What is the correct height of vent pipe?

[AlanD]

I have just moved my central heating header/expansion tank, and am now getting overpumping. I think I have put the open vent pipe too low, at its heighest it is about 300mm above the water level in expansion tank. What should be the correct height?

 

[blueshields]

12" is okay from the top of the water level to the highest point of the vent pipe.
The height of the F&E tank must obviously be the highest point in the system.

Presuming if this is happening from cold and effecting both DHW and CH modes, it sounds to me like you have one or more of the of the following problems ;

Pump speed now too high.
A tap/valve somewhere is closed.
Pump is now in the wrong place (before the vent pipe).
Pump is fitted backwards.
Blockage or system seriously sludged up, this can cause the water to escape via the easiest route.

There are a few other issues that could cause this (cylinder coils perforated and cylinder feed tank now above F/E tank etc..). However, the key to your problem is probably due to the "move".

Let us know if any of the above are possible and send us more info if not.