Honeywell 3 port valve electrical fault

Bit of a head scratcher yesterday, fault on Y plan central heating system.

Problem is, when the CH is turned on (with the room stat turned up) the boiler fires as normal but when you turn the stat down the boiler continues to run. When testing i found there was 109V between neutral and the orange wire of the valve and the same between neutral and the white wire of the valve.

This happened when everything else was disconnected from the system and the only way to clear the voltage was to switch off the FCU and turn it on again.

I've changed the valve and also the valve head again but the problem persists. Another odd thing to note is that when a test meter is put on between the neutral and orange to test the voltage the resistance of the meter is enough to stop the boiler from firing.

Any thoughts gentlemen. i can't believe that i've had 2 faulty valves from different sources.

Boiler - Ideal Icos, Stats + Programmer - Danfoss, Valve - Honeywell

The valves always 'leak' a voltage.

The ideal is one of the boilers that especially don't like this, there is a resistor you can fit to eliminate this problem! Contact ideal.

as londonboy said when the room stat/cyl stat is satisfied their willbe 230v on the grey and approx 100v on the orange.

Thanks for the replies, after a bit of digging on the net i found that this 109V is normal as has been said. Contacted Ideal and the grunt on the technical helpline was no help at all. Any idea what size resistor it would need? I assume it wants to go between the SL and N at the boiler.

As far as I can make out the leakage voltage appears through a 270k quarter watt resistor in the valve so placing another of these across orange and neutral should at least halve the "leak".

I have just diagnosed and repaired a Y plan problem where the CH would not switch off except for a power reset. The 3 port valve was replaced but did not solve the problem.

The ~115VAC that the valve has on its CFH output (orange wire) when in the CH off and DHC off position is too high for some boilers and this is enough for a valid CFH and the boiler will run forever.

Solution: Connect a 0.47uF (or larger) AC rated capacitor across the Neutral (terminal 2 blue / black wires) and the call for heat to the boiler (terminal 10 or wherever the orange 3 port valve wire is connected) This will drag the voltage down to zero and prevent the boiler from running when not commanded to.

I hope this helps

Nick