Quick answer
Sudden low water pressure across the whole house is usually caused by a partially closed main valve, a clogged whole-house filter or water softener, a failed pressure-reducing valve (PRV), a municipal supply issue, or a leak. Start by confirming it’s truly house-wide, then measure pressure with a hose-thread gauge, check and fully open valves, replace any clogged filters, bypass the softener, and adjust or replace the PRV if needed. If you’re on a well, check the pressure tank and switch. Call a pro if pressure stays low after these checks or you find signs of a hidden leak.
What’s happening and why
A sudden drop points to a single upstream issue affecting the whole system. The most common culprits are:
- Main shutoff not fully open after recent work.
- Clogged whole-house sediment/carbon filter or softener resin bed.
- PRV stuck or failed.
- City-side problem (hydrant flushing, main break) or a closed curb stop.
- Leak between the meter and the home’s plumbing.
- For wells: failing pump, stuck pressure switch, waterlogged tank.
Target house pressure: 50–75 psi
Below 40 psi: Noticeably weak
Above 80 psi: Too high (risk of damage)
Step-by-step diagnosis and fixes
1) Confirm the scope (5–10 minutes)
- Test multiple fixtures on both floors, and both hot and cold. If only one fixture is weak, clean its aerator/showerhead instead.
- Ask a neighbor if they also have low pressure (municipal clue).
2) Measure your water pressure (10 minutes)
- Attach a hose-thread pressure gauge to an outdoor spigot closest to the meter, or to the laundry faucet.
- Open fully and read static pressure. Open a second indoor faucet to see “flowing” pressure.
- If static is normal (50–75 psi) but pressure collapses under flow, suspect a restriction (filter/softener/partially closed valve).
3) Check valves (5 minutes)
- Locate the city/meter valve and your house main shutoff. Verify both are fully open.
- Ball valve: handle inline with pipe.
- Gate/stop valve: turn counterclockwise to stop, then back a quarter-turn.
- Also verify the water heater cold inlet is fully open. If hot is low but cold is normal, check heater inlet or sediment buildup.
4) Inspect and service point-of-entry equipment (15–30 minutes)
- Whole-house filter: If the housing pressure gauge reads low or the filter’s overdue, replace the cartridge. A clogged filter can drop pressure house-wide.
- Water softener: Put in bypass and retest pressure. If pressure returns, the softener or its prefilter is restricting flow—service or replace.
- Flush lines after changes: run a tub spout for 2–3 minutes.
5) Evaluate the PRV (20–60 minutes)
- Find the bell-shaped pressure-reducing valve near the main. Check pressure again downstream of it.
- Adjustment: Loosen locknut, turn screw clockwise slowly to increase pressure, counterclockwise to reduce. Make 1/2–1 turn, retest.
- If no response or pressure hunts/surges, the PRV may be failing—replace it.
- Replacement is a moderate DIY (cutting pipe/compression unions). If you’re not comfortable sweating copper or threading fittings, call a plumber.
6) Look for leaks (10–30 minutes)
- With all fixtures off, watch the water meter’s leak indicator. If it spins, you have a leak.
- Walk the property: soggy patches, hissing in walls, constantly running toilets. Fixing a running toilet can restore pressure and reduce noise.
7) On a well? (15–45 minutes)
- Check pressure switch cut-in/cut-out. If stuck or points are burnt, replace switch.
- Ensure tank pressure (air side) is 2 psi below cut-in when empty. If the tank is waterlogged (no air cushion), you’ll see pressure swings and poor flow.
- Verify the pump runs and the breaker hasn’t tripped.
Tools and materials
- Hose-thread water pressure gauge (0–100 psi)
- Adjustable wrench or channel-lock pliers
- Screwdrivers, flashlight, bucket, towels
- Replacement whole-house filter cartridge and wrench
- PRV repair kit or replacement PRV (if needed)
- Plumber’s tape (PTFE) and pipe dope for threaded joints
Approximate costs:
- Gauge: $10–20
- Filter cartridges: $10–40
- PRV: $60–200 (pro install $250–600)
- Plumber diagnostic visit: $100–200
Safety considerations
- Turn off water and relieve pressure before opening housings or cutting pipe.
- For gas water heaters, avoid extinguishing the pilot with splashes; if you do, relight per manufacturer instructions.
- Electrical safety on well systems: shut power at the breaker before opening pressure switch or working near wiring.
- Avoid scalds: after changes, run cold first, then re-balance mixing valves.
Tips for best results
- Keep spare filter cartridges and note the change date on the housing.
- Install a permanent gauge downstream of the PRV for quick checks.
- If municipal pressure varies, a pressure-boosting system (pump + small tank) can stabilize flow.
- Exercise your shutoff valves twice a year to prevent them seizing.
Common mistakes
- Leaving the main or water heater valve partially closed after service.
- Over-tightening the PRV adjustment screw—small turns, test, repeat.
- Replacing a filter without depressurizing the housing (cracked canisters are common).
- Ignoring a constantly running toilet; it can mask bigger issues and waste water.
When to call a professional
- Pressure remains under 40 psi after valve checks, filter/softener bypass, and PRV adjustment.
- Evidence of a slab or underground service line leak (wet spots, meter spinning with fixtures off).
- You’re uncomfortable replacing a PRV or working on well electrical components.
- Brown water, air in lines, or knocking noises persist—possible main break or system contamination (contact your water utility first).
With a gauge, a few simple checks, and basic maintenance, most sudden pressure drops can be identified and corrected in under an hour. If the readings don’t make sense or you suspect a leak you can’t locate, bring in a licensed plumber to protect your home and water quality.