Hi my elderly mum has Vaillant ecotec plus 418 on an open vent system falshing F22. the boiler is now outside warranty . the manual says low pressure . but there are no filling loops underneath the boiler. Her neighbour says thats coz its NOT a pressurised system and to look at the pump [ which has never been changed] . Ive changed the pump, bled the radiators and its still has a F22 fault . can anyone suggest what else i could try before parting with ££££. i would be very grateful for any advice.