Newly Installed Toilet Runs Intermittently in the First Two Weeks: Commissioning Fault Decision Tree

Why this matters

A toilet that was silent on installation day but starts running intermittently over the following two weeks is a commissioning fault, not a defective unit. The three most common causes - a flapper seat defect that only shows after thermal cycling, a float set at the overflow threshold, and a supply valve that was partially closed during setup - are all correctable without replacing the toilet. The risk to the technician is that the customer's next call is to a plumber, who charges a diagnostic fee and replaces parts the tech already touched. Catching this on a callback saves the relationship.

Run the food coloring test first

Before diagnosing the cause, confirm that the running is actually a tank-to-bowl leak and not a fill valve refill cycle triggered by a slow leak elsewhere.

Add several drops of food coloring to the tank - not the bowl. Do not flush. Wait 15 minutes. If color appears in the bowl without flushing, the flapper is not sealing and water is seeping from the tank into the bowl continuously (even if silently). If no color appears, the tank seal is intact and the intermittent running is a fill valve issue triggered by pressure variation.

This single test splits the diagnosis cleanly.

Path 1: color appears in bowl - flapper seat issue

A flapper that tested fine on installation day can fail to seal after the toilet has been through temperature cycling. Cold supply water running through a new fill valve drops the tank water temperature, and both the flapper and the seat rim expand and contract at different rates. A flapper that seemed to sit flat on install may micro-warp just enough to allow seepage once it has cycled through a dozen temperature changes.

Inspect the flapper seat (the rim around the drain opening at the tank bottom). Run a finger around the seat rim. Any roughness, pitting, or calcium deposit that built up before the original install counts as a seating defect. Even a small imperfection breaks the seal.

If the seat is rough: clean it gently with a fine abrasive pad, taking care not to score the surface deeper. Re-seat the flapper and re-run the food coloring test.

If the seat is smooth and the flapper still leaks: the flapper itself is warped. Replace it. Use the same flapper type that came with the unit - universal flappers occasionally do not seal correctly on specific flush valve designs.

Path 2: no color in bowl - float height or supply pressure issue

If the food coloring test is clean, the flapper is sealing and the running is caused by the fill valve cycling. This means tank water is reaching the overflow tube and draining silently into the bowl, triggering a refill.

Check the water level in the tank against the marked fill line (or the top of the overflow tube). The water surface should sit approximately one inch below the top of the overflow tube. If the water level is at or above the overflow tube opening, water is constantly or intermittently draining over it.

Float set at the threshold: On install day, the float was set at what appeared to be the correct level with the supply valve partially open and the house pressure at that moment. If the supply valve was later opened further, or if house pressure varies with time of day (common on municipal systems during morning demand peaks), the tank fills slightly higher during a low-demand pressure window and water reaches the overflow tube.

Adjust the float down so the fill line is a full inch below the overflow tube top. Test by flushing three times and checking the final resting water level each time.

Supply valve not fully open on first install: Some technicians open the supply valve partway to slow the initial fill and avoid a surge. If it was left partially open, later household use that draws pressure briefly may allow the partially-open valve to meter water at a different rate. Open the supply valve fully (turn counterclockwise until it stops), then re-adjust the float height.

Verify under variable conditions

After correcting either fault, test under the conditions that trigger the running. For pressure-related float issues, flush the toilet several times at different times of day and check whether the water level holds consistently below the overflow tube. Running that only happens at night or early morning suggests pressure variation is still at play and the float needs to be set slightly lower.

Decision summary

• Food coloring appears in bowl: flapper not sealing - inspect seat for roughness, replace flapper if seat is clean
• Seat is pitted or cracked: seat repair kit or flush valve replacement, not flapper swap
• No food coloring in bowl + water level at overflow tube: lower float height, fully open supply valve
• Running still occurs after float adjustment: check for pressure variation by testing at different times; lower float further if confirmed

References

• Standard flush valve seat inspection and flapper compatibility criteria
• Fill valve float adjustment procedures for ballcock and tower-style fill valves
• Supply line pressure variation patterns on municipal systems and effect on float equilibrium