Struggling to see the advantage of S Plan, although I have seen a number
of
posters supporting this.

Advantages of S-Plan:
1. More reliable
2. Simpler to understand
3. Simpler to install and debug
4. Works with simple SPST thermostats
5. Provides volt free contacts (required by some boilers)
6. Is extensible to cover more than just 2 zones (i.e. rather than just
heating and hot water, you can have independent timing and temperature
control of bedrooms, living rooms, conservatories etc.)

Advantages of Y-Plan:
1. Slightly cheaper (needs one expensive valve, rather than too cheaper
ones).
2. Slightly less space required.

Currently the program is set for 1 hour water heating only before main
heating phase only at start of day.
1 hour water heating mid-day.
In the evening we have 6 hours heating with 1 hour water heating included.

Irrelevent. Both Y-Plan and S-Plan provide completely independent control of
the zones, although Y-Plan is obviously limited to 2 zones.

Christian.