Worcester 24SBi

5 Mar 2006
Reaction score
United Kingdom
system is 5yrs old. Set up as twin zone (s)plan with dual drayton Zone valve actuators (ZA5\\\'s). Problem is CH comes on when HW on but not vice versa.Checked programmer is working ok and actuators click and whir like they are working. HW anf CH should run independantly.Could this be a fault with the actuator on the CH or am I barking up the wrong tree?
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How do I prove the CH zone valve actuator is faulty (i.e allowing a flow when it should be shut) ,other than replacing it? . If I do replace the valve do I need to swap the whole in line compression fitting as well as the motorised valve or does it come as one unit? Any advice is much appreciated.
The zone valve actuators are Drayton ZA5 type.
you've actually answered your own question (as a Google search for Invensys ZA5 would have revealed!).
The ZA5 is actually the actuator only. The valve body it sits on has a different part number.
To prove that it's an actuator problem, turn off the system and remove the CH actuator head (2 screws). Turn it on in CH mode and see if you can see the drive socket on the bottom of the actuator moving round. If you CAN, chances are the switch inside the actuator is broken (= new actuator). If it doesn't move, the Synchron motor inside the actuator will need to be replaced. But at £30-odd for a complete actutor you may feel it's not worth the hassle. Then again: just rerplacing the motor involves much less wiring than doing the whole actuator. Your call.

Before getting a new head, check that the valve spindle will turn reasonably easily with your fingers. If not, complete new valve. With the head removed and the valve in the open position, the system will run as before, with the rads heating up along with the hot water. If you turn the cylinder stat UP a little and the boiler temperature down a little, you will be able to keep the rads going for quite a while before the cylinder stat opens and shuts everything down.
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NO screws, just press the button and lift the head off.
I've removed the CH actuator then switched the HW on and observed the motorised drive socket inside rotating which looks OK. The valve itself does seem slightly stiff, I can't turn it with my fingers but can easily turn it with pliers. I've turned it tight shut with pliers, left the actuator off and put the HW on only and the valve still seems to be letting hot water flow into the CH. Can I assume from this the internal valve assembly is worn and letting a flow through even when the vlave is shut?
The system has 14 radiators that are getting hot when the valve is shut! is it possible for a worn valve to allow sufficient flow through to heat this ammount of radiators?
With the actuator in place, there's a spring holding the valve shut. the motor opens it against the spring. Chances are, pump pressure has pushed it open.

My advice was to live with it jammed open until you could fix the actuator. If that's not OK - then try to work out a different solution.
I see what you're saying but now I'm thinking this has got to be a fault with the valve itself rather than the actuator as I've physically shut the valve tight using pliers with the actuator removed and hot water is flowing through the valve. If the pump was forcing the valve open presumably I should see the lug on the valve spindle move to the open position and it doesn't.

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