I have an oil fired Camray Quartet central heating system, 10 yrs old. The boiler keeps firing then switching off after 5 mins, no heat going to radiators. I read that I may have an air lock and unscrewed the front of the Grundfos Selecteric 15-50 pump and some air with drips of water came out. This seemed to solve it and it worked for a day but now keeps doing it. I have bled the rads but by the gurgling sounds from the pipes near the boiler sounds like air. Also a slight grinding noise from near the pump but when front is off the pump you can see it is working. I am not an expert so any advice appreciated please. The motor and pump on the boiler both replaced 3 years ago.