One For The Thermal Store Boys!

[SimonG]

Was first fixing a new en-suite for a customer out in the sticks. They've recently had a woodburning stove tied in with the oil boiler through a thermal store. The question they asked me whilst I was there was why is the return pipe on the stove hotter than the flow. I said I doubted that that would be the case so they lit the stove. After an hour I checked the pipes and both where very similiar temp wise. About half an hour later they called me through and the return was considerably hotter than the flow. They had all the technical bumph so I spent some time looking at the piping and controls and couldn't see anything wrong. The system had normally open and closed valves to protect the cylinder and for the heat leak.

Any ideas?