based solution will be able to
perform a considerable amount of secondary loop heating service before
this
activates the vessel thermostat, allowing the boiler to perform a slightly
longer more efficient burn.

With an instantaneous combi, the combi must fire every time the secondary
loop pump activates, and in the worst case, if the loop circulation is
slow,
it might even be below the modulation threshold, so it might even have to
cycle to maintain primary temperature, although careful design can
eliminate
this.

Therefore, even in a best case scenario, the instantaneous combi fires
every
time the loop thermostat calls for heat, whilst the storage based system
could service for hours without boiler assistance, with sufficient
capacity
and thermostat hysterysis/double thermostat.

A secondary circulation loop is well, a secondary circulation loop. Whether
there is a large cylinder on the loop or an instant water heat makes no
difference. The loop will cool no quicker on any of them.

You could have a few small DHW vessels on the loop, in series, like under
sink units, have a pressure vessel too. So when you draw off small volumes
of water at the local point, the combi does not kick in as there will be no
flow through the boiler until the pressure has dropped - avoids nuisance
firing. There could be a thermostat on each small vessel to fire the combi
to replenish the vessel of DHW. A Dutch/Belgian company did somthing
similar to this - distributed storage giving instant water at the taps. The
total volume of all the vessels is enough to fill a bath.