I've been trying to track down why one of my radiators in a newly acquired house has no flow. After finding that one side of the radiator is blocked off, further investigation located a motorised valve labeled central heating VB valve at a junction box.
The junction box seems to connect live/neutral and a switch pair (two cables), 4 wires to the valve. The valve is across the flow to a single radiator (probably). With the valve open, the radiator works. There is no obvious thermostat in the affected room (just a wireless remote and TRVs). Further investigation uncovered a flow control valve (just a normal radiator valve I think) between this motorised valve and the radiator. It's all in trunking/behind units so hard to see well.
I traced the cables back to this comedy junction box (included for your entertainment, not because I think it will help in the diagnosis). One set of cables is un-connected here, explaining the lack of action, there are 2 more mechanised valves here for the DHW circuit.
This isn't something I've come across before in the UK, and I'm trying to work out what I expect to find on the other end of this wiring. Most likely the valve motor has failed - and maybe leaving it set on manual will work for the winter, but what behaviour should I be expecting? This is a sealed system.
I'm assuming that VB stands for vacuum breaker - I might be wrong here since I only found the term in a list of abbreviations.
If it's not a vacuum breaker, maybe it is an anti-siphon device. This radiator is the closest (downstairs) to the DHW cylinder, on a vertical spur.