CH pressure fluctuating rapidly with vibrating noise

25 May 2011
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United Kingdom
Keston C40 (don't mock: running perfectly 10 years on), S-plan. It started kettling around last November, or so I thought (no experience). A dose of Sentinel X200 quietened it a bit, or so I thought. Gave it a second dose. No real change. Eventually realised it was not the boiler at all, not least as the noise only happened when the CH was on and not when the DHW was being given priority. I then thought the noise was coming from the CH motorised valve in the next-door cupboard (Honeywell V4043H). Changed the actuator head: no improvement. Changed the valve body: no improvement. Then I realised the nearby pressure relief valve was fluctuating rapidly with the noise. Video here (it starts with the PRV then pans left to the valvegear and left again to the boiler).

None of the pipes is rubbing against anything, nor does the noise stop when I hold the pipes with a gloved hand.

The system loses very little pressure and I'm sure the pressure vessel is OK: it's 1.0 bar cold rising to not much more than 1.5 and only needs a little topping up twice a year, and I repressurised the vessel not too long ago.

Could this be the PRV itself and if so, why would the noise and pressure fluctuation only happen when it's the CH that's being fed?

Thanks in advance for any wisdom.
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