A rental apartment is rarely occupied 100% of the season. Between bookings it stands empty for days, and out of season for months. During that time something happens that the guest smells the moment the door opens: sewer odour from the floor drains. It is not a question of cleanliness — the apartment can be spotless — it is the physics of an empty space.
Why an empty apartment starts to smell of sewage
Every floor drain, shower and sink has a trap — a bend in the pipe holding a small amount of water. That water is a barrier: it keeps sewer gases on the other side. As long as the drain is used regularly, the water is topped up and the barrier stays intact.
When the apartment stands empty, nobody runs the water. The water in the trap evaporates — in warmer, poorly ventilated bathrooms in anything from a few days to two weeks. The trap runs dry, the barrier disappears, and sewer gases (hydrogen sulphide with its rotten-egg smell, methane, ammonia) travel upwards and fill the room. Rugs, towels and bed linen absorb it.
The longer the apartment stands empty, the stronger the smell — and it is at its worst for the first guest after a break, who is then the one who writes the review.
A negative review costs you the whole season
In short-term rentals a single sentence — "clean, but it smells of the sewer" — pulls the score down and pushes the property further down the results on Booking and Airbnb. A drop in ranking means fewer views and fewer bookings, and it lasts for months. The cost of one bad review far exceeds the cost of the solution.
Why the usual tricks do not hold
The most common advice treats the symptom for a few days:
- Running water into the drain before the guest arrives — it helps until the water evaporates again. With frequent changeovers and several units it becomes a permanent job with no guarantee.
- Sprays and air fresheners — they mask, they do not remove. The guest still smells the sewer underneath the perfume.
- Taping over the grate — temporary and unsightly; a fix the guest notices immediately.
- Chemicals and drain unblockers — they do nothing for a dried-out trap; the problem is not a blockage, it is the missing water barrier.
The permanent solution: a passive silicone seal
Instead of relying on water that keeps evaporating, a passive silicone seal with a one-way valve (Green Drain™) is fitted into the drain. The principle is simple: the seal lets water through downwards while it is flowing, and the moment the water stops it closes and blocks the gases, even when the trap is completely dry. It works mechanically, it does not depend on water, and it has no wearing parts.
For an apartment that means, in practice:
- Fitted into the existing drain with no tools, in about 30 seconds per drain
- No flushing the drains before every guest — even after a long break the apartment does not smell
- 11 sizes (1.25″–6″) for every floor drain, shower and bathroom
- No chemicals, no maintenance, medical-grade silicone that does not break down for years
For owners with several units it is a one-off installation that removes a recurring problem and protects the property's score for the whole season.