Toilet Water Level Recedes After Flushing? [Here’s Why & Fixes]

Toilet Bowl Water Level Recedes

You notice that the water level in your toilet bowl has started to recede slightly after a flush. You wonder if this is a problem or a normal occurrence in some cases. If the water level in your toilet bowl suddenly starts to recede, you should be concerned. A toilet bowl that loses water is a sign of problems with the toilet or the vents.

The waste handling system in your home is not as simple as it appears. A complex system of pipes, vents, and fixtures must all work properly to keep the water flowing properly. When the water recedes in a toilet bowl after a flush, the most common causes are a cracked or defective toilet or a blocked vent pipe.

Finding the cause of the water level receding in your toilet after a flush requires a bit of detective work. The most obvious place to look is at the toilet itself.

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Diagnosing a Leaking Toilet

There are numerous places a toilet can leak, causing the water in the toilet bowl to recede. Following a few simple steps can often find the problem. In most cases, you can address these problems with a few simple tools.

Step 1: Inside the Tank – The Flapper Valve

Remove the lid from your toilet tank. At the bottom of the tank is the flapper valve. It is usually connected to the flush handle by a short length of chain. There are other styles of flapper valves with direct connections. However, the concept is the same.

Step 2: Open the Flapper Valve and Inspect the Flapper

Water leaking by the flapper valve can sometimes cause the toilet bowl’s water level to recede slightly. The flapper valve must seal tightly to the valve ring on the bottom of the tank. If the flapper valve is damaged, it may not seal properly.

Inspect the outer edge of the flapper valve for wear, tears, or cracks. Some flapper valves may get so soft with age and use that they won’t make a seal. If you suspect that the flapper valve is leaking, replace the flapper. Flapper valves are available at almost any hardware or home improvement store.

Step 3: Tank Water Level Issues – Adjust the Float

If the water level in your tank is too high or too low, the tank flush valve may not operate properly. When the flush valve doesn’t operate correctly, the toilet bowl’s water level can recede or not flush properly.

Most toilet tanks have a mark inside the toilet tank that shows the recommended water level. You can adjust the float valve in the water tank to bring the water level to this mark. The adjustment may be a screw on the float valve, or, on older systems, you must bend the float’s arm.

Step 4: Seals and Gaskets

Your toilet has several seals and gaskets. Two-piece toilets have a gasket between the tank and the bowl. Over time, this gasket may dry out, shrink, and crack. A damaged seal can allow the toilet to leak. This sort of leak is usually obvious and easy to diagnose. A leak at the seal can cause the tank flush system to no deliver enough water for a proper flush

The toilet has a seal ring at the base where the toilet joins the waste pipe. This seal ring may be wax of a manmade product. This seal, or bowl ring, can be damaged if the bolts holding the toilet to the floor are loose. As the toilet rocks even slightly, the bowl ring loses its seal.

Some homeowners may feel comfortable tackling the job of replacing a tank gasket or bowl ring. However, these are often jobs best accomplished by a licensed plumber. In some cities and towns, local building codes mandate that a licensed plumber make these types of repairs.

Step 5: Cracked or Damaged Toilet Bowl

A crack or other damage to the toilet bowl can allow water to escape from the bowl. You will notice a drop in the water level in the bowl when a leak occurs. Damage to a toilet bowl can occur if objects drop into the bowl or the toilet is subject to undue stress.

Sometimes, a crack or damage is easily visible. Objects falling into the toilet bowl may leave cracks or even holes in the porcelain. However, hairline cracks are often hard to see.

You should never stand on your toilet. Your toilet is not a stepstool and the stresses can cause the toilet to crack or even collapse. Uneven pressure on the toilet’s edges may cause cracks inside the toilet or on the outside at the base. Replace any toilet that has cracks or damage should immediately.

Clogged or Blocked Vent Pipe

A receding water problem may not be in your toilet at all. The receding water level could signal a deeper problem in your waste handling system. In some cases, solving this problem may require the services of a licensed plumber

A clogged or blocked vent pipe can cause the water to recede in your toilet when another drain in your home is in use. The wastewater system in your home must have a clean airflow whenever water is flowing in the drainpipes. To understand this problem, you must understand how the waste system works.

Your Wastewater System – The Basics

The waste handling system in your home is not just pipes that carry the wastewater to the sewer connection or septic system. The waste piping also has vents that allow air into the waste piping and allow sewer gases to escape harmlessly outside your home.

When you flush a toilet or turn on the shower, water begins to flow down the drainpipes. AS the water flows downward and outward, it pushes air ahead of the water. If a vent pipe is clogged, there is no air available to replace this air.

The drain system tries to get the air by pulling sucking the water out of any toilet nearby. You will notice that the water level in the toilet bowl recedes.

Finding the Problem – The Hunt is On

Finding a clogged or blocked vent pipe is not usually a job for the average homeowner. Checking and clearing blockages in vent pipes usually involve a plumber, and a plumbers drain snake.

Using a plumbers snake to clear vent lines means lugging the heavy machine to the roof. Not many homeowners have access to a snake machine or can work on steeply pitched roofs. We recommend that you find a reputable licensed plumber to clear your vent pipes if necessary.

How Does a Modern Toilet Flush?

With a few exceptions, modern toilets use a quite simple method to carry the waste away to the drain system. The process involves gravity, a siphon, and a P-trap built into the toilet.

The water that remains in the bowl after a flush sits in the P-trap of the toilet. This double loop system in the base of the toilet holds the water in the bowl. The water serves as a seal on the waste and drain system to keep sewer gases from escaping into your home.

Keeping an adequate water level in the P-trap and bowl of your toilet is important for you and your family’s health and safety. Sewer gas not only smells bad, but the gas can also be dangerous if your home concentrations get too high.

The Flush – Gravity at Work

When you operate the flush valve on your toilet, the water stored in the toilet tank rushes down to the bowl. A series of channels direct the water into the bowl around the rim to clean away any debris. As the water rushed into the bowl, the water level in the bowl rises. As the water flows over the back of the P-trap, it creates a siphon effect. The siphon pulls the wastewater and debris through the P-trap and into the drain.

After the Flush – Prepping the Bowl

After the flush, the flapper valve closes, and the tank begins to refill. At the same time, a small diverter tube continues to run water into the bowl. The flow of water from the diverter tube refills the bowl and creates a new seal in the P-Trap.

The float valve stops the water flow when the tank is full, and your toilet bowl is prepped and ready for its next use.

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Receding Water in Your Toilet Bowl – Not to be Ignored

If you notice the water level in your toilet bowl receding, don’t ignore the warning. Low levels of water in your toilet bowl can let dangerous sewer gases into your home. Whether the problem is with the toilet or in the drain system, you should immediately diagnose and fix the issue.

We hope that the tips and tricks in this article are of help. Plumbing problems can be a vexation to many homeowners. However, with a little knowledge and helpful hints, many such problems are easily remedied.  As always, good luck and have fun.

Dennis Howard

Dennis is a retired firefighter with an extensive background in construction, home improvement, and remodeling. He worked in the trades part-time while serving as an active firefighter. On his retirement, he started a remodeling and home repair business, which he ran for several years.

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