I've got a Vaillant system boiler and water cylinder. Not long ago, the heating would not turn off, the British Gas man came out and said the paddle in the valve was sticking (????) and replaced it. Since then the water cylinder makes a loud clunking banging sound, but only in the afternoon and only once , usually not long after the hearing and hot water comes on (up to an hour after). It doesn't happen in the morning though. It sounds like it's going to come through the ceiling. Is it an airlock, ? If so, why only does this happen in the afternoon and how easy is it to fix ?
