"Andy Hall" wrote in message
...
On Wed, 24 Nov 2004 21:59:30 +0000, Hugo Nebula abuse@localhost
wrote:

Can someone please explain the meaning and function of different bits
of a central heating system:
-Is there a difference between a by-pass and an automatic by-pass
valve?
-Do all new boilers turn off if the return is the same as the water
going out (which I presume is what would happen with a by-pass)?
-If not, is there such a thing as a flow switch to turn the system off
in conjunction with the by-pass?

A bypass is used with some types of boiler to cover the case when both
(or the active)heating and HW thermostats are satisfied and the boiler
is in full output.

Typically there is an over-run arrangement on the boiler internal
thermostat so that it continues to power the pump when this happens.
The idea is that water continues to be circulated around the system
until the water cools enough to stop it actually boiling in the heat
exchanger because the heat was not dispersed.

If the motorised valve(s) are closed at this point, there isn't a path
for the water, so hence a bypass is used. This goes after the pump
but before the motorised valve and in the simple case is a lockshield
valve slightly open.

An automatic bypass is pressure sensitive and so doesn't open until
the routes around the circuits are closed. This means that there is
no bypass flow except when needed.

Also all bypasses on new or upgraded systems must ow be automatic. Not sure
why as there is no real loss of heat due to it but that's the rulezzzzz