DHW vented tank thermosyphon

[RobLe]

We have a vented DHW system, pic of similar attached.  The tank is nicely insulated, it's even boxed in with poly beads down the back!  It occurs to me though that the vent itself is a horrible heat leak mechanism, as it leaves the hottest part of the tank, then travels upwards into the cooler loft.  Sure that pipe is insulated, but not as well as the tank, and that whole 2m length of that 22mm copper pipe thermosyphons away so is permanently at the maximum DHW temperature.  Is there any reason why there can't be a thermosyphon break (london loop, short down then up segment)?  I've looked in the regs, and they require that the vent be 19mm ID minimum, but there's nothing I can see about it having an upwards only slope.  Every picture I have even seen on the internet shows the vent heading straight up, or horiz then up (hardly any better).  It just seems a huge waste of heat to install that way.  Thoughts ?

 

[Radian]

I can't link you to anything but I do recall the vent pipe being required to rise continually. I had an argument with a plumber about this once. However, the reasoning must be about the ability of the system to relieve pressure in the event of overheating and boiling the water. Would a London loop develop enough back pressure to rupture a cylinder? Give us a break! Obviously the safest pipework would be the simplest with an uninhibited rise to the top but for goodness sake! Of course if there was a 'local minimum' and no flow then sediment might accumulate and bung up the loop. That might just be enough reason for the regulations to prohibit it but I for one wouldn't let it stop me, especially with 22mm pipework.

[Nickfromwales]

Leave it alone, and mummify it with HT neoprene insulation.

Same result, less iffy behaviour. 
Go to pub.