An obscure question for you which flummoxed the bloke who came to service my boiler today.
Why is the "traditional" Honeywell S-Plan designed in such a way that a single failure can effectively bypass both cylinder stats and the programmer?
I've now had two motorised valves (Danfoss, pah!) fail in quick succession - one which failed with the valve open, and another where the microswitch kept sticking.
The only clue to this is the boiler running 24 hours a day. And a huge gas bill and toasty house when you've left it alone for two weeks.
It's probably to save cable runs. I've only taken a quick glance at the S plan wiring layout but it uses the switch outputs on the back of the valve to trigger the boiler/pump. If you didn't use those then I think you'd need additional relays for each valve with cabling fom the demand pin for the central heating (pin 5) and the tank stat. (pin 8).
Rich.
Not a Honeywell valve as they are sprung return normally so unless something wedges them open they won't fail in the open position.
Some Danfoss valves are shoe type and are prone to letting by and can fail open.
Had a few of these in the past.
http://www.electriciansblog.co.uk/2012/02/s-plan-twin-zone-central-heating-system-electrical-control-connections-and-wiring-diagram/s-plan-twin-zone-wiring-diagram/
The grey of the valve switch is permanently live, (1) so if it fails open it will run the pump and boiler with no interruptions.
If the grey is took off the permanent live and connected with the brown of the valve, ( 5 or 8 ) it will only start the boiler if the timeclock or respective stat is in demand, once the valve has travelled to its open position.
