Soil pipe flows into shared manhole serving multiple properties, Despite rodding of public sewer by utility company this manhole frequently backs up above crown of incoming pipe. When this happens trapped air in soil pipe between manhole and durgo causes problems, slow discharge of WCs and shower waste,
Height of building, dormer windows, vaulted ceiling ,slate roof, together with permits and access for scaffold make extending soil pipe to vent 1 metre above top floor windows a major operation,
With durgo removed WCs flush perfect and power shower fully functional with no backup of water in shower tray,
Replacement durgo experienced identical problems.
Looking for advice,compliant with building control, especially an air admittance valve that could cope with the above conditions