It has a zone valve controlled by a different circuit to the boiler /
time switches etc.
There is no call back to the boiler.

So the time switch comes on for the hot water and pumps to that zone.
The zone valve cuts in and out (on a different circuit) and the bypass
valve deals with the fact that there is no call back from the valve to
the boiler.

This is one of the reasons why unvented cylinders must be installed by a
trained competent person.

The zone valve you are looking at is the safety override valve that was
supplied with the cylinder. You must not interfere with it in any way. It
should be lashed up to its own dedicated overheat stat. It is the equivalent
of an immersion heater's overheat stat.

What you need to do is add an additional functional zone valve to your
S-Plan-Plus system. This should be run from your programmer, through a
separate thermostat and with the boiler interlock connections used. The
unvented cylinder is, thus, separated from the boiler by TWO independent
zone valves.

DO NOT disable or interfere with the existing zone valve. It is an important
safety device and MUST NOT be used for functional zoning.

Christian.