Quick Answer:
If water pressure dropped across the house immediately after the main valve was replaced, the most likely causes are a valve that is not fully open or not seated correctly, debris caught in the valve or downstream lines, or an unexpected closed shutoff. First check an outdoor hose bib with a pressure gauge or a simple bucket test to confirm the supply pressure. Then visually inspect the new valve stem and seating. If you can’t safely restore normal pressure, call your water utility or a plumber.
Why This Happens
Replacing the main valve interrupts flow and can introduce dirt, small bits of packing or grit, or cause the new valve to sit slightly off-center. That can restrict flow even though the valve appears open. Other reasons include:
- Valve not rotated fully to the open position.
- Valve stem or seat mis-seated after installation.
- Debris trapped just downstream of the valve.
- Individual shutoffs or angle stops left closed during work.
If the whole system lost pressure and will not come back up after you open the valve, see System won’t repressurize after shutoff for related causes and checks. If some fixtures behave oddly once water is turned back on, check Fixtures malfunction after water turned back on.
Step-by-Step What to Do
1. Confirm there’s a supply and where pressure is lost
- Go to an outdoor hose bib (lowest and easiest point). Attach a pressure gauge if you have one, or run water into a bucket to judge flow. This tells you if the issue is before or after the main valve.
- If the hose bib has good pressure and indoor pressure is low, the problem is inside the house. If the hose bib is low, the issue is at the main valve or supply from the street.
2. Inspect the main valve stem and seating
- Visually check the valve stem: is it aligned, not bent, and does it move smoothly when you operate it?
- Gently cycle the valve: close it fully, then open it fully. This can help seat the mechanism correctly and clear small debris. Do not force a stuck stem.
- Look for signs of leaks around the valve and downstream piping while operating it.
3. Check for debris and downstream shutoffs
- Partially open a nearby fixture (hose bib or laundry faucet) to flush possible debris trapped after the valve change.
- Verify that any isolation valves inside the house are open: water meters, pressure regulators, and appliance shutoffs.
4. Isolate and test sections if needed
- If pressure is present at the hose bib but not indoors, close the house isolation valve and test individual branches (kitchen, bathrooms) to locate the restriction.
- Note any fixtures that sputter, run weakly, or drip—these help pinpoint the affected lines.
5. If pressure remains low, stop and get help
- If the valve stem is loose, or you find a suspected broken seat piece, do not continue to operate the valve—call the water utility or a plumber.
What Not to Do
- Don’t leave a mis-seated main valve — it can cause ongoing low pressure, damage to the valve, and sudden failure later.
- Don’t force a stuck stem with excessive torque; that can break the valve or plumbing.
- Don’t ignore a visible leak or water pooling after the replacement; shut the supply off and get professional help.
When to Call a Professional
- If you cannot get full open/close function on the main valve or the stem feels loose or stripped.
- If pressure is low at the supply side (hose bib) after re-seating attempts, which suggests a valve defect or meter/supply issue.
- If you discover active leaks, damaged piping, or debris that you cannot clear safely.
- When the system shows odd behavior across multiple fixtures after the work — a licensed plumber can diagnose or replace the valve correctly.
Safety Notes
- Turn off the water if you see a leak you cannot control. A second person is helpful for safety when working around wet floors or outdoor equipment.
- Use the correct tool for the valve and avoid strikes or makeshift tools that can damage the stem.
- If the replacement was done by the water utility or a contractor, contact them first — they may be responsible for correcting installation issues.
Common Homeowner Questions
- Q: How do I know the valve is fully open?
A: Most valves have a handle position that aligns with the pipe when open; cycle it fully open and check flow at a hose bib. - Q: Can trapped air cause low pressure after work?
A: Yes — briefly opening several fixtures to bleed air can restore normal flow. - Q: Is a new valve likely to fail right away?
A: Failures are uncommon but can happen if the valve is damaged during install or if debris prevents proper seating; call a pro if problems persist.