Quick Answer:

If a toilet tank refills extremely slowly or stalls while other taps seem fine, a pressure-reducing valve (PRV) that drops pressure under low flow can be the cause. First time the refill with all other water off and clean the toilet fill valve screen. If several fixtures show the same behavior, measure the water pressure while the toilet is filling and consider adjusting or replacing the PRV if the pressure falls too low under that small demand. For more detail, see PRV and slow toilet refill and PRV causes low pressure upstairs.

Why This Happens

• PRVs are installed to limit incoming main water pressure to protect plumbing. Some are set low or have worn internals that react poorly at low flow rates.
• Under a tiny demand like a toilet refill, a failing or misadjusted PRV can drop pressure more than it should, so the fill valve gets little flow and the tank refills slowly or stops.
• Debris in the toilet fill valve screen or the supply shutoff can mimic the same symptom, so check the toilet’s components before assuming the PRV is the problem.

Step-by-Step What to Do

Step 1 — Isolate other water use and time the refill

Turn off every other fixture and appliance that uses water (washing machine, dishwasher, taps, irrigation). Flush the toilet and time how long the tank takes to refill from empty to the fill line. Note whether the refill is a steady slow trickle, pauses, or stops altogether.

Step 2 — Clean the toilet fill valve screen

Shut the small supply shutoff under the tank, remove the supply hose or open the access and check the small mesh screen where the tank connects to the supply. Gently clean any debris and reassemble. Open the shutoff and re-check refill time. A clogged screen is a common, easy fix.

Step 3 — Check if multiple toilets or fixtures are slow

If only one toilet was slow and it improved after cleaning the screen, you’re likely done. If multiple toilets (or other small-flow fixtures) refill slowly, move to pressure checks.

Step 4 — Measure pressure while the toilet is filling

Attach a pressure gauge to an outdoor spigot or to a hose bib near the main to measure static and flowing pressure. Record:

• Static pressure with everything off.
• Pressure while the toilet is actively filling (have someone flush while you watch the gauge).

If pressure drops significantly during the toilet refill (more than a few psi) but static pressure is normal, the PRV may be shutting down or dropping too far under low demand.

Step 5 — Adjust or replace the PRV if needed

If the gauge shows an unacceptable pressure drop, try adjusting the PRV per manufacturer guidance (usually a screw or nut on the valve). Make small adjustments and re-test. If adjustment doesn’t help, the PRV may be worn and should be replaced. Replacing a PRV often requires shutting off the main and draining some water—consider professional help if you’re not comfortable.

What Not to Do

• Don’t replace toilet parts in bulk until you confirm pressure during the refill — many parts won’t fix a pressure problem caused by the PRV.
• Don’t force the PRV adjustment screw without knowing the valve type; you can make the problem worse.
• Don’t ignore a large pressure drop during small demands; it often indicates a PRV issue that will affect other fixtures.

When to Call a Professional

• If pressure testing shows a large drop during tiny flows and adjusting the PRV doesn’t help.
• If you’re not comfortable shutting off the main, draining lines, or replacing the PRV.
• If the PRV is hard-piped into a complex manifold or in a confined space where access is limited.

Safety Notes

• Shut off the main water supply before removing or replacing the PRV, and relieve system pressure by opening a valve or spigot.
• Wear eye protection when working with pressurized lines; unexpected sprays can occur when fittings are loosened.
• If soldering or using power tools near plumbing, follow standard safety practices and ensure there are no flammable materials nearby.

Common Homeowner Questions

• Q: How long should a toilet normally take to refill?
  A: Typically under a minute; anything much longer suggests restricted flow or pressure issues.
• Q: Can a PRV be adjusted without replacing it?
  A: Yes, many PRVs have an adjustment screw, but if it still drops pressure under small demand the valve may need replacement.
• Q: Will a slow refill waste water or cause damage?
  A: It wastes time and can keep tanks at partial levels, but the main concern is inconvenience and potential failure of other fixtures if pressure is broadly reduced.