Quick Answer:
If your shower starts with normal pressure and then the flow drops after a minute, first note whether the drop follows a change in temperature. If the collapse happens as the water warms, the pressure-balance spool or cartridge in the valve can be sticking — you can confirm whether the problem is the valve by removing the shower head and running the water. If the issue repeats or you are unsure, call a plumber.
Why This Happens
Common causes for a pressure that starts normal and then collapses include:
- A sticking pressure-balance spool or cartridge in the shower valve that moves when temperatures change and then jams as it warms.
- A partially clogged shower head or inline screen that changes flow as mineral deposits loosen or shift with temperature and pressure.
- Temporary drops from the supply side — for example, when other appliances run or after the water utility changes pressure. See Shower pressure drops when washer runs and Low pressure after city outage for related supply-side examples.
- Faulty or worn mixing valves in older single-handle shower systems.
Step-by-Step What to Do
Step 1: Watch exactly when the drop happens
Start the shower and note the timing. Does it fall immediately, or only after you change the temperature? If it consistently collapses when the water warms up, that points toward the pressure-balance spool or cartridge sticking as it expands.
Step 2: Test with the shower head removed
Remove the shower head and run the shower at the same settings (temperature and handle position). If the flow still collapses with the head removed, the problem is upstream at the valve or supply. If the flow stays steady with the head off, the head or its screen is the likely cause (clean or replace the head).
Step 3: Check other fixtures and appliances
Run a sink or flush a toilet while the shower is running to see whether other fixtures affect pressure. Repeatable drops when another appliance runs indicate a supply-side issue rather than the shower valve.
Step 4: Inspect the trim and valve (if comfortable)
Turn off the water to the shower at the shutoff valves or the main supply before removing trim. Remove the handle and trim to visually inspect the cartridge or spool for obvious debris or corrosion. If you don’t find anything obvious, don’t force parts; reassemble and proceed to the next step.
Step 5: Decide whether to clean, replace, or call a pro
If you saw debris or buildup and you’re comfortable doing the job, you can remove and clean/replace the cartridge following the manufacturer’s instructions. If the cartridge is old, corroded, or seized, replacement is usually the right fix. If the collapse is repeatable or you’re not confident, contact a plumber.
What Not to Do
- Don’t keep forcing the handle around when pressure collapses—this can worsen scald risk; if collapse is repeatable, a plumber is appropriate.
- Don’t remove valve parts without shutting off the water. You can cause flooding or injury.
- Avoid guessing with valve adjustments that alter hot/cold balance while someone is under the water — that increases scald risk.
- Don’t pour harsh chemical cleaners into the valve to “clear” a blockage; that can damage seals and cartridges.
When to Call a Professional
- The drop is repeatable and you’ve confirmed it occurs with the shower head removed — that indicates a valve problem that often needs a plumber to replace the cartridge or spool.
- You can’t find shutoff valves or you’re unsure how to remove the trim safely.
- There is any sign of leaks behind the wall, corrosion, or a seized valve you cannot move.
- Scalding or significant temperature swings occur when the pressure drops — these are safety concerns best handled by a pro.
Safety Notes
- Always shut off the water supply before working on the valve. If you can’t locate local shutoffs, use the main house shutoff.
- Be cautious of scald risk when testing temperature changes. Have someone stand by to shut water off if temperatures spike.
- If you smell gas or see major leaks while working, stop and call an emergency professional immediately.
Common Homeowner Questions
- Why does the flow only drop after a minute? That timing often means parts inside the valve are moving as the water temperature changes and then sticking when warm.
- Can I just clean the cartridge myself? You can if you’re comfortable and have the right replacement parts and shutoffs, but shut off water first and follow the manufacturer’s steps; otherwise hire a plumber.
- Is this dangerous? It can be — sudden temperature swings increase scald risk, so stop using the shower if you can’t control the temperature safely and call a pro.