expansion tank overflow

I have an oil-fired vented central heating system. Whenever the heating is on there is a constant overflow of hot water from the inverted J pipe over the expansion tank. A new pump and motorised valve have been fitted. The whole system has twice been treated to remove sludge, drained, flushed through and refilled to no avail. The overflow does stop if all the radiators, which are mostly fitted with thermostatic valves, are turned off - which rather defeats the object. In the past someone has fitted a gate valve in the pipe leading to the overflow - this is normally open. I have run out of ideas and would be very grateful for help.

Pump over
The valve on the vent pipe should not be there and must not be turned off(not that it is causing the problem).
It could be down to a few things including poor pipe configuration (how cold feed and vent are joined up) or hard oxide in and around cold feed / vent part of piping which flush wont shift. Try turning pump speed down until sorted properly.

Hi

Thanks for the response. The problem started about eighteen months ago. Before that it was OK, so presumably pipe configuration can be ruled out as this has not been changed. I have tried running pump at both low and high speed. This makes no difference.How would I deal with oxide in cold feed/vent area of piping?
Thanks again for your reply.

Vernon

vedavis said:

someone has fitted a gate valve in the pipe leading to the overflow -.

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As suggested remove the tap: this could be very dangerous as it is now. If tap was closed and water in system boiled then your central heating system will likely explode like a a bomb and take your house out (water expands 1600 times when turns to steam).