I have a 12 year old house with bathrooms at either end, each with their own downpipe. The downpipes are both fitted with Durgo air inlet valves in the loft rather than being open to the air above the roof.
Sometimes when the toilet is flushed, the downward suction seems to drain the water out of the bath trap causing sewer smells to occur in the bathroom. If some water is poured into the bath the problem smells stop. We leave plugs in the bath and on the overflow vent to help prevent this happening.
Is there a special trap I can fit to the bath to prevent this suction effect from happening?