Quick Answer:
If the shower temperature moves when someone opens a nearby tap, run the shower and open that sink while watching the temperature. A sharp change usually points to a pressure-balance valve that isn’t working or to supply lines that can’t meet simultaneous demand. Check the shutoff stops at the shower valve and call a plumber if the shift is repeatable or severe.
Why This Happens
- Most showers use a pressure-balancing or thermostatic valve to keep temperature steady. When another fixture opens, cold or hot water pressure can drop and the valve may not respond fast enough.
- Undersized supply piping or a weak feed (long runs, small diameter, or shared lines) can’t keep the flow and pressure steady when two or more fixtures run at once.
- Partially closed service stops at the shower valve, debris in the cartridge, or a failing pressure-balance mechanism reduce the valve’s ability to correct pressure swings.
- Sudden temperature swings can cause scalding or cold shocks. For related issues, see topics like Shower scalds unexpectedly and Cold bursts during hot shower.
Step-by-Step What to Do
1. Reproduce the issue safely
- Run the shower at your normal setting. Keep someone nearby to help, or do this yourself if you can safely access the sink or other fixture.
- Open the nearby sink or fixture that usually causes the change and watch the shower temperature. Note how quickly and how far the temperature moves.
2. Watch for sharp shifts
- If the temperature drops or spikes immediately and dramatically when the sink opens, that points to pressure-balance problems or limited supply capacity.
- If the change is small and gradual, the system is less likely to be dangerous and may only need minor servicing.
3. Check the shower shutoff stops
- Locate the shutoff stops (service valves) for the shower—usually behind an access panel or under the escutcheon. Verify they are fully open. Partially closed stops mimic supply restrictions.
- Open and close them gently to confirm smooth operation. Don’t force valves that feel stuck.
4. Test other fixtures and patterns
- Try running the tub, dishwasher, or washing machine to see if they cause the same effect. This helps determine whether the problem is the shower valve or overall supply capacity.
- Note time-of-day differences: well pumping or municipal pressure swings can make the issue worse at peak use times.
5. Basic cleaning and inspection (non-invasive)
- If you’re comfortable, remove and inspect the shower cartridge or pressure-balance assembly for obvious debris or corrosion. Many kits have small screens or filters that can clog.
- If the cartridge looks corroded or damaged, replace it only if you have the correct part and instructions; otherwise, stop and call a pro.
What Not to Do
- Don’t ignore sharp shifts when other taps run—pressure-balance failures can worsen; if it’s repeatable, a plumber is appropriate.
- Don’t force or remove valves without knowing what you’re doing. Damaging a valve or its seals can make the problem worse or cause leaks.
- Don’t bypass anti-scald or pressure controls. Removing or defeating safety features increases risk of burns.
When to Call a Professional
- Call a licensed plumber if the temperature shift is repeatable, large, or causes scalding or cold shocks.
- Get help if you find a seized or leaking valve, if the cartridge is badly corroded, or if your home’s piping may be undersized for current demand.
- Ask the plumber to test water pressure, inspect the pressure-balance or thermostatic valve, and check for partially closed stops or undersized feeders.
Safety Notes
- Sudden hot water spikes can scald. Never leave children or vulnerable adults unattended in a shower that shows temperature instability.
- When testing, keep the shower at a moderate temperature so a sudden change is less likely to cause injury.
- If you smell gas or see major leaks while inspecting plumbing, stop immediately and contact the appropriate emergency service.
Common Homeowner Questions
- Q: Why does it only happen with one sink running?
A: That sink may share the same hot or cold feed line, creating the largest pressure change when it opens. - Q: Can I fix this by raising the water heater temperature?
A: No. Raising heater temperature increases scald risk and doesn’t fix pressure or supply problems. - Q: Will replacing the shower cartridge likely solve it?
A: If the valve is failing, a new cartridge or valve can fix it; if the problem is supply size or line pressure, cartridge replacement won’t help.