Reuben Saltzman

How to Correct an Improper Air Gap on a Toilet Fill Valve

The guts of your toilet need to be properly installed to avoid a cross-connection between the water in your toilet tank and the water you drink. If you’re selling a house in Minneapolis and don’t have this little detail correct, the city will require repair as part of their Truth in Housing program. But it’s not just a Minneapolis thing; it’s also required by the Minnesota State Plumbing Code, Table 603.2. It’s also required in the International Residential Code under section P2712.4 Flush valves in flush tanks.

While this is a common defect, the repair is usually quick and easy. The most common fill valve used today is the Fluidmaster 400A, and the diagram below shows the proper height.

Fluidmaster Fill Valve Diagram with Air Gap

To get this right, the C.L. mark must be at least 1″ above the top of the overflow tube. In most cases, this can be corrected by adjusting the height of the fill valve. If the overflow tube is too high, that can also be cut down.

You have the same requirement on older toilets equipped with a ball-style fill valve, called a ballcock (don’t look at me like that).

Standard ballcock diagram

The C.L. mark must still be installed at least 1″ above the top of the overflow tube. Those fill valves don’t have adjustable height controls, so the fix is either cutting down the overflow tube or replacing the fill valve.

If you have an older clamshell-style fill valve at the bottom of your toilet tank, the only acceptable repair is to replace it. Thankfully, it’s a pretty simple project.

7 responses to “How to Correct an Improper Air Gap on a Toilet Fill Valve”

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    January 26, 2010, 5:09 am

    […] Toilet Ballcocks […]

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    April 26, 2010, 7:46 pm

    […] Toilet Ballcocks […]

  3. Toronto Toilet Repair
    May 13, 2010, 10:39 am

    Well, as a professional plumber from Toronto, I would say this article is very useful. I would recommend it to read to any homeowner.

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  7. Alex
    May 29, 2024, 8:05 am

    Great information! I had no idea there was this risk for cross contamination and I had recently adjusted all of my fill valves much lower to reduce the amount of water in each flush (the water was high enough before I couldn’t achieve this by adjusting just the float alone). Now it’s time to recheck the CL lines relative to the overflow tube.

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