Why Your Toilet Flushes Slow—and How to Fix It

Find out the cause behind your slow-flushing toilet.

Human hand flushing toilet
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There are many important systems in a home that help to improve your convenience and comfort, but there are few things more integral than a functioning toilet. So when your toilet doesn't flush properly, resulting in partial flushes and leftover waste, it can be frustrating and even embarrassing when guests come to visit. A slow-flushing toilet is a problem that can usually be resolved by an experienced DIYer, but it depends on what exactly is causing the issue.

There are several possible reasons a toilet may be flushing slowly or inadequately, and determining the cause is necessary before the problem can be fixed. Common causes of a slow-flushing toilet include low water levels, a partially clogged drain, mineral build-up on the jet holes, a defective flapper, and a blocked drain vent. To help solve your problem, take a look at the in-depth breakdown of these toilet issues and the possible fixes below.

Low Water Level

One of the most common reasons for a toilet to have issues flushing properly is that the water level inside the tank is too low. If there isn't enough water in the tank when the toilet is flushed, then the flushing power of the toilet will be greatly reduced because there is a limited force flowing into the toilet. The pressure created by a full tank flowing into the toilet bowl normally provides enough force to move the contents of the toilet bowl into the drain. Without enough water, it's likely that some waste will not be flushed, leaving you to wait for the toilet tank to refill just so you can try and flush the toilet again.

This issue can usually be fixed by changing the level of the toilet float in the toilet tank. A toilet float sits on top of the surface of the water and automatically turns off the fill valve when the water level reaches a set point. Adjust the toilet float so that more water fills the toilet tank before the fill valve is turned off to increase the flushing power of the toilet.

Partially Clogged Drain

A clogged toilet is typically easy to spot, and most people know how to use a plunger to help break up the blockage and force it through the drain, but a partial clog can be more difficult to identify because the toilet will still flush, just with significantly reduce efficacy. If a partially clogged drain is the cause of your slow flushing toilet, there are several methods that you can use to try and clear the clog and restore flushing power.

One of the first things to try is simply plunging the toilet. Oftentimes the plunger generates enough force to clear any clogs or partial clogs, but if this doesn't work, then it's a good idea to consider using a drain snake. Items like kids' toys, floss, wipes, and more can get lodged in the drain line, so if you aren't having any luck moving the clog, then it may be time to contact a plumber to clear the blockage.

Mineral Build-Up

Water flows from the tank into the bowl primarily through the jet holes located on the underside of the toilet bowl rim. However, due to the small size of these holes, they can become clogged or partially blocked, reducing the force of the flush. If you live in an area that has hard water or the toilet is several years old, then minerals like calcium or magnesium can build up in these jet holes and obstruct the flow of water.

Use distilled white vinegar and a stiff-bristle toilet brush to help break up the mineral build-up to restore the flushing power of the toilet. It's also a good idea to consider installing a water softener so that this problem doesn't keep happening in the future.

Defective Flapper or Flush Valve

Problems with a slow flushing toilet can sometimes be traced back to the flapper and flush valve assembly. These parts can be found in the toilet tank and are responsible for releasing the water from the tank into the bowl. The flush valve assembly consists of an overflow tube, a lever, a chain, a flapper, and the flush valve opening in the base of the tank. The flapper is a simple rubber plug or ball that blocks the opening to the toilet bowl, and it's usually off to one side of the overflow tube. When the lever or button is pressed on the toilet, it activates the flush valve and opens the flapper, releasing the water from the tank into the toilet bowl.

There are also flush valves that sit directly on top of the flush valve opening. These assemblies use a flush valve seat to plug the opening to the toilet bowl instead of using a flapper. When the toilet is flushed, the flush valve seat lifts up, allowing the water from the tank to flow into the toilet bowl.

However, if there is a problem with the flapper or flush valve, then the flow of water into the toilet bowl may be restricted. The restricted flow reduces the power of the flush, resulting in a slow flushing toilet. To resolve this problem, inspect the flush valve assembly for any problems. Sometimes the chain can get trapped or stuck, so you may be able to fix this issue relatively easily.

If the problem persists, then it may be best to completely replace the flush valve assembly. This isn't a complicated repair, but if you aren't comfortable replacing this part of the toilet, then consider contacting a professional plumber.

Blocked Drain Vent

Many people don't realize that the plumbing drain lines that direct the flow of wastewater within the home to the septic system or municipal sewer system are actually installed with vents. The vents typically run to the roof, allowing gasses to escape instead of building up inside the pipes and causing foul smells to permeate the home. These vents don't just help with smell and releasing hazardous gas to the outside—they also allow air to enter the waste lines to help the wastewater flow freely through the pipes.

Obstructed vent lines can reduce the flushing power of a toilet and should be cleared immediately to prevent further problems with the wastewater system. Clearing these vents can be difficult because you will need to climb up to the roof to access the vents. This is especially problematic if you don't know exactly what to look for when you get to the roof. For this reason, you should hire a professional plumber to clear the vents.