potterton combi 100. Hot water, but cold rads

[markt]

Hi Guys

My potterton combi 100 is heating water fine, but only fires up for one minute on CH then switches off. Having switched it self off there is the sound of water being pumped round the system. All rads are cold, there is no air in rads and all leds are ok.
Any ideas guys?

 

[doitall]

If you can hear water being pumped around the system then you have air, whats the boiler pressure set at

 

[croydoncorgi]

If the pressure's OK, second guess: automatic air vent (top, lefthand side of boiler, poking out of casing) is blocked / stuck and not venting air. System has collected air from somewhere (removing rad for decorating, some other drain-off of CH water, ...?)and it's found its way into the boiler plumbing and has nowhere to go.

Finding HW OK but CH stopped by an airlock is unusual but if you can definitely hear 'air noise' in CH mode, then that's probably it.

Make sure the dustcap on the AAV is loose. Otherwise, depressurise system (through drain-off NOT thru pressure relief valve!, remove AAV and clean it up. Do NOT use the isolation valves on the Flow and Return under the boiler - they are very likely to leak afterwards and be EXTREMELY careful to counter-hold the AA valve body when you undo the nut. If you turn the vertical tube you'll probably loosen the completely inaccessible nut at the bottom. Tightening it again will be a complete PITA!
Refit AAV, repressurise system and check that air (but not water!) comes out of the AAV.