Baxi 35/80 IE Installation & Servicing Instructions Manual page 39

14. The domestic hot water relay PCB is fed from the domestic hot water zone valve. Central
heating demand is controlled by a live feed to terminal 2 on the boiler terminal strip from the
central heating zone valve.
15. When there is a demand for domestic hot water the boiler will not supply water at a
temperature greater than 65°C irrespective of the setting of any central heating control(s) or if
the cylinder thermostat is set higher.
16. Additional 2 port zone valves may be incorporated in the system as required and wired in
the same manner as the basic system.
Scheme 3
Scheme 3
17. Scheme 3 shows the use of a 3 port two position zone valve and is based on a 'W' plan
system. The boiler will provide hot water to the central heating system or domestic hot water
store.
18. When there is a demand for domestic hot water and the cylinder thermostat is not
satisfied the zone valve will be energised to open port 'A'.
19. The zone valve is installed so that port 'A' is connected to the domestic hot water circuit
and port 'B' to central heating. This is the reverse of a conventional 'W' plan. It ensures that
the valve is energised only when there is a hot water demand, not for long periods i.e. central
heating demand.
20. The boiler is controlled by a live feed from either room or cylinder thermostat depending
upon demand.
21. The domestic hot water relay PCB is fed from the cylinder thermostat. Central heating
demand is controlled by a live feed to terminal 2 on the boiler terminal strip from the room
thermostat.
22. Whilst there is a domestic hot water demand the central heating circuit will not be served.
Once the cylinder thermostat is satisfied the zone valve allows circulation of water through the
heating circuit even if the timer still calls for domestic hot water.