This makes the circulating coil for the hot water storage tank your
unrestricted loop. It is the perfect unrestricted loop between flow
and return with a perfect medium for dissipating heat from the boiler.

This wouldn't comply on an unvented cylinder, though. These have to be zoned
off and capable of shutting down their heating circuit independently. As the
bypass is particularly likely to be used when there is an overheat
situation, using a circuit that can be independently shut down by the
cylinder's safety systems would not be a good idea.

Christian.