How can I stop water pumping out of the airvent pipe on my central heating system?

[Quiet Man]

How can I stop water pumping out of the airvent pipe on my central heating system? I have just fitted two more 28mm pipes (flow & return) to my boiler so I can have hot water only in the summer. It works ok but for water flowing into the expansion tank in the roofspace form the airvent (I think it's called an airvent). All the other radiators are on the other two pipes from the boiler and when isolated I have this problem. The pump is at it's lowest setting, we live in a bungalow so the expansion tank is not too high up. Can anyone suggest a remedy? Thanks

[buildersmate]

The problem is most likely to be occurring because the air separator (the correct term for the device you describe) is positioned in the system close downstream from the pump. It should placed upstream from the pump, between the boiler and pump such that water passing through the boiler gets heated, passes through the device, then onto the pump and hence around the system. What is happening is that the positive pressure of the pump is overcoming the 'head' of water created by the expansion tank. Bear in mind that this positive 'head' of water pressure is only the difference between the level of the water in the expansion tank and the top of the loop that the expansion pipe rises to (maybe 60cm?) above the expansion tank. Since one bar of pressure is equivalent to 10m of water 'head', you can see that the water will very quickly get pumped around the expansion system instead of being pushed around the heating circuit. The elegant solution is to move the relative positions of the 2 devices but this may not be easy. An alternative is to raise the height of the loop above the expansion tank to create a better head. You could do thid using the modern plastic pipe, cutting the existing expansion pipe, using a straight coupling to move into plastic pipe and taking a huge loop right up to the apex of the roof, returning to a few cm above the height of the tank. Even this may not solve the problem.

[Hard@it]

I think that your problem is that your cold feed is connected to the suction side of the pump, and should be changed to the discharge side.
What is happening is that when your system is running normally your pump is drawing down your cold feed supply from the bottom of your tank in the attic, and is then pushing it up your open vent and discharging into your tank.
Cheers Hard@it

[gasman]

I think the problem is that when all the rads are isolated the water has nowhere else to go but up the expansion pipe and into the feed headertank. A differential bypass installed should do the trick available at all good plumbers merchants