l have a conventional vented fully pumped system from an oil-fired boiler on the ground floor. HW tank is in the 1st floor airing cupboard along with the pump and 3-port valve. 22mm vent pipe rises through the airing cupboard up to the F&E tank in the loft where it rises some 600mm before dropping back to the tank. HW feed tees off the rising vent pipe in the airing cupboard through an air separator then to the pump inlet with the cold feed in between. All was working fine until a recent pump failure. This was duly replaced but then found there was a degree of pumping over even on the pumps lowest speed. It is the correct pump for the installation. Rectified this by re-routing the cold feed into the vent pipe approx. 250mm above the air separator. Cleaned out the air separator and pipework to pump while at it. BUT, now have occasional flow stoppage around the CH circuit? Programmer and 3-port valve/controller all checked OK. Local heating engineer scratching his head as is one of his colleagues who he brought along for a 2nd opinion. Would be very grateful for any suggestions as to where o go from here..