Quick Answer:
If one shower weakens only when two (or more) showers run at the same time, the problem is usually the home’s overall water supply capacity or the pressure-reducing valve (PRV), not the individual shower fixtures. First test each shower alone, then test them together to confirm whether the loss of flow only happens under simultaneous demand.
Why This Happens
Homes and municipal supplies can only deliver a limited amount of water flow at a given pressure. Each shower uses a certain flow rate (gallons per minute). When two showers run together, they may exceed what the service, main piping, or a PRV can deliver. That causes a noticeable drop at one or both fixtures. Other common contributors are a partially closed main valve, an aging PRV set low, or pipe sizing that’s too small for simultaneous use.
Step-by-Step What to Do
1. Test each shower one at a time
- Turn on shower A and note how strong the spray feels. If you want a simple measurement, use a one-gallon bucket and time how long it takes to fill while the shower is at your normal setting.
- Repeat with shower B (and any other shower). Record results so you can compare.
2. Test them together
- Run both showers the way household members normally would and note the change. Is one shower noticeably weaker while the other stays similar, or do both drop?
- If the drop only happens with simultaneous use, the fix is often system-level (PRV/supply sizing), not the individual shower.
3. Check simple local issues
- Make sure the main shutoff and any branch valves are fully open.
- Check other fixtures while both showers run — do sinks or toilets also lose pressure? That helps locate whether it’s the house supply overall.
4. Look at the PRV and meter supply
- If you have a pressure-reducing valve, note whether house pressure seems low even when only one shower runs. A plumber can test static and dynamic pressure and adjust or replace the PRV if needed.
- Municipal supply or the service pipe size can limit flow. If your main is 1/2″ branch lines or the service is undersized, simultaneous demand can overwhelm it.
5. Decide next steps
- If individual showers are weak when tested alone, troubleshoot showerheads, cartridges, or local blockages (see the note below about single-shower problems: Low water pressure at shower only).
- If only simultaneous use causes the issue, best fixes are system-level: upsizing service piping, adjusting or replacing the PRV, or increasing municipal supply capacity. These usually require a plumber or utility work.
What Not to Do
- Don’t expect two showers to run strong on an undersized supply; if pressure is unacceptable with normal use, a plumber is appropriate.
- Don’t assume a showerhead swap will fix a problem that only appears with multiple fixtures running.
- Don’t adjust or remove plumbing components you don’t understand—PRVs and service piping work affect the whole house and can cause other problems if changed incorrectly.
- Don’t ignore other signs like ongoing low pressure at multiple fixtures or sputtering (air) — those indicate a broader supply or equipment issue.
When to Call a Professional
- Call a licensed plumber if the drop happens only with normal simultaneous use and you suspect the PRV, main service size, or meter is the limit.
- Hire a plumber to measure static and dynamic pressure and flow, inspect the PRV, and advise on whether service piping needs upsizing or the PRV needs replacing.
- If you see leaks, water discoloration, or loss of hot water when multiple fixtures run, call a pro—these can be signs of failing pipes or other hazards.
Safety Notes
- Turn off the main water before attempting any repairs to valves or piping you’re not experienced with.
- Beware of scalding when testing hot water. Run short tests and adjust temperature settings on the water heater only if you know the consequences.
- If work involves the meter, service saddle, or street-side components, coordinate with your water utility and use a licensed plumber—working on those parts can be restricted or dangerous.
Common Homeowner Questions
- Why does one shower get weak only when both are on?
Because the combined flow exceeds what the house service, pipe size, or PRV can deliver at the same time. - Can I fix it by changing showerheads?
Only if a shower is weak when used alone. If the loss happens with multiple showers, the problem is usually the supply or PRV. - How will a plumber fix this?
A plumber will measure flow and pressure, check the PRV and main valves, and recommend either adjusting/replacing the PRV or upgrading piping or other system components—sometimes coordinating with the water utility.
If you notice a different pattern — for example, the tub spout has weak output while sinks are fine — that points to a different cause and troubleshooting path (Tub spout pressure weak but sinks fine).