A siphon is a small but crucial component of every sink, bath, shower, and floor drain. Its primary role isn't just drainage – the water remaining in the siphon's bent section forms a water seal, preventing sewer gases from entering your home. If a siphon is blocked, leaking, or has dried out, the result is either slow drainage, an unpleasant odour, or both.
In this guide, we'll explore different siphon types, explain how a water seal works, and provide a detailed, step-by-step instruction for disassembly and cleaning. Additionally, we'll discuss what should never be poured down a drain and how to maintain traps and drains that are rarely used – for instance, in a summer cottage or a guest bathroom.
If you're looking for answers to why your entire flat or bathroom smells of sewage, we recommend reading our general overview of odour causes and our separate guide on morning bathroom odours – these explain the broader picture, while this article focuses specifically on siphon maintenance.
Many problems related to siphons might seem complex at first glance, but in reality, they are very simple mechanical components. Once you understand how they work, maintenance becomes logical and quick to learn. Below, we cover all common siphon types, their maintenance differences, and what to do if the problem persists even after thorough cleaning.
Short Answer: How to Maintain a Siphon?
Siphon maintenance consists of three parts: regular cleaning (removing grease and sediment build-up), checking seals (preventing leaks), and keeping the water seal full (preventing odours in rarely used drains). Most domestic siphons – bottle siphons and shallow siphons – can be maintained by the owner, either without tools or with hand-tightened nuts only.
If drainage starts to slow, the first and simplest step is to disassemble and empty the siphon, not to use chemical drain unblockers. If there's an odour but drainage is fine, the most likely cause is a dried-out water seal, which can be resolved simply by adding water.
Recommended frequency: kitchen siphon twice a year, bathroom sink siphon once a year, adding water to rarely used traps once a month.
- Slow drainage → empty and clean the siphon by hand
- Odour, but drainage is fine → check the water seal, add water
- Odour and moisture around the siphon → check seals and nuts
- Unused drain in a cottage or guest WC → cover the trap or add oil to the water
Types of Siphons and How They Differ
There are several types of siphons, each with specific maintenance requirements. Before cleaning, it's worth understanding which type you have, as the disassembly method varies slightly for each.
Bottle Siphon
The most common siphon type found under sinks. Its name comes from the bottle-shaped body, whose lower part unscrews like a cap. The water inside creates a water seal, and most of the sediment – hair, soap, toothpaste residue – accumulates in this lower section. A bottle siphon is the easiest to clean because it can be disassembled without moving the pipes.
P-Trap Siphon
An older solution resembling a curved pipe without a separate collection chamber. The water seal forms within the curve of the pipe itself. Cleaning is a bit more cumbersome as both connections must be detached, but the principle is the same – the internal sediment needs to be flushed out.
Shallow Siphon
Used where space is limited, such as under a low-cabinet sink or for a washing machine drain. Its flat, thin profile fits even when depth is scarce. The water seal is shallower than that of a bottle siphon, meaning it dries out faster and requires more frequent checks, especially if the drain is rarely used.
Floor Drain (Floor Trap)
A drain located in the bathroom or sauna floor, which also contains a water seal – either as a siphon with a small basket and lid, or as a screw-in siphon element. The water seal in a floor drain evaporates faster than usual because the water surface is wide and shallow, so it most frequently needs water added to it.
Dry Siphon or Vacuum Breaker
In newer installations, a mechanical valve is sometimes used instead of water. This valve closes under its own weight and only opens when water flows through. Such a solution never dries out and is well-suited for rarely used drains, but the valve can stick or wear out over time, requiring cleaning or replacement.
How to Choose the Right Siphon for Replacement
If a siphon needs complete replacement, not just cleaning, it's advisable to choose the same type that was previously installed, if space permits – this ensures compatibility with existing pipe connections and slope without extra work. For limited cabinet space, a shallow siphon is often the only practical choice, although it requires more frequent water checks.
