Wire terminals for CH and HW off?

[richig]

Question out of curiosity really! Have recently done a lot of improvements to some rental properties and one job I did was to replace all the CH controls. Noticed when replacing the programmers that all had terminals L N CH off HW off CH on and HW on. In each case there was only need to use the L N CH on and HW on. As this was the case in every house I wondered if someone would explain when there would be call to use these and what sort of wiring would be present when they would be needed (all the ones I replaced were Y or S plan)

Many thanks.

 

[D_Hailsham]

The CH Off terminal is rarely used, but a correctly wired Y Plan requires a connection from HW Off to the valve grey wire. This is in addition to the wire from cylinder stat Sat terminal to valve grey.

 

[simond]

Yes, the Y plan ones were wired incorrectly if there was no cable on the HW off terminal.