Quick Answer:
If water flow is weak right after installing a new faucet, the most common causes are debris trapped in the cartridge or aerator, partially closed or clogged shutoff stops, or kinked/misconnected supply lines. Start by checking the supply stops, removing and flushing the cartridge and aerator, and confirming lines are properly routed.
Why This Happens
- During installation small bits of plumber’s tape, solder, paint, or packing material can enter the valve body or cartridge and block passages.
- Many modern faucets rely on a cartridge with narrow ports; a tiny speck can drastically reduce flow.
- Shutoff stops under the sink may not be fully open, or debris in the stops can restrict flow after work has been done.
- Supply hoses can be crossed, kinked, or trapped in the cabinet so they don’t allow full flow.
Step-by-Step What to Do
Step 1 — Confirm the shutoff stops
- Under the sink, locate the hot and cold shutoff valves (stops). Turn each fully counterclockwise to open them.
- If a stop feels stiff or you only get a little movement, do not force it. Note whether flow improves when the handle is moved.
Step 2 — Check supply lines for kinks or wrong connection
- Visually inspect the flexible supply lines from the stops to the faucet. Make sure they are not kinked, pinched, or looped tightly.
- Confirm the hot and cold lines are connected to the correct faucet inlets and not crossed in a way that introduces unexpected restriction.
Step 3 — Remove and inspect the aerator
- Unscrew the aerator from the spout tip and look for debris or mineral buildup. Rinse it under water and run the faucet without the aerator to see if flow returns.
Step 4 — Flush the lines and cartridge
- Turn off the stops, remove the cartridge or valve stem per the manufacturer’s instructions, and look for shavings, tape bits, or packing material.
- With the cartridge out, briefly open the shutoff stops to flush water into a bucket — this clears debris from the supply and valve body. Close the stops before reinstalling parts.
- Reinstall the cartridge and test flow again with the aerator off.
Step 5 — Inspect for installer tape or debris inside
- Check the valve body and cartridge ports for plumber’s tape fragments or manufacturing debris. Even tiny flakes can block a port.
- Clean parts gently with water and a soft brush; avoid harsh tools that could damage seals.
Step 6 — Test and narrow the problem
- Compare flow at the faucet to other fixtures. If only one side (hot or cold) is weak, focus on that supply stop and cartridge inlet.
- If the issue varies by time of day, think about municipal pressure or system-wide issues — see Pressure fine in morning but poor at night for related causes.
- If only the cold side is affected at this fixture, that situation is covered in Low pressure on cold side only at one fixture.
What Not to Do
- Don’t assume the new faucet is defective—debris in the cartridge is far more common.
- Don’t overtighten fittings or force stuck stops; that can damage parts and make repairs harder.
- Don’t use improvised tools that can chip valve seats or crack plastic cartridges; use the correct screwdriver or cartridge puller if needed.
When to Call a Professional
- If flushing and cleaning do not restore normal flow after you’ve checked lines and stops.
- If shutoff valves are seized, leaking, or damaged and you are not comfortable replacing them.
- When you suspect a deeper plumbing issue (blocked supply in the house, sudden big drop in pressure across multiple fixtures) that simple faucet work won’t fix.
Safety Notes
- Always turn off the shutoff stops before dismantling the faucet. Keep a bucket or towel under the work area for residual water.
- If you need to turn off the house water main, know how to do that and where it is before you start.
- Handle cartridges and seals gently; some parts are plastic and can break if pried with metal tools.
Common Homeowner Questions
- Q: Can mineral buildup cause this right after installation? A: It’s unlikely immediately after a new faucet installation unless old lines were flushed; debris from installation is more common.
- Q: Will running the faucet full blast clear the blockage? A: Sometimes flushing with the cartridge removed will, but persistent blockages usually require removing and cleaning the affected part.
- Q: Is a pressure regulator involved? A: If the low flow is at many fixtures, your home regulator or municipal supply could be involved; for a single newly replaced faucet, the issue is usually local to the fixture.
For more related articles, see the Low Pressure at Single Fixture hub.