The last tee

[Mike Jackson]

To get reverse circulation when heating is off the water would have to flow down the heating return the wrong way. It would then have to circulate through 1 or more rads the "wrong" way before getting into the flow pipe. Assuming that the zone valve is closed correctly the only way to complete the circuit would be to flow the correct way through 1 or more rads and back down the same return to the boiler. For this to happen would involve water flowing through the same section of pipework (ie between last heating return tee and cyl return tee) in opposite directions at the same time.

You don't need complete reverse circulation for warming to take effect. If hot water is pushed up the pipe far enough to flow below a radiator the radiator will heat up by gravity. This will obviously only effect the radiators on the same level or higher than the return connection but I've seen it plenty of times. It tends to be worse when an old boiler is changed for a modern one as the newer heat exchangers are much more restrictive leading to an increase in back pressure.

You may not have seen it happen but I've seen it a lot and altering the return configuration always solves the problem. You've obviously not come across as many poorly installed systems as I have.