Occassonal noise in CH

[bobapcoed]

I have a conventional (not combi or condensing) central heating system with 11 radiators, all with thermostatic valves.
Following noise in the system and the age (approx 18 yrs) I drained the system, added a desludging agent, refilled it and left it for 4 days. It worked perfectly during this time. I then drained it, added inhibitor re-filled it, bled it via the radiators numerous times.
All radiators heat up as does the hot water. However, after about 2 hrs 10 minutes (yes that regular) a vibration sets up. Opening a thermostatic valve usually stops it. I've noticed that in the region of the pump/3-way vave/header tank feed the sound of running water is heard and about 3 seconds later the vibration sets up in the nearest radiator and water can be heard running in the radiator.
One thought was that a slug of air was going past the header tank feed and cold water was getting into the hot system, hitting the thermostatic valve causing it to try and open (because it's cold) but can't because its partly shut in and opening it makes it happy. However this could be a load of rubbish and I can hear all you plumbers laughing. Any help would be greatly appreciated.Cheers.

 

[oilman]

You say all radiators have thermostatic valves. Do you have a bypass valve fitted? if not at least one radiator should have a conventional valve and be left open. If you don't do this, when all the valves shut after about 2 hours (or so), the pump can't circulate water, so it makes a noise.

 

[bobapcoed]

Thanks oillman, that makes a lot of sense. Where would the by-pass be fitted?Can I get away with leaving the thermostatic valve fully open which I understood would always let water pass?Or would it be best to fit a conventional valve?

 

[oilman]

Bypass goes between pump output and boiler return. If you leave the thermostatic valve on, I'd take the head off so do-gooders intent on saving the planet don't do it by destroying your CH system. You could close down the lockshield valve to 1/2 turn or so.