In the first half of 2025 Croatia recorded 22.6 million overnight stays by foreign tourists, according to the Croatian Bureau of Statistics (DZS). Winter is a different story — tens of thousands of rooms along the coast close in October and reopen in April. Six months of empty rooms = dried-out traps in every unit = a sewer smell the moment a guest first walks into the room in May.
What happens in a room over six months
The average Adriatic hotel room has two to four floor drains (shower tray, bathroom floor, sometimes the terrace). In six months of complete inactivity, every trap dries out. The sewer pipe under the hotel does not stop — the rest of the installation keeps working, water keeps flowing, and gases circulate through the collector.
Those gases look for a way up — and they find it through the empty trap of an empty room. Hydrogen sulphide (the rotten-egg smell) and ammonia build up over days. Carpets, curtains and bed linen absorb all of it.
When the guest arrives in May, the room does not smell 'a bit' — it smells badly. And housekeeping with air freshener only makes matters worse.
The physics of a dry trap
The trap is the U-bend immediately below every floor drain, shower tray and washbasin. Its only job is to hold a column of water — typically 50 to 100 ml — that physically separates the room from the sewer network. While the water is there, sewer gases and biofilm aerosols cannot get into the room.
Water in a trap evaporates continuously. The rate depends on four things:
- Room temperature — rooms kept tempered out of season dry the trap faster than cold ones
- Relative humidity — seaside rooms dry more slowly, rooms with air conditioning and underfloor heating much faster
- Air movement — negative pressure in the ventilation system pulls water out of the trap
- The temperature of the sewer pipe below — a warmer collector means faster evaporation
In an empty room with ventilation running, a trap can dry out completely in two to three weeks. The problem therefore does not start in March; it starts in late October, and gases enter without interruption for the rest of the winter. This is not a housekeeping failure, it is physics. More detail in what a trap is and why it dries out.
What the reviews say
Reading Booking.com reviews of Adriatic hotels from the first week of the season turns up the same phrases again and again:
- "The room is clean, but there is a terrible sewer smell"
- "We changed rooms — and the second one smelled exactly the same"
- "Reception says it is normal when the hotel opens — that is not acceptable"
- "Great shower — but the morning smell in the bathroom was unbearable"
A single negative review in the first month of the season affects the hotel's ranking for the whole season. Multiply that by dozens of rooms and you get a measurable drop in bookings.
What one odour costs in operations
The cost is not only reputational. Three items show up in day-to-day operations:
- A drop in score. Research from the Cornell School of Hotel Administration shows that a 0.1-point fall in the aggregate score goes with a 4 to 6 per cent fall in conversion. A review containing the words "sewer smell" usually pushes a property below the psychological 8.0 or 9.0 break.
- Disruption to the shift. The front desk takes the complaint, housekeeping investigates, maintenance is dispatched, sometimes the room is changed or the night is written off. The usual cycle is two to four staff hours per report.
- Repeats on the same unit. Once a trap has dried out, it dries out faster the next time — biofilm stays in the pipe, changes the evaporation pattern and smells in its own right.
Across a group with several properties on the coast, that is a predictable, recurring cost every season.
The pre-season protocol that does not work
The standard response: before opening, the maintenance team goes through every room, runs water into every drain, waits and checks. For a hotel with 200 rooms that can take two to three days with a team of four.
The catch: even once the traps are refilled, the water starts evaporating again. In rooms that are not sold in the first week of the season, the problem comes straight back. Pre-season preparation turns into a permanent job.
The other options and where they stop
Before settling on a mechanical barrier, most hotels work through three options:
- Chemical drain treatments. Enzyme or chlorine-based products poured into the trap. They hold for 24 to 72 hours, then evaporation carries on. They mask the odour; they do not change the cause.
- Glycerine in the trap. A layer of glycerine slows evaporation for a few weeks. That is far too short for a six-month closure, and in greasy drains it behaves unpredictably.
- Automatic trap primers. Devices that top the trap up with water periodically. They work, but they cost €80 to €200 per unit plus plumbing work, they consume water all year round and they have a valve that fails. In the Adriatic hotel stock they are practically non-existent.
A passive seal — one installation, a five-year life
The Green Drain™ silicone seal works mechanically; it does not depend on water sitting in the trap. In its closed position the seal blocks sewer gases even when the trap is completely dry.
In practical terms, for an Adriatic hotel:
- 200 rooms × an average of 3 drains = 600 installations
- 30 seconds per unit = five hours of work for one person
- No pre-season flushing ritual
- No water consumption (relevant for LEED and BREEAM certification)
- No automatic trap primers to break down
The cost of installation pays for itself with the first negative review you avoid.
When to fit them
The common mistake is fitting the seals in April, during the run-up to opening. By then gases have been entering the rooms for six months and the soft furnishings have absorbed them. Seals are fitted when the property closes in October, on the same walk-through that shuts off the water and empties the minibars. The work is faster because the rooms are empty, and the room smells in April exactly as it did in October.
Full overview: Green Drain™ — how a passive one-way drain seal works — sizes, materials, certification and how it is used in hotel rooms, kitchens, schools and hospitals.