Central Heating circulating through header tank

[dinoprint]

When central heating is on water vents through overflow pipe into header tank which does not fill up. Venting stops when pump is turned off. Pump is Grundfoss 15/60 on lowest speed on 22 mm return to boiler. All radiators are hot and there is no air in the system

[stoneyboy]

dinoprint,
Increase the height of the top of the overflow pipe.
end

[rosebery]

Your system is "pumping over". Do not leave it as all you are doing is circulating oxygenated water which will cause corrosion in the system. Pumping over is a symptom of a problem not the problem itself which could be:

1) An incorrectly placed pump,
2) Pump speed too high
3) Insufficient static head
4) Incorrectly configured system
5) System blockage

This topic has been covered many times before on this forum. Use the search functon and you will find the answers. Here are some threads for example:

http://www.diydoctor.org.uk/forums/viewtopic.php?t=7733

http://www.diydoctor.org.uk/forums/viewtopic.php?t=7738

http://www.diydoctor.org.uk/forums/viewtopic.php?t=7702

Cheers