During the absence (6months) of my apartment. the bowl of the W.C. empties. I am on the 1st floor 2 levels above what to do?
There is a combination of a double phenomenon: evaporation and especially aspiration due to a deficit of primary ventilation of the big downspout, which must lead to the open air on the roof. Clearly, as this ventilation is insufficient or non-existent, there is a phenomenon of aspiration when your neighbors use their flush... Ask the trustee of the building to have this point checked and possibly to comply with the installation.
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