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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?

wiring diagram of our system

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Here in the USA. we now only use 120volt control systems since it has been found that 230 volt control systems can cause problems similar to what you are describing. Many years ago we utilized 240 volt controls since it was easier to use 2 legs of 240 volt 3 phase systems to get control power. After it was discovered that these systems caused numerous drastic system failures , they were outlawed in favor of 120 volt control systems. I witnessed many failures; (overheatings, explosions, meltdowns, etc.). The wiring print you show is pretty simple so I have to ask "is this the whole wiring system showing all the controls or just a small portion of the whole system". One part of the problem of 240 volt controls is that if a short to ground develops after a gas valve opens then the valve may be held open if only one leg of the power is switched off. The valve could be held open indefinitely. There are other possible problems with your system, this is just 1 of them. I would also recommend that all the wiring be checked by a master electrician with extensive control training/service to make sure that everything is wired correctly, especially if the controls have been replaced or serviced recently. Trying to service your system from Pittsburgh, Pa. USA. is impossible but I can only render some possibilities.

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  • You're barking up the wrong tree without a paddle there - UK mains is single-phase 240v (as per most of Europe) with very strict earthing, fusing, and RCBO/MCB on all domestic circuits. It's a basic standard household system.
    – John U
    Oct 16, 2017 at 8:45
  • I may be barking up the wrong tree but my answer was correct at least here in the USA.. Maybe one of the master electricians will provide some insight.
    – d.george
    Oct 17, 2017 at 11:54
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Well it looks like the 3-Way valve (Drayton MA-1) was the culprit - or rather the microswitch inside it feeding a voltage back into the system and failing to drive the valve fully to position.

Having replaced the head, all seems well.

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