I have a Sundial Y-plan configuration. The 3 port midposition valve is a Honeywell V4073A and have a Worcester Greenstar 18ri.

After replacing an older 3 port valve which had become stuck with the Honeywell V4073A , I found that the boiler was no longer switching off when in heating only mode and the room thermostat was satisfied.

When the thermostat was demanding heat (pin 5/white live), I was seeing 230V on the orange wire from the 3-port valve (when disconnected from the wiring centre). When temperature was reached at the thermostat (pin 5/white 0V), I found that the orange wire from the 3 port valve was reading 48 volts! As the orange wire is connected to pin 8 and therefore the boiler live (L/R on pump overrun configurations), the boiler continues to fire.

Measured during Hot water was off, pin 7/grey live.

If I switch HEATING off, then WATER on, then water off on the programmer, then the boiler does switch off. The orange wire then reads 0V.

2 Answers 2


According to V4073A FAQ:

When this Heating demand is satisfied, during a Heating ON period, 50V to 150V (voltage can vary according to the supply) will remain on the orange wire...

So the voltage you've found is there by design. Unfortunately, it's enough voltage to trigger the boiler's relay when it's supposed to be off.

(As the programmer is in HEATING only mode, the grey wire will be live which keep the central heating/A port open.)

You have two options:

  1. Use a relay - Wire in a strict 230V relay (Such as http://uk.rs-online.com/web/p/products/4334076/) between the boiler switched live and pin 8 on the wiring centre

  2. Use the official "RC" device - The RC device (Worcester part 87161092750) pulls the voltage down below the tolerance of the boiler's relay. Only when a full 230V is present will the boiler fire. The RC device is wired between Neutral and the L/R (switched live) terminal. There's no polarity, so use either wire for each. In a wiring centre, it would be installed into terminals 2 + 8

I chose the RC device so that it was supportable and maintainable by others in the future.


The official RC (resistor–capacitor) device is actually a Roxburgh XEB 1201 noise suppressor. I guess the resistance of this (120Ω) causes the voltage on pin 8 to now read 21V. It comes in a Farnell bag, so pretty confident it's resold without modification. Buying direct would have saved me £20!


I'm just putting this answer here to backup Alastair's answer to his own question. My situation was almost identical; a newish V4073A 3 way valve (fitted 2017) and an old Worcester Bosch Greenstar 24RI boiler installed in 2005. Boiler/pump would not stop running, I measured about 70V on my orange wire.

I like to cross reference 'stuff I read on the internet' (some bits are wrong!) and whilst I couldn't validate the Worcester part 87161092750 (people sell it but I couldn't find a spec) I did find a Honeywell article that says to use an XEB1201 if you have this problem https://ensupport.getconnected.honeywellhome.com/s/article/Does-Honeywell-provide-a-snubber-capacitor-for-the-V4073-3-way-valve?language=en_US

I picked up a pack of 5 XEB1201s from RS Components for about the same price as 1 of the Worcester parts - worked perfectly.

Thanks very much to Alastair!

FYI, I found various comments that said newer boilers in the same line don't have this problem but don't know for sure how true this is.

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