Toilet tank, lid off
What's wrong here?
- The fill valve's critical-level (CL) mark sits below the top of the overflow tube instead of at least 1 in. above it
- The refill tube must not be clipped into the overflow tube; it should end in the tank water
- Water must not stand higher than the top of the fill valve's float cup
- Nothing. This installation is compliant
Show the answer and the code reference
The answer
The fill valve (ballcock) was set too low: its CL mark is below the top of the overflow tube, so the anti-siphon valve can end up under water when the tank fills to the overflow.
Correct choice: 1. The fill valve's critical-level (CL) mark sits below the top of the overflow tube instead of at least 1 in. above it
A toilet fill valve is an anti-siphon device only when its critical level (the CL line molded on the valve) stays above the highest water the tank can hold. The highest possible water level is the overflow rim, because water spills down the overflow before it can go higher. UPC 603.5.2 therefore requires the ballcock to be installed with its critical level at least 1 in. above the full opening of the overflow pipe. Adjustable-height fill valves are often run all the way down to fit a short tank, which puts the CL under the overflow rim, as here. With a stuck flapper or a slow fill, the valve's vacuum breaker sits in tank water, and a drop in supply pressure can pull that water into the house piping.
How to fix it
Raise the fill valve's telescoping shank until the CL mark is at least 1 in. above the top of the overflow tube, then reset the float for the water line. If the valve cannot be raised that far in this tank, replace it with a model sized for the tank, or cut the overflow tube down only if the flush valve maker allows it.
Code references
UPC 2018 603.5.2 — Ballcocks
Water closet and urinal tanks must be equipped with a ballcock, installed with its critical level at least 1 in. above the full opening of the overflow pipe. Where the ballcock has no hush tube, the bottom of the water supply inlet must be at least 1 in. above the full opening of the overflow pipe.
Drills citing 603.5.2UPC 2018 603.1 — General
Cross-connection control must be provided in accordance with Chapter 6. Equipment, mechanisms or substances that could pollute or contaminate the potable water supply are permitted only where the supply is protected by an approved backflow prevention device or assembly.
Drills citing 603.1
Minnesota
Minnesota adopts the 2018 Uniform Plumbing Code with Minnesota amendments as the Minnesota Plumbing Code (Minn. Rules ch. 4714, effective Dec. 17, 2021). Minnesota amends parts of UPC section 603 (reportedly in Minn. Rules 4714.0603), but no Minnesota amendment to the ballcock critical-level rule is known; check ch. 4714 before relying on it.
About the Minnesota Plumbing CodeCitation under review. The 1 in. critical-level rule is well established (UPC and ASSE 1002 both put the CL at least 1 in. above the overflow). The section number 603.5.2 rests on one search excerpt from a Hawaii code based on the 2018 UPC, and the heading 'Ballcocks' is from memory; nearby 603.5 subsections (atmospheric vacuum breakers, flushometer valves, heat exchangers) make an off-by-one numbering error possible. Confirm both against the 2018 UPC.
Why the other choices are wrong
- The refill tube must not be clipped into the overflow tube; it should end in the tank water. It is the other way around. The refill tube is supposed to discharge into the overflow tube above its rim. Pushing it down into the tank water would make it a submerged inlet that can siphon tank water.
- Water must not stand higher than the top of the fill valve's float cup. The float cup is meant to ride in the tank water; the water level is set by the float and is normally about 1 in. below the overflow. The control point the code cares about is the CL mark, not the float.
- Nothing. This installation is compliant. If the flush valve leaks and the tank fills to the overflow rim, the water rises above the fill valve's critical level, and its anti-siphon feature can no longer stop tank water from being drawn back into the supply.