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I have a gas boiler (Ferroli OPTIMAX HE PLUS 25S) which has separate hot water and central heating control, connected to a timer (programmer - Sauter R27-HW).

The problem is, the water is always heating, even when the programmer shows it should be off, either because it is not within the correct time, or because I have selected that it should be "off" completely.

I have also tried turning the cylinder thermostat down to minimum (20 C) and it remains on even after the water heats well above that.

The only way to turn it off is to completely switch off the power from the mains.

By following wires from the hot water cylinder thermostat I managed to locate the zone valve, which is a DETACH V221D.

There is a switch on the side which goes between "AUTO" and "MANUAL OVERRIDE". This switch appears to be completely loose - i.e. I can slide it back and forth between the two options without feeling any resistance whatsover. OTOH, the central heating zone valve (same model) does offer resistance when trying to slide to "MANUAL OVERRIDE" and automatically moves back to "AUTO" when I let go. This makes me think the hot water one is broken. Could this be the cause?

I suspected the programmer was faulty, but even when I disconnected the "hot water" wire, the hot water still came on.

I also can't tell from looking at the boiler's wiring diagram how it can tell the difference between whether you want the hot water or the central heating turned on - the wires from both controls lead to the same connection, which makes me think that the zone valve alone is responsible for determining whether the hot water specifically is on, and therefore the damage to it is the likely cause, and I should be able to fix it just by replacing this part. Am I right in this assumption?

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    If the switch seems to be mechanically disconnected from its internal workings, then it's entirely reasonable to think that it's stuck in the "Manual Override" setting, causing your boiler to heat at all times. It also strikes me as odd that the functioning one is spring loaded to immediately return to "Auto" when releasing pressure on the switch. I'd think you'd want to be able to put it in "Override" and have it stay there until you set it back to "Auto" - seems impractical to have to have someone hold it in "MO" mode while you take an emergency shower...
    – FreeMan
    Sep 12, 2022 at 12:53

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Sounds plausible.

For a zone-valve system, the heat source (Boiler) would quite normally get a "call for heat" signal when any zone is on, which starts a circulating pump and the the zone valves open to deliver that heat where called for. Thus the wires from both controls lead to the same connection is normal and expected for that part.

Depending on layout of pipes you might get thermo-syphoning delivering heat to the water heater circuit without a pump running - normally that pump would only be running if at least one of the zones is calling for heat. So if it's running all the time there might be something more than a bad zone valve going on.

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  • Can you post some pictures and links to technical information on the valves and a schematic of your system if you have one.
    – Gil
    Oct 12, 2022 at 17:14

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