Quick Answer:
If a single faucet won’t flow unless another tap is opened, it usually means a severe restriction, a stuck check valve (softener or backflow), or a failing pressure regulator (PRV). Start by removing aerators and testing with other taps, then confirm the main and meter valves are fully open. If the problem points to a stuck check or PRV, get a plumber involved.
Why This Happens
Plumbing systems rely on steady pressure and unrestricted paths. When one fixture only flows while another is open, something in the supply line or pressure-control devices is preventing normal flow. Common causes:
- Severe restriction or partial blockage in the pipe or a fitting.
- A stuck or partially closed check valve on a water softener, backflow preventer, or pressure-reducing valve (PRV) that won’t pass full flow until pressure changes.
- A failing PRV or regulator that needs a second tap to shift pressure balance.
This pattern can be related to other issues, including **Intermittent whole-house pressure drops**, so don’t ignore it.
Step-by-Step What to Do
1. Test the faucet basics
- Remove the aerator or screen from the problem faucet. Sediment or mineral build-up there can mimic worse problems.
- Open the faucet fully and note flow. If flow improves dramatically without the aerator, clean or replace the aerator.
2. Check if a second tap affects flow
- Open a second nearby cold-water tap (bathroom, kitchen, or an outside spigot). If the first faucet’s flow returns only when the second is opened, suspect a restriction or device that changes behavior with different flow/pressure conditions.
- This behavior points to valve/check/PRV interaction rather than a single faucet issue.
3. Verify main and meter valves
- Locate the home shutoff (main) valve and make sure it is fully open. A partially closed main can cause odd pressure effects.
- If you can access the water meter valve and it is within your property, confirm it is fully open. If the meter is in the street or locked, contact your water utility if you suspect it’s not open.
4. Check for a stuck check valve or softener/backflow issue
- If you have a water softener or backflow preventer, find the bypass lever and put the device in bypass (only if you know how to do this safely). If normal flow returns with the device bypassed, the check valve or internal part is likely stuck.
- Do not disassemble these devices unless you are trained; a plumber should inspect or service them.
5. Isolate hot vs. cold
- Test cold water only. If cold behaves differently from hot, the issue is on the cold supply side. If both sides are affected, it is likely upstream (main, PRV, or check valve).
6. Note patterns and document
- Take notes: which taps, which times, whether an outside spigot changes things, and whether municipal work or weather seems related. This helps a plumber diagnose quickly.
What Not to Do
- Don’t treat ‘needs two taps’ as normal—this often indicates a serious restriction or failing regulator.
- Don’t force or hammer valves, the meter, or attached devices. That can break seals or cause leaks.
- Don’t attempt to remove or modify a backflow preventer, PRV, or meter if you are not qualified; these devices can require licensed work and may be regulated by your water utility.
- Don’t keep using the workaround of opening two taps as a long-term fix—this masks an underlying failure that can get worse.
When to Call a Professional
Call a licensed plumber if:
- You confirm the device bypass test points to a stuck check valve, PRV, or internal softener/backflow problem.
- You cannot access or safely confirm the meter or main valve position.
- The problem started suddenly at all fixtures or coincides with a loss of supply; if that occurs alongside no flow anywhere, refer to **No water at any fixture suddenly** and contact your utility or plumber.
- There’s any sign of leaks, strange noises, or if work would require digging, removing the meter, or altering regulated devices.
Safety Notes
- Turn off fixtures before working on them and relieve pressure by opening a faucet.
- Do not attempt to open or modify a water meter or municipal shutoff on public property—contact the utility.
- When in doubt about bypassing a softener or handling backflow devices, stop and call a professional to avoid contamination or code violations.
Common Homeowner Questions
- Why does opening another tap help?
Because it changes pressure and flow patterns, allowing a restricted or stuck valve to move or pass flow temporarily. - Can I fix a stuck check valve myself?
If it’s on a softener or backflow device, avoid self-repair unless you are trained; bypassing for a short test is OK, but repairs should be by a plumber. - Will the water company fix this?
If the issue is inside your home plumbing the homeowner is responsible; if the meter or mains are closed or damaged, contact your water utility.