Quick Answer:
A dishwasher filling can cause nearby taps to lose pressure because the machine draws a short, high-volume flow from the same supply line. Start by testing pressure with fixtures isolated, watching your water meter for flow patterns, and checking the pressure-reducing valve (PRV) setting. These checks will tell you whether the issue is local, a household-supply limit, or a city-supply issue.
Why This Happens
- The dishwasher’s fill valve opens briefly and draws a lot of water quickly. If the supply branch or municipal line can’t keep up, pressure at nearby fixtures drops.
- A stuck or improperly set PRV can reduce available pressure under flow conditions even if static pressure looks normal at rest.
- Supply constraints from the street or peak-demand times can look like a dishwasher problem when it’s really a system-wide limitation. See Pressure drops during peak city hours for more on supply-side limits.
- If hot and cold mix in the plumbing under combined demand, you may notice hot water behavior similar to other multi-tap problems — for a related pattern, see Hot water disappears when multiple taps open.
Step-by-Step What to Do
1. Test pressure with fixtures isolated
Confirm whether the dishwasher is pulling down the whole house or just a nearby tap:
- Turn off all faucets, showers, and appliance valves in the house.
- Attach a pressure gauge to an accessible hose bib or use a quick-connect gauge at an under-sink valve. Note the static pressure (no flow).
- With the gauge in place, start the dishwasher’s fill cycle (run a wash). Watch the gauge for pressure drop under flow.
- If static pressure is normal but drops sharply during fill, the issue is demand-related rather than a permanent loss of supply.
2. Observe meter flow patterns
Use the house water meter to confirm when and how much water is flowing:
- With all fixtures off, watch the meter. It should be still. Start the dishwasher and watch the meter dial or digital readout.
- A short, fast movement shows a high instantaneous flow. A long, steady movement during multiple fixtures indicates broader demand.
- Record when drops happen — if they match city peak times or other appliances, the meter pattern helps locate the source of demand.
3. Confirm PRV setting
Locate the PRV near the main shutoff if your house has one. A PRV can be mis-set or failing:
- Check static pressure at a hose bib. Then open a high-flow device (dishwasher or washing machine) and compare the pressure under flow.
- If pressure falls far below the PRV’s setpoint under flow, the valve may need adjustment or replacement.
- Adjusting a PRV is straightforward but can require a wrench and experience. If you’re unsure, record your readings and call a plumber.
4. Inspect local valves and the dishwasher fill assembly
Rule out simple local restrictions before assuming a supply problem:
- Check the dishwasher inlet valve and the supply hose for kinks or partial blockages.
- Confirm the shutoff valve to the dishwasher is fully open.
- If pressure tests show demand-related drops and the dishwasher hardware looks suspect, replace the supply hose or inlet valve as a next step.
What Not to Do
- Do not assume fixture failure when pattern is demand-related.
- Don’t close or partially restrict nearby valves to “fix” a pressure drop — that masks the real issue and can create other problems.
- Avoid opening or modifying the main supply or PRV without proper tools and knowledge; improper adjustments can cause overpressure or damage.
When to Call a Professional
- If adjusting or testing the PRV is needed but you’re not comfortable doing it.
- If meter and gauge tests show large pressure drops at modest flows — this can indicate a supply-line restriction, failing PRV, or main-line issue.
- If you see inconsistent pressure combined with leaks, noisy pipes, or repeated appliance problems.
Safety Notes
- Turn off power to the dishwasher at the breaker before inspecting electrical components or removing panels.
- Use a proper pressure gauge rated for household water systems. Do not use improvised fittings that may leak under pressure.
- Wear eye protection when working near valves and use towels to catch small leaks.
Common Homeowner Questions
- Why does my faucet go weak only when the dishwasher fills?
Because the dishwasher draws a short, high flow from the same pipe, reducing pressure at nearby outlets during the fill cycle. - Can I fix this myself?
Yes for basic tests and simple fixes (check supply hose, valves, and observe the meter). Call a plumber for PRV work or if tests point to supply-line problems. - Will a larger supply line solve it?
Sometimes, but only if the bottleneck is the house branch line. If the limiter is the street supply or PRV, a larger internal line won’t help; have a pro diagnose before repiping.
For more related articles, see the Pressure Drops When Multiple Fixtures Run hub.