From your sketches the system has a constant hot water circulation through
the pump any time the boiler fires up. This means that there is a
thermostat on the hot water cylinder. Correct ?

The motorised valve is only activated when you call for heat to the
radiators.

The wiring sketch shows a "?" on a pair of wires and is most likely the
connection to the hot water cylinder thermostat.

The frost guard 'stat will control the system by over riding the hot water
circulation and only allow the heating to be called for by the radiators.

Everything on the wiring will live mains voltage and should run at no more
than 5 amps at total load. It may even be 3 amps depending on the pump size
and switching loads of the 'stat and things.

The motorised valve will only open when the programmer calls for heating to
the radiator circuit. Unless you've seen things in the wrong places and the
valve is actually a three port, two way valve, which is connected to the
flow pipe after the pump and will swing an arm inside it between the hot
water and heating circuits, or can be set by the system to stop in a mid
point between the two circuits to allow heated water to flow through both
circuits.

A three port valve has three pipes going to in to it. One is the flow from
the boiler, one is the flow to the hot water circuit and one is flow to the
radiator circuit.

From your sketch the system relize on the pump to circulate the heated water
around the two circuits and is fed from a header tank. It looks to be a
very basic, and very easily maintained, system.


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