I'm hoping someone here can help with a mysterious fault with our central heating system, which sometimes gets stuck and runs all day even when the timer is off.
It's a standard (for the UK) low-pressure system with cold water tank in the loft & hot water tank in a cupboard, 3-way valve, gas boiler, radiators, and Honeywell ST9400C timer / controller.
The system is wired as per the diagram below.
The problem is that, sometimes, the system will not shut off even though both timer relays (HW + CH) are OFF - the 3-way valve stays in the 'M' position (HW + CH) and the boiler & pump keep running.
I have changed the timer so pretty sure the relays aren't sticking there, I have checked the room thermostat isn't sticking, which leaves me looking at the hot water cylinder stat as the likely culprit - would others agree with this or have I missed something?
The reason I'm doubting myself a bit is the valve being stuck in "M" (HW+CH) rather than just "H" (HW only), although I'm not sure if this is thrown off slightly by the controller relays being OFF?