I am currently specifying a system that will provide back-up CH/DHW to a proposed solar thermal installation. The current boiler setup is W-Plan (CH-ON powers the valve giving CH only, 3-wire Cylinder Stat "call" cuts CH-ON power to diverter and diverter goes to DHW only).
The Solar DHW will be provided by a Plate Heat Exchanger from the Thermal Store, but with an S-Plan DHW zone valve to call the DHW circuit from the boiler as back-up, activated by a two-wire flow switch. While a 3-wire Cylinder Stat could cut the live feed to the CH Zone valve while simultaneously powering the DHW valve (so diverting boiler all heat to DHW) it would seem that I will need to specify a relay to achieve the same with the two-wire flow switch in an S-Plan circuit, (or can it be achieved by using the CH OFF terminal of the controller)?
The Solar DHW will be provided by a Plate Heat Exchanger from the Thermal Store, but with an S-Plan DHW zone valve to call the DHW circuit from the boiler as back-up, activated by a two-wire flow switch. While a 3-wire Cylinder Stat could cut the live feed to the CH Zone valve while simultaneously powering the DHW valve (so diverting boiler all heat to DHW) it would seem that I will need to specify a relay to achieve the same with the two-wire flow switch in an S-Plan circuit, (or can it be achieved by using the CH OFF terminal of the controller)?