Quick Answer:
If the shower feels weak only on the cold side, first compare cold pressure at a nearby sink to confirm it’s the shower, then check that the shower’s cold-side shutoff (angle stop) is fully open. If the valve has small inlet screens or a cartridge, they can collect debris — cleaning those screens often restores flow. If ports inside the valve look clogged, call a plumber rather than forcing parts.
Why This Happens
Two common causes produce low cold flow at just one shower: a restricted cold supply upstream (closed or partially closed stop, debris in the line, or a clogged mixer port) or internal valve blockage (screens, cartridge passages). Imbalance can also come from a failing pressure-balancing cartridge that favors hot over cold. It’s usually local to that fixture, which is why you should compare pressure at a nearby sink first.
Step-by-Step What to Do
Step 1 — Compare pressure at a nearby sink
Run the cold tap at a sink close to the shower. If the sink has normal cold pressure, the problem is likely at the shower valve or its immediate supply. If the sink is weak too, the issue is upstream and may affect the whole line.
Step 2 — Verify the cold-side shutoff is open
Locate the cold-side shutoff (angle stop) that feeds the shower. Many showers have a small valve in an access panel or under a nearby cabinet. Turn it fully counterclockwise. If it was partly closed, opening it should restore flow.
Step 3 — Test the shower with the cartridge removed or the handle off (if comfortable)
With the water turned off at the supply, remove the trim and handle to access the cartridge only if you are comfortable doing so. Briefly turn the cold supply back on to see if raw cold water flows freely into the valve body (catch water in a bucket). No or very weak flow here points to a clogged inlet or valve port.
Step 4 — Inspect and clean inlet screens and cartridge screens
Many valves have small mesh screens on the cold inlet or on the cartridge itself. These catch sediment and can reduce flow. Turn off water, remove the screen(s) and rinse them under running water, brushing gently with a soft brush. Reassemble and test. Cleaning these screens often fixes the problem.
Step 5 — Check for buildup in cartridge ports
If screens are clean but flow is still weak, the cartridge ports or valve passages may have mineral deposits. You can try soaking metal screens or non-delicate cartridge parts in a vinegar solution to loosen mineral scale, but do not force or pry plastic parts. If ports still appear blocked, stop and call a professional.
What Not to Do
- Don’t ignore a cold-side restriction — imbalanced flow can cause unstable temperature and sudden hot spikes.
- Don’t force the stop valve or internal parts if they feel seized; excessive force can break fittings or create leaks.
- If the cold stop or cartridge ports seem clogged, a plumber is appropriate rather than attempting aggressive drilling or reaming of passages.
When to Call a Professional
Call a licensed plumber if:
- You confirm the cold inlet flow into the valve is weak after the stop and screens are checked.
- Cartridge ports or valve passages look clogged and cleaning didn’t help.
- There’s corrosion, broken parts, or you’re not comfortable shutting off supplies and removing the cartridge.
A pro can disassemble the valve safely, clear internal passages, or replace the cartridge and restore balanced flow.
Safety Notes
- Always shut off the water supply before removing trim or a cartridge. Turn the supply off at the angle stop or the main if needed.
- Beware of scalding when testing hot or mixed flow — open taps slowly and stand back from spray.
- Use basic tools and avoid excessive force. If a part won’t budge, stop and call a plumber to avoid damage.
Common Homeowner Questions
- Shower pressure weak in morning only — Could that be related? Answer: Yes; morning low pressure is often from shared municipal supply patterns or a tankless heater issue, which is different from an isolated cold-side restriction.
- Low pressure upstairs shower only — Is that the same problem? Answer: It can be similar, but a single upstairs shower may have its own shutoff or clogged inlet screens; check those first.
- Can I fix a clogged cartridge myself? Answer: You can clean screens and rinse parts, but if passages are calcified or the cartridge is seized, a plumber is safer.