In article ,
news writes:
A friend of mine has just had a (leaking) 25 year old HW tank replaced -
standard vented HW system gas heated by a secondary coil.

The house is just like mine and on the output of the secondary heating
coil there was a tap like body - neither hers or mine has ever had a
handle and I assume that it is the equivalent of the lock-shield valve
on the rads. It is designed (I assume) to stop the HW tank hogging all
the heat when both HW tank and rads are both demanding heat - but once
set during installation it can be forgotten (in just the same way that
the lock-shields are forgotten).

Anyway as part of the new tank installation this tap has been removed
and not replaced with anything. Is this normal for modern installations
or has there been some sort of drop off?

Problem with older cylinders is they didn't have many turns in the
coil, so ended up sending the water back without much of the heat
being removed. So yes, they short-circuited the heating system
without dumping much heat into the tank, so boiler output was much
reduced when running both heating and HW.

Newer cylinders tend to have larger coils, or even fast heat recovery
where the coil splits into several thinner coils for larger surface
area and higher heat input into the cylinder. So although this will
short out the central heating, it will absorb most of the boiler's
output resulting in a very fast reheat of the tank. The tank stat will
switch back to heating-only before the house starts cooling down.
Some systems nowadays don't even have the mid-position for both heating
and HW - when there's demand from the cylinder, they divert 100% of the
boiler output to a fast recovery coil, which will reheat the cylinder
in minutes, before switching bask to central heating - the radiators
won't even have time to get cold.

OTOH, if the cylinder only has a few turns of the coil, without the
balancing gate valve, it won't work optimally. I don't know if these
cylinders are still available - maybe someone else in the group might?

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]