For rarely used drains, such as in a summer cottage or guest apartment, consider a dry siphon (vacuum breaker), as it doesn't rely on water remaining in the water seal, thus preventing odour problems even after several months of disuse. Siphons with vacuum breakers are slightly more expensive than conventional ones, but in the long run, they reduce the maintenance burden, especially in rarely used households.
How a Water Seal Works and Why it Needs Maintenance
A water seal is the standing water in the U-shaped or bottle-shaped part of a siphon that prevents sewer gas from entering the room. It’s a simple but effective solution – as long as the water stays in place. If the water disappears, so does the barrier, and odours come directly from the drain into the room, even if the pipework itself is perfectly fine.
Water disappears from the water seal in two main ways. The first is evaporation – if the drain isn't used for a few weeks, the water evaporates into the air, especially during heating periods when the air is dry. The second is siphoning, which happens when a large volume of water moves through the pipework all at once, and the resulting negative pressure draws the water from a smaller siphon along with it.
Maintenance essentially means keeping the water seal full and the siphon clean. A clean siphon doesn't change the physics of the water seal but prevents sediment build-up, which in turn accelerates evaporation and provides an independent source of odour – even if there's enough water, a dirty siphon itself can smell.
- The height of a water seal is typically 50–60 mm – this should be sufficient for normal protection.
- The shallower water surface of a floor drain and shallow siphon evaporates faster than that of a bottle siphon.
- Regular use is the best maintenance – the water renews itself, and sediment doesn't accumulate.
How Often and When to Clean a Siphon
The frequency of cleaning depends on the type of drain and how much it's used. A kitchen sink siphon accumulates residue most quickly, as it receives grease, food particles, and detergent residue, which together form a sticky layer. A bathroom sink and shower siphon primarily collect hair, soap, and skin cells.
The clearest sign that a siphon needs cleaning is slowing drainage: water stands in the sink longer than before it drains away. Another sign is an odour that intensifies when hot water is run – this indicates an organic layer in the siphon that begins to ferment when heated.
It's best not to wait until the drain is completely blocked. This pre-slowing stage is the ideal time to disassemble the siphon, as the sediment is still soft and easily removed, without the need for harsher methods.
- Kitchen siphon: clean approximately twice a year.
- Bathroom sink siphon: once a year, more frequently in households with long hair.
- Traps and shower drains: check once a year, along with cleaning the hair trap.
- Rarely used siphon (cottage, guest WC): check water once a month.
Step-by-Step Siphon Cleaning
For most bottle and shallow siphons, no special tools are needed for cleaning – most nuts are designed for hand-tightening. Before starting, prepare a bucket or a wide container, as there will always be some water in the siphon, and rubber gloves if you want to avoid direct contact with the sediment.
- 1. Place a bucket or container directly under the siphon to collect water and sediment.
- 2. For a bottle siphon, unscrew the bottom collection part (usually by hand, counter-clockwise); for a P-trap, loosen both connecting nuts.
- 3. Remove accumulated sediment from the siphon – hair, grease clumps, debris – and wash the part clean with a brush and warm water.
- 4. At the same time, check the seals: if the rubber seal is hard, cracked, or flat, replace it with a new one before reassembly.
- 5. Also rinse the initial part of the drainpipe where the siphon attaches, and check that the overflow opening (if present) is not blocked.
- 6. Reassemble the siphon, ensuring the seal sits correctly, and hand-tighten the nut firmly – excessive force with a tool can crack the plastic part.
- 7. Run a few litres of water from the tap and check the assembled connections for leaks before considering the job complete.
If the Sediment is Hard and Won't Come Off with a Brush
Soak the siphon part in hot water with dish soap for some time – this will soften the grease and soap layer. Baking soda and vinegar can be used against light odours and thin films, but they are not a substitute for mechanical cleaning if the layer has already thickened.
Seals, Leaks, and Siphon Disconnection
Apart from odours, another type of problem with siphons is a leak at the bottom of the cabinet or behind the wall. The cause is usually a rubber seal that has hardened over time and lost its elasticity, or a nut that was left loose during assembly. Both are easy to check and inexpensive to fix.
