Water Pressure Drops: Why It Happens and How to Fix It

Pressure drops when the toilet flushes, when the dishwasher runs, or throughout the day without apparent cause — each pattern points to a different cause. Identifying which type of pressure drop you have is the first step to the right fix.

Why Does My Water Pressure Drop When Someone Flushes the Toilet?

A brief pressure drop when the toilet flushes — a momentary reduction in shower flow, for example — is normal in most plumbing systems. The fill valve draws about 1–3 gallons per minute while refilling the tank, which is a meaningful additional demand on the supply pipe.

When it’s normal: A brief, momentary reduction lasting 10–30 seconds while the toilet refills, then returning to normal — this is expected behavior in most homes.

When it’s a problem:
– The pressure drop is significant (shower becomes barely usable)
– The drop lasts more than 60–90 seconds
– The same thing happens with any additional water use

Causes of excessive toilet-flush drops:
– Undersized supply pipe: 1/2-inch supply pipe throughout the house doesn’t handle simultaneous demands well. In older Seattle homes, original supply piping may be undersized for modern fixture counts.
– Galvanized pipe restriction: Corroded galvanized pipe has reduced interior diameter — less total flow capacity available. Adding the toilet fill demand to a running shower exceeds the available capacity.
– PRV set too low: A pressure-reducing valve set to 45–50 PSI leaves less headroom — any additional demand causes a noticeable drop.
– Partially closed main valve: If the main shut-off valve is not fully open, the reduced flow capacity becomes apparent under concurrent demand.

Water Pressure Keeps Dropping Throughout the Day — What Causes It?

Pressure that varies throughout the day without any pattern tied to fixture use:

City supply pressure variation: Municipal water pressure isn’t constant — it fluctuates with demand. Peak demand periods (morning, evening) can reduce street pressure by 10–20 PSI. This is normal system behavior.

PRV response to supply variation: If the PRV is at or near its lower limit when city pressure drops, the regulated pressure inside the home may become inadequate. A PRV adjustment or replacement addresses this.

Partially failing PRV: A PRV that’s deteriorating may not maintain consistent pressure — it sets pressure inconsistently, causing the random variation you’re experiencing. PRV replacement solves this.

Slow leak: A leak in the supply line (inside or outside the home) reduces available pressure as water is lost. Check your water meter: shut off all fixtures and read the meter. Wait 30 minutes. If the meter has moved, there’s an active leak.

Galvanized pipe with heavy restriction: If supply pipe restriction is significant, any variation in city supply pressure is amplified — you’re working with less headroom and the fluctuations become more noticeable.

Why Does Pressure Drop When the Washing Machine Runs?

Washing machine fill cycles draw 2–4 gallons per minute — one of the highest instantaneous demands in a residential plumbing system. The fill valve opens, a large volume of water is demanded simultaneously, and pressure at other fixtures drops.

If the drop is moderate and brief (lasts only during the fill cycle): This is normal behavior in many homes. It indicates the supply pipe is handling close to its capacity, but not necessarily failing.

If the drop is severe (other fixtures become barely functional):
– Supply pipe is undersized (1/2-inch throughout an older home)
– Galvanized buildup is restricting the main supply line
– PRV is set too low
– Main valve is partially closed

Practical approach: If the pressure drop during the washing machine fill is bothering you, check the main valve (confirm it’s fully open) and test the supply pressure at a hose bib. If pressure is adequate at the hose bib (above 60 PSI) and the main valve is fully open, the cause is likely pipe restriction from galvanized buildup or undersized pipe — both of which are infrastructure issues rather than simple adjustments.

Water Pressure Drops Suddenly — Could It Be a Leak?

A sudden, significant pressure drop throughout the house — not tied to fixture use, happening at once — warrants immediate investigation for a leak.

