Quick Answer:
If the toilet refills slowly only while the washing machine fills, the washer is pulling enough water to drop pressure at the toilet. Recreate the condition by running the washer and watching the toilet refill. If the refill slows, check overall water pressure and the supply sizing; many fixes are possible without replacing fixtures.
Why This Happens
When the washer fills it opens a wide valve for several minutes. That sudden demand can lower pressure in the branch that feeds the toilet. Possible causes include:
- High simultaneous demand from the washer and other fixtures on the same branch.
- A partially closed shutoff, clogged supply line, or small-diameter branch that can’t deliver both flows.
- A pressure-reducing valve (PRV) or low municipal pressure that can’t hold steady under load.
- Toilet components that need a higher minimum inlet pressure to refill at the normal speed.
If you’ve recently worked on toilet parts or installed automation, see related issues such as Tank refills slow after replacing fill valve and Slow refill after installing smart shutoff for how changes to the toilet or controls can change refill behavior.
Step-by-Step What to Do
1. Recreate and observe
Run a normal wash cycle and watch the toilet tank while the washer fills. Note whether the refill slows only during the washer fill and how much slower it becomes. This confirms the timing and rules out intermittent faults.
2. Check simple supply problems
- Ensure the home main and both fixture shutoffs are fully open. A partially closed valve can mimic low pressure.
- Open a cold tap at a sink while the washer fills. If that tap also drops to a trickle, the issue is systemic rather than the toilet.
3. Check overall pressure and flow
Observe static pressure (tap closed) and dynamic feel when the washer runs. If several fixtures dip or taps run weak under load, the house supply or pipe sizing may be the limit. At this point consider the supply may be undersized for simultaneous loads.
4. Inspect the toilet and washer supply points
- Confirm the toilet fill valve is working and the valve inlet is not blocked. Clean any screens on the supply stops.
- Check the washer inlet hoses and valve for debris or flow-restricting screens.
5. Try practical temporary fixes
- Stagger laundry runs so the toilet doesn’t need full refill while the washer is in its fill cycle.
- Open a distant cold tap slightly while the washer fills to see whether sharing flow differently reduces the toilet slowdown (diagnostic only).
6. Consider supply sizing or pressure equipment
If the house feed or branch lines are too small to carry both flows at once, a permanent fix is to increase pipe capacity, add a dedicated branch, or adjust/replace a faulty PRV. These steps usually require a professional assessment.
What Not to Do
- Don’t accept competition-related slow refill as unavoidable—supply issues can be corrected.
- Don’t try to raise municipal pressure or tamper with the water meter or street-side equipment yourself.
- Don’t ignore persistent problems; if the slowdown continues after basic checks, a plumber is appropriate.
When to Call a Professional
Call a licensed plumber when:
- The toilet and other fixtures consistently lose pressure while the washer runs despite shutoffs being open.
- You suspect the home supply or PRV is limiting flow, or you want proper flow and pressure measurements.
- You need pipe upsizing, a dedicated feed, or changes to the PRV or meter connection.
A pro can measure dynamic flow, inspect the PRV, and recommend whether upsizing or rerouting is the right solution.
Safety Notes
- Turn off appliance water supplies before disconnecting hoses or working on valves.
- Do not attempt to alter the municipal connection, meter, or curb stop — contact your water utility or a professional.
- If you’re uncomfortable with plumbing tools or diagnosing pressure issues, call a plumber rather than risking damage.
Common Homeowner Questions
- Q: Is this normal? A: Not necessarily; some drop is normal but severe or persistent slowdown should be checked.
- Q: Can I fix it myself? A: You can check valves and screens, but supply sizing or PRV work is best left to a plumber.
- Q: Will bigger pipes solve it? A: Often yes, if the cause is undersized branches, but a pro should confirm before altering plumbing.
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