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.

THe normal arrangement is then for a signal wire from the motorised
valve to go to the boiler switched live. This wire is live if either
thermostat demands and the motorised valve is open accordingly, and
fires up the boiler. Once this live goes away because the
thermostats are satisfied, the boiler is shut off apart from being
able to run the pump to disperse heat.

You could incorporate a flow switch, but normally the other controls
do the job.





--

..andy

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