Checking the pump torque is down to experience but if it's low then that could be the cause.
When the heating and hot water demands are satisfied the zone valves will close...the pump can continue to circulate water via the bypass whilst it cools the heat exchanger (but only if the bypass is correctly set).
Have you tried removing the red pump valve handles and using a spanner on the spindle...
When the heating and hot water demands are satisfied the zone valves will close...the pump can continue to circulate water via the bypass whilst it cools the heat exchanger (but only if the bypass is correctly set).
Have you tried removing the red pump valve handles and using a spanner on the spindle...