Potterton locking out (hot water circuit only)

[PPK]

Morning All,

I've a Potterton Suprima 60 that's locking out when heating the hot water cylinder only. It tends to cycle on again (once cooled?) with some minor clucks and hissing inside boiler, before shutting down again.

Pump's less than a year old and if I turn the thermostat on the HW cylinder down it stops the boiler.

If I run the CH at the same time all is good. Recently had a new gas valve installed by a gas engineer.

Have put some Fernox F3 in yesterday afternoon, anything else I can check?

Thanks

 

[PPK]

No ideas?

 

[CBW]

Do you live in a hard water area?

 

[PPK]

Yes

 

[CBW]

Could be scaled up cylinder?

 

[PPK]

Thanks, any way to be sure?

Going to swap out the thermostat today on the cylinder as I think it’s suspect...