The meter test:
1. Shut off all water in the house (close all faucets, ensure no appliances are running)
2. Go to the water meter and read it
3. Wait 15–30 minutes without using any water
4. Read the meter again — if it has moved, there’s an active leak somewhere

What sudden pressure drops with a leak look like:
– Pressure drops noticeably at all fixtures simultaneously
– The water bill has increased without a change in usage
– There may be a wet spot in the yard above the service line, or unexplained wet areas in the basement or crawl space

What to do: If the meter test confirms a leak, shut off the main water supply and call a plumber. A leak in the main service line requires professional diagnosis and repair.

Pressure Drops Only Upstairs — Is That a Pipe Problem?

Pressure at upper floors is naturally lower than at ground level — for every foot of elevation, you lose approximately 0.43 PSI. A two-story home with 8-foot ceilings may have 3–4 PSI less at the second floor than at the first.

When it’s structural (not a problem): Small pressure differences between floors are inherent. If second-floor pressure is slightly lower but still adequate (above 45–50 PSI), no action is needed.

When it’s a problem:
– Second-floor pressure is dramatically less than first floor (more than 10–15 PSI difference at the same flow)
– Upstairs fixtures barely work while downstairs is normal
– The issue developed recently (wasn’t always this way)

Possible causes:
– Partially closed isolation valve on the upstairs supply branch
– Galvanized pipe restriction in the branch that serves the upper floor
– Supply line sized too small for upper-floor runs (longer pipe = more friction loss)

Diagnosis: Compare pressure at an upstairs hose bib or fixture with the downstairs measurement. If the difference exceeds 10–15 PSI with no flow, there’s a restriction in the upstairs supply branch.

Whole House Water Pressure Dropped Overnight — What Happened?

City supply event: Main breaks, scheduled pressure testing, or valve operations on the city distribution main can temporarily reduce street pressure. Check the Seattle Public Utilities website or call SPU to ask if there’s a known event in your area.

PRV failure: A PRV that fails can cause sudden pressure reduction (if it fails closed or partially closed) or complete loss of pressure regulation. Check pressure at the hose bib closest to the meter — if it’s adequate there, the PRV has failed on the downstream side. If it’s low there too, the issue is in the city supply.

Partial main valve closure: If someone performed any work near the main shut-off valve, it may not have been reopened fully. Check that the main valve (near the meter or at the house entrance) is fully open.

Overnight temperature drop and pipe expansion: Rarely, severe overnight cold can affect supply lines in unconditioned spaces. More likely to affect outdoor hose bibs than indoor supply.

Meter valve issue: The meter valve (which the city controls) may have been partially closed during maintenance. Contact SPU if you suspect a meter issue.

Why Does My Shower Pressure Drop When the Dishwasher Runs?

Same mechanism as the toilet and washing machine — the dishwasher adds concurrent demand during its fill cycle. Dishwashers typically fill with 3–6 gallons per cycle, demanding 1–3 GPM from the supply.

Why the shower is particularly sensitive: A shower valve is already drawing 1.5–2.5 GPM. Adding the dishwasher demand can push the system close to its flow capacity limit — especially if:
– Supply pipe is 1/2-inch throughout
– Galvanized pipe has significant scale buildup
– Main valve is partially closed
– PRV is set low

Pressure-balancing shower valves: Required in new construction (and most renovated bathrooms), these valves maintain the ratio of hot to cold water when pressure fluctuates — so you don’t get scalded when the cold pressure drops due to a dishwasher fill. They don’t prevent the pressure reduction, but they prevent the dangerous temperature shift.

How Do I Tell If a Pressure Drop Is From My Pipes or the City Supply?

The hose bib test:
1. Connect a pressure gauge to an outdoor hose bib (the bib is typically on the main supply line before the distribution system branches through the house)
2. Test with all fixtures off — this is the baseline street pressure you’re receiving
3. Compare to the measured pressure at interior fixtures with the same conditions

If hose bib pressure is adequate (65–75 PSI typical Seattle municipal) and interior pressure is significantly lower:
→ The problem is inside your house (PRV, main valve, galvanized restriction)

If hose bib pressure is also low:
→ The problem is in the city supply or the service line from the main to your meter

The neighbor comparison: Ask a neighbor on the same block if their pressure is normal. If their pressure is also low, it’s a city supply issue. If their pressure is fine, the problem is in your service connection or internal plumbing.

