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I have been having problems with air in the system ever since I converted from gravity HW to a pumped system with 2 valves (1 CH & 1HW).
To overcome the air problem I have done some re -piping and put in a ventair device.
The Feed from the top of boiler rises vertical through the ventair device , the pump is on a tapping on the side of the ventair and the cold feed is between the Ventair and the pump (The vent, cold feed and pump are all within about 6 inches of pipe) The pumped feed then splits to HW and CH circuits. The returns are through the two valves to the bottom of the boiler.
I have just filled and bled the system. Running the pump (without heat)what is happening is that the water is pumped through the heting system and the bolier then up the vent pipe to the tank, down the expansion pipe and back into the pump. (The expansion tank does not overflow)
Altering the speed of the pump has no effect on the problem.
Where do I go from here?????

Pumping Over - an incorrect system layout can lead to a state of hydraulic unbalance, allowing water to be pumped over via the open vent pipe into the header tank.

The OV pipe should run directly from the boiler rising continuously to discharge over the feed and expansion tank. Try extending vent pipe, to about 18" above tank.

Re-site the pump, without adversely affecting the heating efficiency of the system.

Expansion pipe tee'd in before the pump, not after ?


htg

Thanks for the reply.
The layout I had (Boiler, vent pipe , f&E pipe, pump in that order) seemed to be quoted by most people as the optimum layout, however the amount of water that was being pumped into the tank was full bore 22mm flow - I even thought there was a blockage in the pipe between the vent and F&E pipe. I doubt that extending the vent pipe 18 foot over the header would have stopped it.
Since the last post I have swopped the F&E pipe to just before the vent pipe (the order is now boiler, F&E, Vent, Pump ) and this seems to have improved the situation, there is still some water coming over from time to time - I believe this is happrening when it is clearing air out of the system gennerally when running the HW only. I guess it would be worthwhile increasing the height of the vent pipe a bit to reduce the problem.