loud thump when central heating goes off

[carole1]

I have a pressurised central heating system and every time it turns off because it has reached temperature there is a loud thump noise. I have traced this noise to the zone valve actuator in the airing cupboard. It first makes a whirring noise as the switch travels from A to B, then the thump. The heating still works, but I am afraid it may explode at any minute. Any ideas?

 

[outinthestix]

Sounds like the actuator is worn . Replacement may be possible without changing the whole valve dependant on make and model.

 

[dal5band]

What does the valve do ?

 

[chrishutt]

If it's one of a pair of zone valves, you may need a by-pass to allow the flow generated by the pump overrun to be accommodated. Do you have two zone valves and is there a existing by-pass?

 

[corgiman]

We had this problem with some tower three ports

it turned out the ball that acts as a stopper was loose on the arm and it was that that was causing the thumping

 

[ChrisR]

Also on quite new drayton 3 ports, with the rubber wedge shaped pipe-blocker. Clipping the pipe reduces the noise a lot.

 

[corgiman]

top tip thanks chris

 

[carole1]

The actuator is a Drayton and it says on it Actuator Code ZA6 if that helps.
As for what it does, I was hoping someone would tell me! The switch on the side slides from A to B whenever the heating comes on and from B to A (with a thump) when it goes off, so I guess it diverts the hot water away from the radiators?
I can't see another box like it in the airing cupboard so I don't think it is part of a pair.
Excuse my ignorance, but what is a "tower 3 port". Is it the same sort of thing? If so, did you replace the whole valve or was it fixable?

Thanks for all the replies!