Extractor hood with exhaust, sometimes bad odor from the bathroom in the attic

Hello everyone, We have a new building since 2019. On the ground floor, we have a kitchen with an extractor hood, exhaust air, meaning the whole thing goes out through a wall box. Not always, but occasionally, we have noticed that with (longer) use of the extractor hood with closed windows, a smell comes through the house from the bathroom in the attic (in between there is our first floor which also has a bathroom). It smells like a drain. The bathroom on the first floor is currently not in use; occasionally we just run water briefly, but actually the bathroom has not been used since we moved in. I am not sure where the smell is coming from in the bathroom (shower, washbasin, or toilet). What we do notice is that with the extractor hood on the ground floor and windows closed, even when there is no smell, wind can be felt from the toilet flush tank or the flush button (but I cannot say if the smell also comes from there when it occurs). Do you have any experience with this or is there a logical explanation? Is there anything to criticize regarding the execution by the general contractor or otherwise? What is the reason for the draft or the "smell" and what could or should we do? Thanks in advance

The logical explanation is that new buildings are airtight. That means an extractor hood must draw fresh air into the building from somewhere; otherwise, a negative pressure will develop inside the building, or the hood cannot function properly (noisy, increased power consumption, reduced throughput). Existing ventilation systems may go out of sync and suffer. If there is a stove that is not designed to be room-air independent, there is even a risk to life.

The siphons in the mentioned bathroom will be dry, so the necessary "fresh air" will be drawn into the house from the drainage pipe. Even if they are not dry, it is possible that the negative pressure in the building is so strong that air is pulled through the siphon from the drainage pipe.

Open a window when you use the hood to provide the necessary supply air. Otherwise, convert to recirculation mode.

Regardless, you will need to flush all drains in the unused bathroom with water every few weeks to prevent the siphons from drying out.

Hello,

Is there anything to criticize about the execution by the GU or anything else?

On the contrary, that shows that you have a wonderfully airtight house.... Seriously, the exhaust hoods pull such a high volume flow that you should also open a window directly in the kitchen. Otherwise, drafts will come in at all corners and back into the house, and even the drains in the bathroom can become forced ventilation.

Best regards.

Andreas

I am not sure where the stench in the bathroom is coming from

Then google Kittybob (the construction infotainment podcast) Episode 07 "the incense stick affair" - that will help you ;-)

Thank you all for your replies! : I listened to it, that could indeed be the explanation for the wind from the push button, but if it is not actually a defect and it just depends on ventilation, I think we can live with that. Thanks again to everyone

I listened to it once, [...], but if the whole thing is not a defect after all

If you listened to it until the end: defect yes, but also fixable.