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Tankless Toilet Won’t Flush: How to Troubleshoot and Repair

Tankless toilets are becoming increasingly popular because of their sleek design. They are also much more eco-friendly. The engineering behind the design allows the toilet to flush with enough pressure without a tank.

We have prepared helpful information that can help you figure out why your tankless toilet won’t flush and how to fix it. Let’s get started!

Tankless toilet won’t flush? The most common reason is a malfunctioning flush valve. You should also check the control stop and the main valve supply. Be sure they are in the open position. Another reason may be a worn-out handle assembly from normal wear and tear.

Understanding How Tankless Toilets Work

Before we get into troubleshooting your tankless toilet, first, you must understand how they work. In case you didn’t know, tank toilets are gravity-powered. They can operate with water pressure of as little as 10 psi.

On the other hand, tankless toilets send water through the toilet at high pressure. The amount of water used is the same for both types of toilets. The water shoots with high pressure without mechanical assistance.

These types of toilets are commonly seen in commercial facilities or residential properties that have high water pressure. The industry calls this type of toilet a flushometer (

How Much Pressure Does My Tankless Toilet Need?

The water pressure requirements depend on the brand and type of toilet you have. The average water pressure required by a flushometer toilet is 15 to 20 psi. A high level of pressure is even better.

Common Reasons Why Your Tankless Toilet Doesn’t Flush

Generally, tankless toilets stop flushing due to issues with the flush valve. Tankless toilets work with an electric pump. This is how it generates pressure to empty the bowl. The force helps the waste be siphoned down the drain and into the sewage system.

Tankless Toilet Won't Flush

What exactly does the flush valve do? The flush valve connects to the toilet to limit the quantity of water. This process ensures that the bowl does not overflow. The flush valve of a commercial toilet is called a flushometer. They essentially have the same function.

The flush valve helps to minimize the probability your toilet becomes clogged. It decreases your long-term maintenance costs. The part also ensures the toilet maintains a high-pressure flush each time you use it.

Let’s explore the other possible reasons why your tankless toilet doesn’t flush.

Low Pressure

To increase flow pressure, adjust the control stop. You can also get an inspection of your plumbing system. There could be deficiencies that can be addressed.

The water supply line connected to a tankless toilet provides direct water pressure. This mechanism is controlled by the flush valve. This is a manually-operated piece along with the control stop screw. Together they create the flushing process.

The control stop screw meters the water supply. If your toilet won’t flush, you can attempt to adjust the flushing pressure. Adjust the control stop screw located inside the body of the flush valve.

To make this adjustment, you may follow these steps:

Low Pressure

Step 1

To find the valve control stop screw, you must unscrew the access cap. This piece is located at the elbow. It is where the flush valve meets the water supply line.

Step 2

Use a screwdriver to turn the control stop screw in a counter-clockwise direction. Turn it until you sense it completely opens. To close the valve, rotate the control stop screw by turning it clockwise.

Step 3

You have to take your time when adjusting the control stop screw. Rotate it counter-clockwise only 1/4 turn each time. Perform a test flush after each 1/4 rotation to assess the flushing pressure. If you need to continue to adjust the pressure, keep opening the control stop screw.

Resume opening the control stop screw in 1/4 rotation increments. Do this until you achieve the flushing pressure and volume you desire. Finish by reattaching the access cap.

Step 4

If you don’t achieve the desired flushing pressure after adjusting the control stop screw, you can check the property’s overall water pressure.

Check Piston – Repair or Replace if Necessary

To restore proper sealing and flushing, the piston assembly may need attention. This part regulates the flush valve’s inner pressure. It utilizes a cylinder that seals the valve seat after flushing.

If the piston assembly needs repair, you’ll notice that the toilet keeps running. Also, if it doesn’t flush or flushes incompletely, the piston assembly must be fixed.

Flush valves are self-closing valves that are automatically timed. When the toilet is at rest, the water has equal pressure both at the lower and upper chamber of the valve. This allows the piston to stay tightly closed on the valve seat.

Next, the inlet releases water at high pressure lifting the piston. The water then travels down the flush pipe. When you flush, some water forces its way to the upper chamber of the valve. It passes through a hole in the piston.

The flush remains steady until the pressure equalizes. The piston is pushed down to the valve seat. As time goes on, the piston’s rubber seals wear out. This causes the water in the upper chamber to leak, preventing it from pressurizing.

When the piston gets stuck to the valve seat, the toilet doesn’t flush. To repair this problem, the piston may need new rubber seals or need a complete replacement.

Replace Diaphragm

The toilet’s pressure is regulated by the diaphragm assembly. The diaphragm is a rubber disc that sits close to the valve seat. It is a flexible piece that is used after each flush. When the toilet is at rest, the water has equal pressure in the lower and upper chambers.

This allows the flexible rubber diaphragm on the valve seat to close tightly. When you flush, the relief valve opens and releases water from the upper chamber. This decreases the pressure.

The water from the inlet comes out with high pressure lifting the diaphragm. It then travels down the flush pipe. As the toilet flushes, some of the water is pushed into the upper chamber. It passes through a small hole in the diaphragm.

Tankless toilet

The diaphragm is then pushed back onto the valve seat as the flush continues. During this time, the pressure equalizes. If the diaphragm or relief valve is worn out or gets stuck, the toilet won’t flush.

Normal wear and tear warp the diaphragm causing the toilet not to flush. If you see that the diaphragm has any damage, replace it and see if that fixes the flushing problem.

Tankless Toilets Are Low Maintenance

The great news is that tankless toilets require less maintenance than those with tanks. They rarely need repairs or replacement of faulty parts. Without a tank or a cistern, tankless toilets are not likely to require much maintenance.

This article has provided various clues that will help you troubleshoot your toilet. Remember to start by checking the flush valve. This piece is the one that is most related to flush issues.

Of course, when it’s too challenging to do it yourself, most plumbers are now experts with tankless toilets. Never doubt to call a professional when you need help!

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