Quick Answer:
If the shower starts with good pressure but drops quickly, first check whether the drop happens after you change the temperature. If the flow falls when you move the handle toward hot or cold, the shower cartridge or pressure‑balance spool is a likely cause. If the drop happens regardless of temperature or affects other fixtures, the issue may be a restriction, a supply pressure problem, or equipment elsewhere in the house.
Why This Happens
- Cartridge or pressure-balance spool wear: Many single-handle mixers use a cartridge or a pressure‑balance mechanism that can stick or wear. When the valve moves to adjust temperature, worn parts can restrict flow.
- Clogged showerhead or flow restrictor: Mineral buildup reduces flow over time but usually causes steady low pressure rather than a sudden collapse.
- Household or municipal pressure changes: A running appliance, a closed or partially closed valve, or low supply after a utility outage can cause intermittent drops.
- Pressure-reducing valve or main supply issues: If a home pressure regulator is failing, pressure may fall when demand rises.
Step-by-Step What to Do
1. Observe exactly when the pressure drops
- Start the shower and let it run. Note whether pressure falls immediately or only after you move the handle to change temperature.
- If the pressure drops after you adjust temperature, suspect the cartridge or pressure-balance spool; these parts sit inside the valve and control both flow and mix.
2. Check other fixtures and timing
- Turn on a sink or another shower briefly while watching the shower. If the shower falls only when other appliances run, it could be a supply limitation. For a related situation, see Shower pressure drops when washer runs.
- Note if low pressure started after a recent city outage or repair; municipal work can leave debris in pipes or change pressure—see Low pressure after city outage.
3. Inspect the showerhead first
- Unscrew the showerhead and run the valve briefly. If flow is strong at the valve but weak through the head, soak or clean the head and remove the flow restrictor.
- Reattach and test. A clogged head often causes consistent low flow, not a quick collapse tied to handle movement.
4. Test the valve without the handle moving
- Set the handle to one temperature and leave it. If pressure stays steady until you move the handle and then collapses, that points to the valve internals (cartridge or spool).
- If you’re comfortable removing the handle and trim plate, you can visually inspect for loose parts or mineral buildup, but stop and shut off water before attempting deeper work.
5. Consider supply-side checks
- Check the home main shutoff and any local shutoffs to the bathroom. Make sure they’re fully open.
- If you have a pressure regulator, note whether pressure seems low in multiple fixtures; a failing regulator can cause intermittent collapses under demand.
6. Decide on repair or professional help
- Replacing a cartridge or pressure-balance spool can fix the problem, but matching the correct part and installing it without leaks requires care.
- If you find the valve internals look corroded, or if you’re unsure, move to the next section on calling a professional.
What Not to Do
- Don’t keep adjusting temperature to fix pressure collapse — that’s only masking the symptom and can lead to scalding or sudden cold shocks.
- Failing cartridges often need replacement by a pro; do not force or repeatedly tap the valve in hopes it will fix sticky internals.
- Don’t remove valve parts without shutting off the water supply and confirming you have the right replacement parts and tools.
When to Call a Professional
- Pressure collapses that clearly happen after you move the handle — a licensed plumber should replace or rebuild the cartridge/pressure-balance assembly to avoid leaks and scald risk.
- If multiple fixtures are affected, or the main regulator may be failing, call a pro to test house pressure and the regulator.
- If you find active leaks, exposed corroded plumbing, or if you’re not comfortable shutting off and disassembling the valve, hire a plumber to avoid water damage.
Safety Notes
- Always shut off the water at the local shutoff or the main before removing a valve cartridge or spool. Leaving the supply on can cause flooding and injury.
- Be cautious of scalding when testing temperature. Make small adjustments slowly while standing away from the spray.
- If you smell gas, see electrical issues, or find large leaks, stop and call the appropriate emergency services or a qualified contractor immediately.
Common Homeowner Questions
- Why does the shower pressure drop only when I change the handle? That behavior usually points to a worn or stuck cartridge/pressure‑balance spool inside the valve.
- Can I fix a cartridge myself? If you have the exact replacement and plumbing experience, yes, but many homeowners prefer a plumber because of part matching and leak risk.
- Could the water heater cause this? Rarely. The heater can affect temperature but not a quick pressure collapse tied to valve movement; check the valve first.