Automatic Bypass Valve

justworriedabit

Thank you. Can I ask, is it then nothing to do with the 'bypass'?  In every house we lived in you always get one rad without the tvr.
So, when we removed the TVR head and when we switch on the CH, effectively that rad will act as a bypass ie not get switched off if room gets too hot.

Jeepers_Creepers

As far as I understand it, it depends on what's causing the rumbling and when it happens.

For instance, a 'by-pass' is required to allow CH water to keep on circulating when all the TRVs have closed off, but the boiler is still being asked to run (by the room stat or programmer) - there is otherwise nowhere for this circulated water to go unless one rad is kept permanently open (eg has no TRV) or an auto-by-pass has been fitted (usually at the end of the rad run). Most modern boilers have by-passes built-in.

The other scenario is when the boiler is asked to shut off when demand has been met, but it still has a fair amount of residual heat within it, in that large - usually sheet or cast - steel exchanger (I'm talking oil boilers). So, if that heat isn't shifted, I'd expect a few rumbles as the water inside boilers and is ejected out the expansion pipe (If it's a vented system). This is usually taken care off with a pump-overrun facility, which means the pump keeps on running even after the boiler has shut off, and circulates away this excess heat through the rads/by-pass until it cools enough. Again, most boiler with built-in pumps will have this feature. The OP's oil boiler certainly does.

So, if the rumbles occur when the CH demand has gorn off, then I'd suspect a possible failure in the pump-overrun feature, possibly coupled with a lack of by-pass (eg an 'open' rad). If the rumbles occur at seemingly more random times - when the system is nicely hot and the rooms nicely warm - then it might be solely down to a lack of by-pass.

The above is 85% speculation... With the help of this: https://library.plumbase.co.uk/flipbooks/RE/worhea2632rs_24191_t/mobile/index.html#p=4