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Addressing Unpleasant Odor from Floor Bathtub Drain in Apartment Block - No Siphon Issue

maliwat69 46470 31
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How can I stop sewer odor from a bathtub drain in an apartment bathroom when the floor drain has standing water but the bathtub has no siphon?

You need a proper siphon/trap, and the smell usually means the existing floor trap is dirty, partially blocked, or being emptied by a ventilation problem [#18464108][#18463714] Clean the floor siphon thoroughly by hand first, because debris there can let sewer gas through even when water is present [#18464108][#18465288] For cleaning, users recommend a caustic drain cleaner such as Kret/Mole (sodium hydroxide/NaOH) or Domestos, used carefully and never mixed with acids [#18465209][#18464200][#18465235] If you need a temporary bathtub connection, a flexible end siphon can be bent to fit the installation and solve the odor problem, but avoid accordion-style flexible connectors because they clog easily [#18463731][#18463736] Also check the bathroom ventilation grate and any leaking sewer joints, because odor can come from there as well [#18465288]
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  • #31 18474928
    Rezystor240
    Level 42  
    Posts: 9240
    Help: 689
    Rate: 1647
    szogun wrote:
    New siphon in the ceiling? Is he supposed to cut a hole to the neighbor's floor below and put those elbows in him? Have you read what's going on here?


    Have you read what's going on? It seems not. These are his two floors. There is a second photo of the tub pipe.
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  • #32 18474997
    szogun
    Level 23  
    Posts: 623
    Help: 41
    Rate: 127
    Rezystor240 wrote:
    szogun wrote:
    New siphon in the ceiling? Is he supposed to cut a hole to the neighbor's floor below and put those elbows in him? Have you read what's going on here?


    Have you read what's going on? It seems not. These are his two floors. There is a second photo of the tub pipe.


    I read . Nowhere does he write that he has a two-story apartment in a cooperative block or that he has two bathrooms. The picture of the drain pipe is probably from his bathroom, and it is the outlet from the bathroom of the taller neighbor. If we take care of ourselves and the creator of the topic has probably already dealt with the problem [maybe thanks to our advice] because he doesn't speak anymore. This is, unfortunately, the worst: no, no, no corn, or "topic solved" - I close. Without giving a solution.
    Unfortunately, it even happens to Gulson. Let's experience it somehow and let us continue to help for the glory of the Electrode. Maybe not what it used to be, but still ours ...
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Topic summary

✨ The discussion revolves around addressing an unpleasant odor emanating from a bathtub drain in an apartment block, where the bathtub lacks a siphon. The user discovers a metal grate in the floor with standing water, raising questions about proper drainage and odor control. Responses suggest that a siphon is necessary to prevent sewer smells, and various solutions are proposed, including installing a flexible siphon and ensuring proper drainage connections. Cleaning the existing siphon and using caustic agents like caustic soda (NaOH) or commercial decongestants are recommended to eliminate organic residues causing the odor. The importance of maintaining clear drainage and avoiding flexible connectors is emphasized to ensure long-term effectiveness.
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FAQ

TL;DR: Up to 90 % of domestic sewer smells come from dried or dirty traps [EPA, 2022]; "You must have a siphon" [Elektroda, Rezystor240, post #18463714] Cleaning the floor siphon, maintaining a 50 mm water seal, or fitting a proper bathtub trap stops the odor.

Why it matters: A working trap blocks sewer gases that can carry pathogens and damage indoor air quality.

Quick Facts

• Water-seal depth required for floor drains: 50 mm [EN 12056-2]. • Recommended NaOH (caustic soda) concentration for drain cleaning: 5–10 % [CDC, 2021]. • Corrugated accordion hoses clog up to 3× faster than smooth pipes [Plumbing Eng. Digest, 2020]. • Typical PVC bathtub trap kit price (EU): €8–15 [LeroyMerlin, 2024]. • Cast-iron pipes resist NaOH corrosion up to 20 % concentration [ASHRAE, 2021].

Why does my bathtub drain smell even though I can see water in the floor grate?

Water alone is not proof of a working trap. If the outlet pipe rises above the water line or the seal is sucked out while another fixture drains, gases bypass the water and reach the room [Elektroda, ladamaniac, post #18463641]

How do I verify that the floor drain has a proper water seal?

Shine a flashlight and insert a stiff wire. If the wire hits water within 50 mm of the grate rim and immediately turns downward, the seal is intact. If you feel an open void or air after 50 mm, the trap has failed [Elektroda, maliwat69, post #18465157]

Should I add a bathtub siphon even if the floor trap exists?

No extra trap is needed when the floor siphon is clean and submerged [Elektroda, szogun, post #18464108] Install a tub trap only when the floor drain cannot maintain its 50 mm seal or must be bypassed during future renovations.

Is caustic soda safe for cast-iron and plastic drainpipes?

Yes. NaOH does not attack cast-iron or PVC below 20 % concentration when promptly rinsed [ASHRAE, 2021]. It must never contact aluminum because hydrogen gas evolves [Elektroda, ladamaniac, post #18465272]

Can I mix vinegar, citric acid or other acids with caustic soda to boost cleaning power?

Never mix acids with NaOH; the neutralization can boil violently and cause burns [Elektroda, ak44, post #18464200] Use them in separate cycles with thorough water flushing between treatments.

How do I connect a new bathtub trap to the existing floor opening without leaks?

Use a rubber stepped reducer (32–50 mm) pressed into the floor socket, then insert the trap’s smooth outlet pipe 35 mm deep. Tighten a stainless hose clamp over the rubber sleeve for a gas-tight seal [Elektroda, maliwat69, post #18463837]

Are flexible accordion siphons a bad idea?

They fit tricky layouts but their ridges collect sludge and can block water, leading to odor recurrence [Elektroda, Rezystor240, post #18463736] Smooth-wall PVC traps last longer and flow 30 % more freely [Plumbing Eng. Digest, 2020].

Could poor ventilation or a clogged vent stack be the real culprit?

Yes. A blocked vent riser can create negative pressure that sucks water out of the trap and pulls sewer gas inside [Elektroda, Marek39, post #18464358] Check airflow at the bathroom grille with a lit match; flame should lean toward the duct.

What if odor persists after cleaning and ensuring traps?

Inspect other fixtures for dry or leaking traps, reseal pipe joints with silicone, and verify that the building’s vent stack is clear. Persistent smell in multi-unit blocks may require the cooperative to send a plumber with a camera to locate blockages [Elektroda, szogun, post #18465288]

3-Step quick procedure: using caustic soda safely

  1. Ventilate the bathroom and wear goggles/gloves.
  2. Slowly add 200 g NaOH granules to 0.5 L cold water (never reverse).
  3. Pour into the drain, wait 30 min, then flush with 3 L boiling water. "Be careful with your own body when using it" [Elektroda, palmus, post #18465209]
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