Quick Answer:
When the toilet refills slowly only while the washing machine fills, the most common cause is a temporary drop in water pressure or flow to the toilet while the washer draws water. Confirm the behavior by running the washer and watching the toilet refill, then check pressure and consider whether the household supply piping or valves are undersized or partially closed.
Why This Happens
Both the washer and the toilet draw water from the same household supply. If the supply pressure or flow is limited, running the washer can reduce the available water for the toilet and slow its refill. Usual reasons include:
- Shared branch lines that are too small (undersized piping).
- Main or branch shutoff valves not fully open or partially clogged inlet screens.
- Low overall house water pressure from the utility or well pump.
- Recent changes or devices that alter flow; for example, if you recently replaced the toilet fill valve or added an automatic shutoff, those items can affect refill behavior — see Tank refills slow after replacing fill valve and Slow refill after installing smart shutoff.
Step-by-Step What to Do
Step 1 — Reproduce the problem
- Run a normal wash cycle (or just the washer’s fill stage) and flush the toilet at the same time.
- Watch the toilet tank refill. Note whether the refill rate only drops while the washer is filling and recovers afterward.
Step 2 — Check obvious valves and screens
- Make sure the toilet shutoff valve is fully open.
- Check the washer inlet hose screens and the toilet inlet screen (if visible) for debris. Clean if needed.
Step 3 — Assess overall pressure and flow
- Observe other fixtures while the washer fills — sinks or showers may also show reduced flow. This helps tell if it’s a house-wide pressure drop.
- If you have a water pressure gauge or an inexpensive test gauge, measure pressure at a hose bib with and without the washer running. If pressure falls significantly when the washer runs, the supply or pressure source may be undersized or weak.
Step 4 — Consider supply sizing and improvements
- If the house plumbing uses small-diameter branch lines (common in older homes), those lines can’t deliver enough flow to run multiple fixtures at once. Upsizing a branch line or the main supply can fix this.
- Also check that the main house shutoff and any branch valves are open and that the water meter or pressure regulator (if present) isn’t restricting flow.
Step 5 — Test after simple fixes
- After opening valves and clearing screens, repeat the washer + toilet test. If the toilet refill is still unacceptably slow when the washer runs, document what you tried and the results to share with a plumber.
What Not to Do
- Don’t accept competition-related slow refill as unavoidable—supply issues can be corrected; if persistent, a plumber is appropriate.
- Don’t randomly replace parts in the toilet or washer without confirming the real cause; replacing the wrong component wastes time and money.
- Don’t try to restrict the washer or toilet flow as a permanent “fix.” That reduces appliance performance and can cause other problems.
When to Call a Professional
Call a plumber if any of the following apply:
- The toilet still refills slowly while the washer runs after you’ve checked valves and inlet screens.
- Multiple fixtures lose pressure at the same time, suggesting a main supply or pressure regulator problem.
- You suspect the house piping is undersized and you want an estimate for upsizing or rerouting supply lines.
A plumber can measure pressure, inspect the entire supply system, and recommend whether valve replacement, pipe upsizing, or a pressure boost is the right solution.
Safety Notes
- Turn off water at the fixture shutoff or main before disconnecting hoses or working on valves.
- If you test pressure with a gauge, follow the gauge manufacturer’s instructions and avoid over-tightening fittings.
- If you aren’t comfortable working near plumbing connections or altering supply lines, hire a licensed plumber. Don’t work on gas appliances or electrical components unless you’re qualified.
Common Homeowner Questions
- Why does it only happen when the washer runs? The washer draws a large burst of water during fill. If supply flow or pressure is limited, that burst temporarily reduces what’s available to the toilet.
- Can I fix this myself? Possibly—start by checking valves and screens and running the washer to observe the effect. If problems persist or if pipe resizing looks necessary, call a plumber.
- Is this dangerous? No. Slow refill is usually a nuisance rather than a safety hazard, but persistent low pressure can indicate a failing pump, a closed valve, or other issues that a professional should evaluate.