Quick Answer:
If your shower loses pressure only while the washing machine fills, the two fixtures are sharing the same supply capacity. Run the shower alone, then run it while the washer fills to confirm. If the drop happens only when both run, the cause is most likely limited supply line sizing, a partially closed valve, or a pressure regulator restricting flow.
Why This Happens
Most homes use one main cold-water supply that branches to several fixtures. When the washer opens its fill valve it can take a large share of the available flow. If pipes or valves on that branch are small or restricted, the shower mixer doesn’t get enough flow and pressure falls. Other causes include a partially closed main shutoff, a stuck washer fill valve that draws too much water, or a pressure-reducing valve (PRV) set too low.
If the issue started after a recent event, check related conditions like Low pressure after city outage or winter work that might have changed valves.
Step-by-Step What to Do
1. Observe the difference
- Run the shower alone and note the feel of the spray and how strong the flow is.
- Stop the shower, start the washer’s fill cycle, then run the shower again while the washer is filling. Watch how much the shower pressure changes.
- If the pressure is fine alone but drops only when the washer runs, you’ve confirmed a supply conflict between the two fixtures.
2. Check simple supply issues
- Make sure the washer’s inlet hoses are not kinked and the washer isn’t using hot-only or cold-only by mistake.
- Verify the house main and any branch shutoff valves are fully open.
- Open another cold-water tap while the washer fills to see whether other fixtures also lose pressure.
3. Look for valve or regulator restrictions
- If your home has a pressure-reducing valve (PRV), it may be set too low or partially clogged. Adjusting or replacing a PRV is usually a job for a plumber.
- Older or corroded shutoff valves and fittings can restrict flow even when “open.”
4. Consider pipe sizing and routing
- Smaller-diameter branch lines feeding both the washer and the bathroom can’t supply both at full strength. If the shower is fed from the same small branch as the washer, the built-in capacity is the limiting factor.
- If the problem points to sizing or routing, a professional can evaluate options like re-piping a branch, adding a dedicated feed, or installing a different mixing valve.
5. Try simple mitigation
- Use washer delay or run large-fill appliances at different times from showers to avoid the conflict while you decide on a permanent fix.
- If winter work affected plumbing, check guidance for Shower pressure worse after winterization before doing more troubleshooting.
What Not to Do
- Don’t repeatedly start and stop appliances to “test” pressure — doing that can stress mechanical parts and waste water.
- Don’t force open or wrench valves beyond their normal movement; you can break stems or fittings.
- If conflicts persist after basic checks, don’t delay calling a professional — some fixes require tools and permits.
When to Call a Professional
- When multiple fixtures lose pressure at once, or the drop is severe.
- If the main shutoff, PRV, or hidden plumbing needs inspection or replacement.
- When you suspect re-piping, upsizing a supply line, or installing a dedicated feed is necessary.
- If you’re uncomfortable working near the washer’s controls or plumbing connections.
Safety Notes
- Turn off the water at shutoff valves before working on hoses or fittings to avoid sudden flooding.
- Be cautious of hot water when testing the shower — sudden pressure changes can produce scalding.
- Do not attempt electrical work on the washing machine yourself. If the washer’s electrical systems are involved, call a qualified technician.
Common Homeowner Questions
- Why does the shower only drop when the washer runs?
Because the washer and shower are sharing limited supply flow; the washer’s fill uses enough water to reduce what’s left for the shower. - Can I fix this myself?
Do basic checks (valves open, hoses not kinked). For PRV adjustments, re-piping, or unclear causes, hire a plumber. - Will a bigger pipe solve it?
Often yes — increasing pipe size or adding a dedicated feed raises available flow, but a professional should confirm the right solution.