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Discuss Overflow running when heating or hot water on in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

The thermal expansion coefficient of water changes with temperature. Water in heating systems typically expands around 1.5% when heated to say 70°c, not 4 %. Having said that something here doesn't quite add up.
 
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It can't just keep overflowing each time the pump comes on or due to the expanded volume without external make up after the first overflow.
Then either the initial level is far too high, or the ball valve isn't shutting off fully.
The system volume expands by about 4 per cent when heated, so a system containing 100 litres would expand by 4 litres in volume.
 
The system volume expands at different percentages at different temperatures. For it to expand roughly 4% you would be heating the system water to close to 90-95°c. Most systems will expand around 1.5-2% when heated between 60-70°c and even less in low temperature systems which are becoming ever more frequent.
Looking at the OP initial diagram again it is possible the resistance of the HW and CH circuits are greater than through the boiler and combined cold feed and vent, which will cause the water to take the path of least resistance and back up said pipe. Of course an excessively high cold fill line would aid in the problem. If it was a faulty ballvalve I'd expect it to overflow regardless of whether or not CH or HW were on.
 

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