I have an oil fired, vented central heating system. The upside down J pipe which vents into the expansion tank continually runs with hot water when the central heating is on. A new pump and three way valve has been fitted without any effect. The whole system has been treated twice to remove sediment and flushed through following draining before being refilled to no effect. If all the radiators are turned off (they are all mostly fitted with thermostatic valves) then the overflow stops - which rather defeats the object! Help would be very much appreciated as I don't know where to go next.
This is a topic that gets posted to this forum more or less every week. Your problem is caused by poor system design. Specifically the cold feed and expansion pipes are placed too far apart and in effect become part of the system with a pressure differential between the two. In your case this differential is sufficient to lift the water in the expansion by the foot or so it needs to cause an overflow. Conversely in other systems the reverse problem may be seen where water is sucked out of the expansion allowing air to be constantly drawn into the system.
Examine where these pipes connect to the circuit, and move the feed to a closer (within 1 foot) to remove the imbalance.
You will see an improvement if the pump speed is lowered, but of course this reduces the overall flow, and may reduce the temperature of some radiators.