boiler discharges to header tank

[stathe]

My heating system boils up into the header tank from the boiler when the
domestic hot water , has heated up.This is since I have changed the program
for the water to be heated before the Central Heating now the weather is better ( Previously switched
on together) I think maybe the Drayton hot waterTank thermostat HT S 3 may be at
fault, or what else may cause this? The boiler is a Warmflow oil Cabinpak and the pipes run into my loft to distribute around the bungalowThere are two electric valves in the system also in the loft.Not a sealed system. Any ideas appreciated Regards E

 

[jobloggs]

Is the hot water coming out of the vent pipe (pumping over is common problem) or up the feed pipe into the F & E tank (unlikely)? It shouldn't be doing either.

What is happening is that when the cylinder stat is satisfied it is closing the zone valve on the HW circuit. Water has nowhere to go as CH circiut is also closed off by its zone valve. With both valves closed the pump and boiler should not be getting power. Fault is likely in the microswitch in the HW zone valve.

In any case best practice is to have the feed and vent pipes on the low pressure side of the pump (between boiler & pump) and to have an automatic bypass valve fitted between pipework on the pressured side of the pump and the return pipework to the boiler.