Quick Answer:
If one shower runs with normal pressure but becomes weak when another shower is used at the same time, the house is most likely hitting the flow limit of the water supply or the fixtures are competing for volume. First, test pressure with only one shower running. If pressure drops significantly when multiple showers run, supply limits are the likely cause rather than a single shower valve. You should also check individual shower heads and cartridges for clogs before assuming a supply problem.
Why This Happens
There are two common reasons you’ll see one shower weaken when another is used:
- The total available water flow from your service line, pump or well is limited. When two showers run, each one gets a smaller share, so one or both feel weak.
- Pressure-balancing valves and cartridge issues inside a shower can make one fixture react more noticeably when demand changes elsewhere. If a valve or aerator is partially blocked, it will show up as a big drop when flow is shared.
If the symptom is only in the shower and not other fixtures, read about Low water pressure at shower only for related fixes. If a single tub or spout behaves differently than fixtures at sinks, see Tub spout pressure weak but sinks fine.
Step-by-Step What to Do
Step 1 — Test single-shower pressure
- Turn off all other water fixtures in the house.
- Run the shower you’re testing and note how strong the flow feels.
- If you have a simple flow gauge or a 1‑gallon bucket, time how long to fill it — this gives a rough gallons-per-minute (GPM).
Step 2 — Test with two showers running
- Run the first shower at the same setting as before, then start the second. Note whether the first drops significantly.
- If the flow on the first shower falls a lot, you’re likely seeing a supply or flow-capacity limit.
Step 3 — Check other fixtures and valves
- Run other fixtures (sinks, laundry) while one shower is on to see if the system-wide flow is low or the issue is specific to the second shower.
- Check the main shutoff and any local shutoffs to ensure they are fully open.
Step 4 — Inspect shower heads and cartridges
- Remove and inspect aerators and shower heads for mineral buildup or debris. Clean or replace if clogged.
- If one shower uses a pressure-balancing or thermostatic cartridge, a failing cartridge can make that shower lose flow more noticeably under shared demand.
Step 5 — Consider supply-side limits
- For municipal supply: a low incoming pipe size, old piping, or a faulty pressure regulator can limit flow.
- For wells/pumps: the pump capacity or pressure tank size may be too small to support multiple showers simultaneously.
- If testing shows acceptable single-shower flow but large drops when multiple fixtures run, supply limits are the likely cause.
What Not to Do
- Don’t assume every weak shower is a bad valve — test one fixture at a time first.
- Don’t expect multiple showers to run well on an undersized supply; some homes simply cannot support simultaneous high-flow fixtures without upgrades.
- Don’t take apart or replace plumbing parts if you’re unsure — improperly reassembled valves can leak or cause scalding.
- Don’t ignore the problem if the falloff is large; if pressure is inadequate, a plumber is appropriate.
When to Call a Professional
Call a licensed plumber if:
- You confirm the house has adequate single-fixture flow but major drops when multiple fixtures run and you want a solution (service-line upsizing, booster pump, or re-piping may be needed).
- There is low pressure throughout the house, the pressure regulator appears faulty, or the well pump cycles unusually.
- You’re uncomfortable working on valves, the water heater, or the main shutoff. A plumber can measure system flow and recommend proper upgrades safely.
Safety Notes
- Be careful when testing hot water — run tests at safe temperatures to avoid scalds.
- Turning the main water supply off can release pressure; open a faucet to relieve pressure before disconnecting fittings.
- If you have a well pump or electrical controls, shut power to the pump before doing any pump work. Leave electrical troubleshooting to a professional.
Common Homeowner Questions
- Why does only one shower get weak? Running multiple fixtures reduces the available flow; a restricted supply or competing high-flow fixtures cause this.
- Can I fix this by replacing the shower head? Sometimes — if the shower head or cartridge is clogged. If the system is hitting a supply limit, replacing heads won’t solve the overall shortfall.
- Is a booster pump a good solution? A booster pump can help on some homes, but a plumber should assess supply, pipe size, and system design before installing one.