Mr Fizzion wrote:
Can you have such a thing or is this one of Doctor Evil's fantasies?

Assuming youre referring to a secondary circulation loop on the hot
water supply system, then;

The combi fires on a draw off from the hot water outlets. The only way
I could imagine installing a circulation loop would be to install a
heat exchanger (tube-in-shell, preferably) on the secondary return and
pipe the primary side of the HX as a zone on heating. You'd need the
heating to operate 365 days. You could use an electric heater.

There are also electric trace heating system for hws, which are
installed under thermal insulation to minimize the heat loss from the
pipes. These don't need a circulation loop.

If you're doing that you'd be better off installing an unvented hot
water storage cylinder ( which has got a heat exchanger & has got a
large volume of water, so it won't discharge scalding water if the
controls overshoot the set-point slightly). If you're doing that, you
might as well scrap the combi and put in a conventional hot water
storage system.

You should realise that Drivel is a moron and his rantings should be
treated accordingly.