Pumping over, only on CH

[commercialman]

. The water level in the F/E tank isn't exactly high as it is, only a couple of inches over the fill pipe, and the Vent pipework already goes all the way to the rafters, so no additional height available there either

I was suggesting to increase the diameter of 22mm OSV above the water line ie: a short piece of 42mm , this will normally cure any surge issues due to the increased volume of pipework , or maybe fit an air separator?

 

[Gasguru]

Have you bled all the rads?

Then cut out the tee where the vent is connected and check for blockage...a magnet won't always find a restriction.

 

[mysteryman]

Your mock-up is exactly how it should be done, make sure the vent and cold feed are not more than 150mm apart.