GeoffB wrote:
replying to John Rumm, GeoffB wrote:
I've recently replaced a faulty Honeywell V4043H motor. While investigating
the wiring etc. to see how the whole S Plan system worked, I found myself
asking the same question: why not connect the grey wire (normally permanently
live) to the brown wire, on each of the two (HW and CH) zone valves? If you
do that, the only connection between the two valves is the orange wires that
feed live to the pump & boiler. For each of the zone valves, the brown and
grey would go live together when switched on by the timer. But there must be
a reason why the grey wire is permanently live , otherwise Honeywell would
just have bridged the brown & grey in the valve.

No, it wouldn't work. Consider the situation where both CH and HW call
for heat. Both valves motor on, opening the valves and closing both
microswitches to energise the orange wires, so the boiler runs.
Then, the HW stops calling for heat. The thermostat no longer supplies
power to the brown (and grey) wires. However, because the microswitch
is closed, power flows from the orange wire (energised from the other
valve's orange) to the grey. The grey is connected to brown, so brown
is still energised, and the valve stays open. The system continues to
heat the hot water when not required. If the CH stops calling for heat
earlier, the same applies in reverse.

Mike