Is a Sudden Pressure Drop Always a Sign of a Problem?

Not always — but sudden drops warrant a quick check.

Usually not a problem:
– Brief drops during washing machine or dishwasher fill cycles (lasting 2–5 minutes)
– Modest pressure reduction during toilet refill (lasting 30–90 seconds)
– Slight pressure variation at different times of day (city supply demand fluctuation)

Warrants investigation:
– Sudden pressure drop at all fixtures simultaneously with no fixture in use
– Pressure drop accompanied by a running water sound in walls
– Pressure drop that doesn’t return to normal
– Sudden drop accompanied by wet spots or unexplained water

Warrants immediate action:
– Pressure drop accompanied by visible water damage or active flooding
– Pressure drop with the sound of running water in walls or under floor

How to Stop Water Pressure From Dropping When Multiple Fixtures Are On

Step 1: Confirm main valve is fully open. The main shut-off valve (at the meter or entry to the house) must be completely open. A partially open gate valve reduces flow capacity significantly.

Step 2: Test and adjust PRV if present. A pressure-reducing valve set below 60 PSI gives limited headroom for concurrent demands. Adjusting to 65–70 PSI may be all that’s needed.

Step 3: Clean aerators and fixtures. Clogged aerators in individual fixtures increase apparent restriction and amplify the pressure drop effect.

Step 4: Check for galvanized pipe restriction. If the home has galvanized supply pipe, interior scale buildup is the most likely cause of severe concurrent-use pressure drops. Compare pressure at the meter vs. inside the house — a large difference (more than 15 PSI) indicates pipe restriction.

Longer-term solutions:
– Repiping galvanized supply: Removes the restriction permanently
– Pressure booster pump: An electric pump that boosts supply pressure — appropriate when city supply pressure is inadequate and pipe restriction isn’t the cause
– Main supply line upsizing: Increasing the service line from 3/4-inch to 1-inch (if the service line is undersized) improves flow capacity

FAQ

Q: Why does my water pressure drop when someone flushes the toilet?
A: The toilet fill valve adds 1–3 GPM to the concurrent demand. If the supply pipe is at near-capacity, this additional demand causes a brief pressure drop at other fixtures. A momentary reduction while the tank refills is normal. A severe drop that lasts more than 90 seconds warrants checking the main valve, PRV setting, and whether galvanized pipe restriction is limiting total flow.

Q: Water pressure keeps dropping throughout the day — what causes it?
A: Most often city supply pressure variation (normal — pressure is lower during peak demand hours), or a partially failing PRV that doesn’t regulate consistently. Test pressure at various times and compare morning vs. evening readings to establish whether it’s demand-related.

Q: Water pressure dropped suddenly — could it be a leak?
A: Yes. The meter test confirms it: shut off all fixtures, read the meter, wait 15 minutes, read again. If the meter moves, there’s an active leak. A sudden whole-house pressure drop with no city supply event is a leak until proven otherwise.

Q: How do I know if a pressure drop is from my pipes or the city supply?
A: Test pressure at the outdoor hose bib (which is typically on the main supply before your interior distribution branches). If hose bib pressure is adequate and interior pressure is low, the problem is inside your house. If hose bib pressure is also low, the city supply or service line is the cause.

Q: How do I stop water pressure from dropping when multiple fixtures are on?
A: First, confirm the main valve is fully open. Adjust the PRV if it’s set below 60 PSI. Clean aerators. If galvanized pipe restriction is causing the problem (compare meter pressure vs. indoor pressure), repiping is the long-term solution. A pressure booster pump is an alternative if the city supply itself is inadequate.