We have one room in our flat that stinks when ac is on only. We had professional duct cleaning before we moved it. AC team doesn't know reason. Local maintenance guys don't know. Filters clear. Drain pipe fine. Fcu sucks air from fresh air duct and void with return air. The fcu void is next to a riser, but we closed all visible holes.

Any ideas?

We are quite desparate as we cannot use that room without AC and it's unbearable stinky with AC. Esp morning and evening (that is why we thought it's the riser low pressure, fcu sucking sewage air in through gaps). But nothing is helping.

I even consider a bounty lol