A flushometer toilet that flushes weakly is usually short on pressure or flow, or its valve is out of adjustment or worn. Check the control stop first, then the diaphragm or piston and its bypass, then the bowl's rim holes and the drain. Zurn lists 25 psi running pressure for its valves, and some fixtures need more.
Start with the supply: control stop, pressure and flow
The first cause on Zurn's troubleshooting chart for a fixture that is not flushing with enough water is a stop valve that is not open enough, with the fix to open it for the desired volume. Kohler's flushometer guide likewise lists a supply stop that is not allowing sufficient flow. Zurn also warns that the control stop should never be opened to allow flow greater than the fixture can evacuate, so the stop is set for a full flush, not wide open.
If the stop is right, look at what the building delivers. Zurn's data sheet for its sensor flush valves gives an operating water pressure of 25 psi running and 80 psi static, and adds: "Higher running pressure may be required for some fixtures." Kohler's roughing-in table for its flushometers lists a 1-inch supply pipe and 25 gallons per minute for a toilet, and a static pressure window that starts at 35 psi for the toilet and 25 psi for the urinal. Kohler tells installers to "Confirm that water pressure and flow meet the minimum requirements."
The plumbing code sets a floor too. Section 604.3 of the 2021 International Plumbing Code requires the water system to deliver the capacities in its Table 604.3 at peak demand, and that table lists a flow rate and a flow pressure for flushometer-valve water closets that is higher than for tank toilets. Section 604.7 says a booster system is required where street pressure is insufficient to provide those flow pressures. The edition and amendments your city enforces can differ from the 2021 text.
Zurn's guide describes a field test when gauges are not available: remove the working parts, open the stop and let water pass through the empty valve. If the supply proves unsatisfactory, it says to increase the pressure or supply. That is a job for a licensed plumber or building engineer, not a stop adjustment.
The valve: diaphragm, piston, bypass and wrong parts
A worn valve can cut the flush short. Zurn's chart lists "Flush valve shuts off too quickly" with these causes: a damaged or punctured diaphragm, an enlarged bypass orifice, and diaphragm and cylinder guide assemblies that are not tight. The remedy for the first two is a new replacement kit. Kohler's guide lists a worn or damaged piston among the reasons for low water flow.
Parts can also be wrong. Zurn's chart names a urinal kit installed in a closet valve, or a 1.0 gallon urinal kit in place of a 1.5 gallon one, as causes of insufficient flush volume. A toilet valve needs the kit made for a toilet, and the gallons-per-flush rating has to match; the kit choice is covered in why a Sloan flushometer keeps running.
The bowl: clogged rim holes and jet
Even with a good valve, a bowl with restricted rim holes washes poorly. Kohler says "Poor flush or rim wash may be caused by obstructed rim holes in the toilet," and that hard water deposits can restrict flow through them. Kohler's fix is to shut off the supply, ream the holes and soak them with a lime-dissolving cleaner for 8 to 24 hours. Kohler's chart for its pressure-assist toilets also asks whether the jet in the bowl is obstructed.
The drain: a slow line looks like a weak flush
If the bowl fills and drains slowly, the trouble may be downstream. Kohler's chart for its pressure-assist toilets tells you to pour 2 to 3 gallons slowly into the bowl and see whether it drains as you pour or the water level gets higher, and to treat a rising level as a blockage. EPA's WaterSense program set a 1.0 gallon-per-flush minimum for labeled flushometer-valve toilets after stakeholders worried about blockages in commercial buildings with longer drainlines. For repeat stoppages, see why a toilet keeps clogging.
Quick order of checks
- 1Confirm the control stop is open enough for a full flush.
- 2Compare supply pressure and flow to the valve and fixture specs.
- 3Inspect the diaphragm or piston, bypass and kit match.
- 4Clean the rim holes and check the jet.
- 5Test the drain.
For a home toilet with a tank, why a toilet won't flush all the way covers the tank-side causes. If a building keeps losing flush strength, HQ does commercial plumbing across the Valley, licensed and insured, with free estimates and upfront pricing, and can measure the pressure at the valve before anything is replaced.