Quick Answer:
When your washing machine starts and the whole house sees a brief pressure drop, the most likely cause is a short, high water demand from the washer that outpaces supply or the pressure regulator reacting. Before assuming a broken pipe or a bad fixture, run a few simple checks: Test pressure with fixtures isolated, watch the meter to observe meter flow patterns, and confirm the PRV setting. These will tell you whether it’s a demand-related issue or something that needs repair.
Why This Happens
Washing machines fill quickly in bursts. If the supply side (incoming municipal pressure, service line size, or a reduced PRV setting) can’t keep up, pressure at every tap drops briefly. The same basic pattern shows up with other cyclical loads like ice makers or irrigation—short pulses of flow that reduce system pressure during the fill.
Step-by-Step What to Do
1. Prepare and isolate
- Turn off the washing machine and any other appliances that use water (dishwasher, icemaker, irrigation) so you can run controlled tests.
- Close the shut-off valves at other fixtures or simply make sure no other taps are open. The goal is to test one condition at a time.
2. Test pressure with fixtures isolated
- Open a single cold-water tap (preferably at a laundry or utility sink) and note the steady pressure and flow.
- With other fixtures isolated, start a washing-machine fill cycle and watch how that one open tap behaves. If pressure dips only while the washer fills, the issue is demand-related rather than a single faulty fixture.
3. Observe meter flow patterns
- Stand near the water meter and watch the register or flow indicator while the washer fills. Short, repeated pulses are normal for fill valves; long continuous flow or rapid spinning suggests a larger demand or leak.
- Record how long the pressure stays low and whether it recovers quickly after each fill pulse.
4. Confirm PRV setting
- Locate the pressure-reducing valve (PRV) near the main shutoff or where the supply enters the house. Check the system pressure with a gauge while the system is idle and while the washer is filling.
- If pressure drops a lot during demand but the PRV is set unusually low, the PRV may need adjustment or replacement. If you’re not comfortable adjusting it, note the readings and call a pro (see below).
5. Interpret results and next steps
- If isolated testing shows normal pressure except during washer fills, this is a demand-related pattern and not a failing fixture.
- If pressure is low even with no demand, or meter shows continuous unexpected flow, investigate leaks, service line restrictions, or a failing PRV.
What Not to Do
- Do not assume fixture failure when pattern is demand-related.
- Do not immediately replace the washing machine or other appliances without confirming the pressure pattern with the tests above.
- Do not force or overtighten plumbing components while diagnosing; that can cause damage or leaks.
When to Call a Professional
- If the meter shows continuous flow when all fixtures and appliances are turned off (possible hidden leak).
- If adjusting or testing the PRV is needed but you’re not comfortable doing it—PRV work can require specific tools and experience.
- If pressure drops are large, prolonged, or happening at multiple times beyond washing cycles; that can indicate service-line limitations or municipal supply issues.
Safety Notes
- Turn off electrical appliances at the wall before working around them or turning valves nearby to avoid shocks or damage.
- When closing the main shutoff, do it gently—sudden valve movement can surge the system and damage fixtures.
- If you suspect a gas appliance or complex plumbing work is involved, call a licensed technician rather than attempting risky repairs yourself.
Common Homeowner Questions
- Is a brief pressure drop normal when the washer runs?
Yes. Short dips during fill cycles are common if supply or PRV settings are marginal. - Will a new washer fix the problem?
Unlikely—new washers have similar fill rates. Fixes focus on supply capacity or PRV adjustments. - Can I adjust the PRV myself?
Sometimes, but if you’re unsure, hire a plumber to avoid incorrect settings or damage.
For more related articles, see the Pressure Drops When Multiple Fixtures Run hub.