Most motorised valves have an inbuilt switch
which
is used to control the pump, if these are connected together then the pump
will
only run when a valve is open.
The proper CH ones do, but I was
assuming Phil was going to use small solenoid type valves inline with the
radiator (there are some that various people including Ian Bird have found
from
RS). These I believe are simply a solenoid operating a valve - either
open
or closed.
The proper CH ones have a small
motor
and microswitches inside - when power is applied, the motor operates and
moves a
lever which operates the valve. When the valve is opened, the lever
operates a microswitch at its limit which signals the pump/boiler to
fire.
The lever is spring loaded, so that when power is removed from the motor
(which
runs continuously whilst the valve is open, typical power consumption
6-10W) the
lever returns and the microswitch is opened, switching the boiler/pump
off. There are 2 port and 3 port mid-position flavours - the above
certainly applies to the 3 port ones as I've disassembled one
;)
James H
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