In article ,
"Set Square" writes:

What you need is, in effect, an S-Plan-Plus [see
http://content.honeywell.com/uk/homes/systems.htm] with 3 zones, 2 for
heating and one for hot water.

Use a standard programmer and cyl stat to control the zone valve for the hot
water - with 2 or 3 on/off periods per 24 hours, as desired - but always the
same temperature unless you manually twiddle the cyl stat (which is probably
acceptable). On the same standard programmer, set CH to permanently on and
use its CH ON contact to feed the programmable room stats. The actual on/off
periods (and different temperatures if desired) are then controlled by the
room stats - which in turn control their respective zone valves.

The volt-free contacts on all 3 zone valves are connected in parallel - with
the output side controlling the boiler and pump. That way, the boiler and
pump will run whenever one or more zones requires heat and will shut down
when they are all satisfied - achieving the desired interlock.

You will need to allow for pump overrun with most boilers.
The Honeywell schematic does not.

--
Andrew Gabriel