Quick Answer:
If the shower loses force only while the washing machine is filling, the house supply is being shared and the washer is reducing flow to the shower. First run the shower by itself, then start a washer fill and watch the change. If the drop happens only during washer use, reduce simultaneous demand and consider checking the pressure-reducing valve (PRV) or overall house pressure.
Why This Happens
Most homes have a limited water flow available at one time. When two fixtures draw water at once—from the shower and the washer—the available flow divides. A few common reasons for a noticeable drop:
- Shared supply lines or narrow piping limit simultaneous flow.
- A partially closed shutoff or mixing valve can magnify the perceived drop by restricting one branch more than another.
- A city supply dip or a pressure-reducing valve set too low will reduce baseline pressure.
- Appliance fill valves add demand instantly when they open, causing a temporary dip.
If you see pressure issues that started after municipal work or seasonal changes, related articles like Low pressure after city outage or Shower pressure worse after winterization may help explain specific causes.
Step-by-Step What to Do
Step 1 — Observe the shower alone
Run the shower with no other water-using appliances on. Note how strong the flow is and whether it stays steady for a minute or two.
Step 2 — Start the washer fill and watch
With the shower still running, begin a washer fill (or start a cycle that fills). Watch how the shower pressure changes immediately and over 30–60 seconds. This confirms whether the washer draw is the cause.
Step 3 — Compare cold vs hot supply
Try the same test with only hot water and then only cold (if your faucet lets you). If one side drops more, a valve, mixer cartridge, or the water heater branch might be contributing.
Step 4 — Check visible valves and shutoffs
Inspect the main shutoff and any local shower or branch valves for partially closed positions. Open them fully if safe to do and observe whether pressure improves.
Step 5 — Reduce simultaneous demand
If the washer is clearly the trigger, stagger laundry and showers so they don’t run at the same time. This is the simplest household fix when supply capacity is the limit.
Step 6 — Consider PRV setting and house pressure
If you have a pressure-reducing valve or consistently low pressure even with no appliances running, note the pressure reading if you have a gauge, or hire a plumber to check the PRV and overall supply capacity.
What Not to Do
- Don’t repeatedly run washer and shower together to “test” under stress — doing so can overwork the washer fill valve and waste water without giving useful long-term information.
- Don’t assume a single partial valve is the only cause without checking other fixtures and the main supply first.
- If the supply can’t keep up, a plumber is appropriate to assess PRV/sizing rather than buying pumps or devices without a diagnosis.
- Don’t try to increase municipal pressure by tampering with city connections or unsafe modifications.
When to Call a Professional
Contact a licensed plumber if one or more of the following apply:
- Pressure is poor even when no appliances are running.
- You find a stuck or leaking PRV, or suspect the PRV needs adjustment or replacement.
- There are signs of leaks, pipe damage, or unusual noises when multiple fixtures run.
- You want an assessment of supply capacity, pipe sizing, or the feasibility of installing a booster pump.
If the issue started after municipal work or a seasonal change, a review of situations like Low pressure after city outage can be useful alongside a plumber’s inspection.
Safety Notes
- Hot water can scald—avoid testing hot flow full blast where you or others might be exposed.
- Turn off electrical devices near water before inspecting leaks or making changes.
- If you’re unsure about adjusting valves or the PRV, leave changes to a professional to avoid code or safety problems.
Common Homeowner Questions
- Why does the shower only dip when the washer fills? — The washer creates additional demand on the same supply, reducing flow available to the shower.
- Will tightening a valve fix it? — If a valve is partially closed, opening it can help, but systemic supply limits won’t be fixed by valve tweaks.
- Is a booster pump the answer? — Possibly, but only after a plumber confirms the problem isn’t a PRV setting, pipe sizing, or municipal supply issue.