Why Does the Water in the First Floor Toilet Get Sucked Down When the Basement Bathroom Sewage Pump Operates?

Key Symptoms to Check First

Please check if any of the following symptoms occur together.

• Water level in the first-floor toilet drops the moment the sewage pump operates
• Air sounds such as 'glug' or 'gurgle' occur in the toilet
• Toilet water shakes or bubbles rise
• Water seal level in the toilet is lower than usual after pump operation
• Sewer odors rise in the bathroom
• Symptoms started after replacing the pump or modifying the piping

The water seal in the toilet acts as a water barrier that prevents sewer odors and gases from rising indoors. This water seal can be lost due to negative pressure, positive pressure, or siphon effects inside the pipes, so the vent pipe must maintain stable pressure.

Possible Main Causes

Why Does the Water in the First-Floor Toilet Get Sucked Down?

When the basement sewage pump operates, sewage is rapidly pushed through the discharge pipe into the shared sewage pipe.

At this time, the water column moving with the sewage pulls air down inside the pipe. If the vent pipe does not immediately replenish the necessary air, a momentary negative pressure can form inside the pipe.

If this negative pressure reaches the first-floor toilet drain pipe, part of the water seal inside the toilet is pulled toward the pipe, making it appear as if the water is being sucked down. The vent pipe is installed to prevent such siphoning and pressure changes.

Conversely, if air cannot escape and is compressed inside the pipe, instead of water going down, bubbles may rise or water may slosh due to positive pressure.

Is a Too Large Pump Flow Rate the Direct Cause?

If the pump’s instantaneous discharge volume is excessively large compared to the piping conditions, it can worsen the symptoms.

However, in most cases, it is not only the pump flow rate but a combination of the following factors that contribute.

• Actual discharge volume of the pump
• Vertical lift and horizontal conveyance distance
• Diameter of the discharge pipe and shared sewage pipe
• Location and direction where the discharge pipe joins the shared sewage pipe
• Presence and diameter of the vent pipe
• Slope and blockage condition of the existing sewage pipe
• Distance between the first-floor toilet drain pipe and the pump discharge pipe
• Installation direction and operating condition of the check valve

Therefore, rather than simply deciding “the pump is too strong” and replacing it with a smaller pump, you should first verify whether the pump and piping are properly configured as one system.

What Is the Inspection Sequence On-site?

Possible Improvement Methods

Based on on-site diagnosis, the following improvements may be necessary.

1. Install or reinforce vent pipe: Review the vent pipe diameter and connection location to maintain stable pressure in the shared sewage pipe.
2. Change pump discharge pipe junction location: If the discharge pipe is connected directly below or near the first-floor toilet drain pipe, consider changing the junction location with the shared sewage pipe.
3. Improve junction direction and pipe fittings: Adjust the junction direction and fitting structure so the discharge water does not conflict with the existing sewage flow.
4. Check shared sewage pipe diameter and blockage: If the pipe diameter is insufficient or partially blocked inside, pipe cleaning or diameter enlargement may be necessary.
5. Inspect check valve and shut-off valve: If the check valve does not close properly or is installed in the wrong direction, backflow and repeated starts can occur.
6. Reevaluate pump capacity: If problems persist after improving piping structure, reselect pump flow and performance based on actual lift and conveyance distance.

Will Replacing the Pump with a Smaller One Solve the Problem?

It is not recommended to reduce the pump capacity without checking the site first.

If the pump is selected too small, it may not discharge sewage sufficiently, increasing discharge time and causing repeated operation. Since sewage from toilets contains solids, you need to consider not only flow rate but also passage performance, lift, piping length, and diameter.

This issue is more likely a system problem where the pump discharge and existing pipe pressure balance do not match, rather than a pump malfunction.

Information Needed for MyPump On-site Consultation

The following information is necessary for an accurate assessment.

• Manufacturer and model of the installed sewage pump
• Vertical height from the pump to the shared sewage pipe
• Total horizontal piping length
• Diameter of the pump discharge pipe
• Diameter of the shared sewage pipe
• Junction location of the discharge pipe and existing sewage pipe
• Presence and diameter of the vent pipe
• Distance between the first-floor toilet and the junction point
• Check valve installation location
• Video showing symptoms in the toilet during pump operation
• Photos of on-site piping or a simple cross-sectional sketch

At MyPump, rather than arbitrarily replacing the pump, we analyze the vertical lift, horizontal conveyance distance, pipe diameter, vent pipe, junction location, and number of users on-site to identify the cause of the problem.

If the vent pipe or piping structure is the cause, we first propose piping improvements. If the pump performance does not match the actual site, we select a suitable sewage pump model and proceed with supply consultation.

In conclusion, the phenomenon of water in the first-floor toilet being sucked down when the basement sewage pump operates is not solely due to pump flow rate. The vent pipe and discharge pipe junction structure should be checked first, and then pump capacity should be determined according to piping conditions for a proper solution sequence.