Check the location of the leak with a dry paper towel: wrap it around the connection point briefly and see if moisture appears there within a few minutes of running water. This way, you can pinpoint whether the leak is from the nut, the seal, or elsewhere, such as a cracked plastic body.
If the leak is from a crack in the body itself, there's no point in replacing the seal – the entire siphon needs to be replaced. Plastic siphons are inexpensive and readily available, so replacement is usually a quicker and more sensible solution than repairing a broken part.
- Do not overtighten the nut – plastic cracks usually occur due to excessive force, not pressure from the pipework.
- Replace the seal if it is hard, flat, or shows cracks.
- Also check the connection of the sink's overflow hose – it's part of the same system as the siphon and often leaks for the same reason.
Common Mistakes in Siphon Maintenance and Disassembly
Many problems related to siphons arise not from a lack of maintenance, but from incorrect maintenance. The following mistakes are most frequently encountered by ToruFix OÜ in homes where people attempt to fix siphons themselves.
- Overtightening nuts with tools – plastic siphons are designed for hand-tightening; excessive force creates cracks that will later leak.
- Reinserting the seal in the wrong position or upside down – it may appear to seal initially but will leak under the first significant flow of water.
- Reassembling the siphon without testing – discovering a leak only the next day, when water has already soaked the cabinet base or floor.
- Pouring chemical drain cleaner into an already disassembled and empty siphon – the cleaner will simply sit there and do nothing, as the siphon is already open.
- Buying the wrong size spare part – siphon diameters and body shapes vary by manufacturer, so it's advisable to take the old part to the shop or write down its measurements.
What You Should Never Pour Down a Siphon or Drain
The condition of a siphon is most significantly affected by what goes down it daily. Some common habits accelerate sediment build-up and significantly shorten the siphon's lifespan.
- Cooking oil and frying fat – these solidify on the siphon walls when cool, forming a layer to which other waste later adheres.
- Coffee grounds and food waste – the most common cause of blockages in kitchen sink siphons.
- Construction dust, sand, and cement residue – these settle at the bottom of the siphon and won't flush out with normal water.
- Hair and hygiene products in the WC and shower drain – get tangled in seals and flaps.
- Strong corrosive drain cleaners as a regular solution – these damage seals and older plastic parts if used too frequently.
Rarely Used Drains: Cottages, Guest WCs, and Seasonal Properties
The most common odour problem directly related to siphon maintenance occurs in drains that are rarely used. In a cottage, garage, basement, or guest bathroom, the water seal can completely evaporate over several weeks, even if the siphon itself is perfectly fine and clean.
The solution is simple: once a month, you should run a few litres of water into every rarely used drain. If the place will be completely unused for an extended period, such as a cottage for winter, adding a tablespoon of cooking oil to the water helps – the oil film stays on top of the water and slows evaporation by weeks.
For cottages and seasonal properties, it's worth checking all siphons in the autumn before a long period of disuse: clean them, check seals, and add an oil film if necessary. In spring, at the beginning of the season, it's wise to inspect all drains in the same way before problems have a chance to arise.
- Pour water into rarely used drains at least once a month.
- For longer breaks, add an oil film to the water or cover the trap with a stopper or film.
- Before the end of the season, clean all siphons, not just those used daily.
When to Call a Professional Plumber
Most siphon-related maintenance tasks can be done yourself without special knowledge. However, there are situations where it's advisable to call a plumber: if the siphon is an older type with corroded metal parts, if the drain repeatedly blocks even after cleaning, or if the cause of the leak is in the pipework behind the siphon, not the siphon itself.
It's also worth calling a specialist if the entire drain assembly needs rebuilding instead of just replacing the siphon, for instance, due to an incorrectly sloped connection or an unsuitable pipe diameter. This is common when renovating a WC or bathroom, where plumbing appliances are installed holistically, not just as a siphon replacement.
ToruFix OÜ installs and maintains plumbing appliances in Tallinn and Harjumaa, and can also help when recurring blockages or odours require broader diagnostics, not just siphon cleaning.