Faulty zone value actuator?

25 Feb 2008
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United Kingdom

I have a zone value actuator that will not stay in the open position. With the heating "on" the actuator tries to open the valve, but when open it immediately snaps back to the closed position. I've taken the actuator off the valve and it continues to do this.

I looked at the bottom of the actuator with the heating on, and I can see the motor turning the "key", but I cannot see any reason why it should suddenly snap back shut.

My temporary solution has been to remove the actuator and manually turn the valve to the open position. I suspect this is not very efficient, but at least it heats the house.

What I am trying to get a feel for is whether the behaviour of the actuator is symptomatic of a problem elsewhere in the system, or whether it is symptomatic of a broken actuator. If the latter, it seems a relatively simple job to fix.

There is a second value in the system - I assume this is the HW feed, and this seems to work fine - stays open when I put just hot water on.

Brief details of the system are as follows:

Boiler is: Ideal Classic FF370.
Closed system (no header tank - use mains to keep pressure "topped-up").
Controls are: Invensys Lifestyle
Actuator is: Drayton ZA5.

Any advice would be really welcome - even if it is as simple as "get the plumber in".
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THe motor in the actuator is controlled by the Room thermostat. When the stat clicks on to heat up the house 230v is applied to the brown wire of the valve. This activates the motor to open the valve; it also closes a switch within the valve which connects the grey wire to the orange. This sends 230v to the boiler which ignites.

If the valve is opening then shutting immediately, either the motor is slipping or the 230v to the brown wire is being disconnected - a thermostat fault. If you have a meter, check the voltage at the brown wire of the valve.

If the boiler stays alight, even though the valve snaps shut, the actuator is slipping but if the boiler does not stay alight it is probably the thermostat.

Take the actuator off and check that there is no wear where the key fits.

Thanks for the advice.

I've checked the brown wire and it stays live when the system is switched on, and after the valve snaps shut. Also, I've checked that the brown wire is not live when I leave the CH system switched on, but turn down the thermostat to a very low temperature.

Just to be clear, the boiler is not firing up when it's just the CH on; the pump/fan starts to whirr, the valve starts to open, but when it's reached fully open it immediately snaps shut, and the pump/fan stops. The "key" under the motor looks absolutely fine, and the valve itself is easy to turn on and off.

In the absence of any other suggestions I was thinking of swappng the actuators around i.e. put the HW one on the CH and vice versa and seeing whether the same problem occurs. A bit of a pain/fiddly doing the wiring, but should be conclusive re. the actuator.

Many thanks.
It is the teeth worn or motor gone, new valve head required don`t waste your time swapping them
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Thanks for the advice.

I couldn't resist having a look. I undid the motor and low and behold half the teeth were missing on the small cog on the rotor. I suspect I'm going to fork out for a whole new part rather than just the knackered cog. At least I've saved on the plumber, so I'm going to bed a happy man.

Many thanks.
Sounds like I have the same problem. :(

:?: Did you manage to find a source of replacement cogs, or did you have to buy a new actuator?

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