Quick Answer:
A slow, steady drop in water flow that worsens over hours or days usually means a progressive restriction in the supply line or control device. Common causes are a clogged sediment filter or cartridge, debris or corrosion collecting at a pressure-reducing valve (PRV) or shutoff valve, or a gate valve that no longer opens fully. Check filters, test a hose bib over time, and inspect service valves and the PRV screen/seat. Plan to replace any failing gate valve rather than trying to force it open.
Why This Happens
When flow fades gradually it’s not an instant break — it’s something narrowing the path. That narrowing can come from:
- Sediment or scale building up in a filter cartridge or screen.
- Debris or mineral deposits lodging at a PRV screen or seat, or the PRV itself losing function.
- A partially closed or deteriorating main shutoff or gate valve that no longer moves freely or seats correctly.
- Debris migrating from nearby work on the service line or from a failing piece of pipe that sheds material slowly.
Symptoms can overlap with other pressure problems; if the issue behaves differently when multiple taps are open, check the guidance in Water works only when multiple taps open. If you see an abrupt cutoff while showering, compare notes with Sudden loss of pressure during showers to separate causes.
Step-by-Step What to Do
1. Confirm the pattern and isolate the problem
- Test several fixtures. Note whether the fade happens everywhere or only in one branch.
- Open the main indoor shutoff fully, then test an outdoor hose bib nearest the service entry. This gives a simple benchmark of supply-side flow.
2. Test flow at a hose bib over time
- Use a bucket and stopwatch to measure gallons (or liters) per minute at the hose bib. Record the rate immediately, then check again hourly or at intervals if the fade continues over hours/days.
- If flow drops steadily at the hose bib, the restriction is upstream (service line, meter, filter, PRV, or main valve).
3. Check and clean/replace the sediment filter cartridge
- Locate the sediment filter (usually near where the service enters the house or before a water softener). Shut off the inlet shutoff valve, relieve pressure by opening a faucet, then remove the housing.
- Inspect the cartridge for compacted sediment or brown staining. Clean reusable cartridges or replace disposable cartridges. Reassemble with a good O-ring and test flow again.
- If the housing is damaged or the cartridge is repeatedly clogged, replace the filter assembly or fit a finer/pre-filter depending on source water.
4. Inspect the PRV screen/seat if serviceable
- Some PRVs have a removable cover or a small screen you can clean. Follow the manufacturer’s instructions: relieve downstream pressure, remove the cover or bonnet slowly, and inspect the screen and seat for deposits or debris.
- Clean gently with water and a soft brush. Reassemble and re-set the outlet pressure if needed. If the PRV is not serviceable or it still restricts flow after cleaning, replacement is the safer long-term fix.
5. Check the main shutoff and gate valves — plan replacement if failing
- Gate valves can appear “open” but the internal gate may bind or corrode, preventing full flow. If a valve is stiff, leaks, or you suspect it isn’t opening fully, plan to replace it rather than forcing it.
- Ball valves are a common upgrade because they provide a reliable full bore when open. Valve replacement may require shutting off water service or calling your water utility if work is on the service side.
6. If cleaning and basic checks don’t restore flow
- Contact your water utility to ask if there’s work or a blockage at the meter or in the street. They can sometimes isolate the problem on their side.
- Document your flow tests and steps taken — this helps a professional or the utility find the issue faster.
What Not to Do
- Don’t wait for total failure — progressive fade is a warning sign you can catch early.
- Don’t force or hammer valves that are stuck; that can break parts and make repairs harder or cause a sudden loss of service.
- Don’t disassemble a PRV or service device without relieving pressure first or without knowing the part is serviceable — springs and parts may eject under pressure.
- Don’t operate plumbing work that requires the water utility’s isolation without coordinating with them.
When to Call a Professional
- If the flow measured at the hose bib drops and you can’t find a cleanable filter or screen; the issue may be inside the meter or service line and needs trained equipment.
- If the PRV appears damaged, leaks, or cannot be safely serviced, call a plumber who can replace and properly set a new PRV.
- If the main shutoff or gate valve needs replacement and you’re not comfortable isolating or draining the system — a plumber or the utility can perform the replacement with minimal disruption.
- When you suspect the municipal side (meter or main) is the problem — the water utility should be involved.
Safety Notes
- Always relieve pressure before opening filter housings, PRV bonnets, or valve covers by opening a downstream faucet.
- Shut off power to appliances (water heaters, pumps) if you’ll be working on supply lines feeding them.
- Use gloves and eye protection when handling dirty cartridges or cleaning screens.
- If you’re unsure about cutting pipe, soldering, or working on the service side of the meter, hire a licensed plumber — mistakes can cause flooding or contaminate drinking water.
Common Homeowner Questions
- Why does pressure drop slowly instead of suddenly?
A slow drop usually means a partial blockage or a device losing capacity rather than a complete break or valve shutoff. - Can I replace a PRV myself?
Maybe, if you’re comfortable relieving pressure and following the manufacturer’s instructions; otherwise hire a plumber because incorrect installation can cause water damage or pressure issues. - How urgent is a fading water supply?
It’s moderately urgent — address it within a few days. Early attention can avoid a complete loss of service and a more expensive repair.