Quick Answer:
If one shower goes cold when more than one is running, the system is usually being asked to supply more hot water or pressure than it can. Run a single shower by itself to see if hot returns. If it only fails when several fixtures run at once, check the water heater’s capacity and recovery and look at household water pressure and valve behavior. For persistent capacity or valve problems, get a plumber to diagnose and recommend upgrades or repairs.
Why This Happens
Two common things cause this: supply limits and valve behavior. A tank water heater can run out of stored hot water or be too slow to recover when more than one shower demands hot water. A tankless unit can be limited by its maximum flow rate. Pressure-balancing or thermostatic shower valves reduce hot flow if cold pressure changes, so a drop in system pressure from another shower can make one fixture go cold. Shared piping and partly closed valves also change how hot water reaches each shower.
Step-by-Step What to Do
1. Run one shower at a time
- Start by running only the affected shower and set it to the warm temperature you expect. Let it run for a minute to reach steady temperature.
- If the shower stays hot when it’s the only one running, that strongly points to a supply/pressure issue rather than a single broken shower valve.
2. Repeat the test with multiple fixtures
- Run another shower (or a faucet) while the first is on and note what changes. Does the affected shower cool immediately, or does the whole house lose hot water?
- Observe whether only one shower is affected or if all hot outlets soften—this helps separate local valve problems from system supply limits.
3. Check the water heater
- Identify the type (tank, tankless, hybrid) and approximate size or flow rating. A small tank or a tankless unit with limited GPM can be overwhelmed by simultaneous showers.
- Look for obvious signs: the tank runs out quickly, the heater cycles rapidly, or there’s no hot water after extended use. Consider whether the heater’s capacity/recovery matches your household peak demand.
4. Check household pressure and valves
- Test other hot-water taps while showers run. Low incoming pressure or a pressure-reducing valve that’s set low can reduce hot flow to one fixture.
- Make sure supply valves to the water heater and to branches are fully open.
- If hot water is fine elsewhere but one shower feels weak, see Hot water fine but shower lukewarm for more focused checks.
5. Inspect the shower valve and cartridge
- Thermostatic and pressure-balancing valves can limit hot water if cold pressure falls or the cartridge is clogged or worn. If you recently worked on the valve, see Shower cold after replacing valve for related troubleshooting notes.
- If the valve is old or sticky, replacing the cartridge or valve body may be needed—this is often best handled by a plumber unless you’re confident with plumbing repairs.
6. Temporary and long-term fixes
- Short term: stagger shower times so the heater can recover between uses, or lower other hot-water uses while showers run.
- Long term: consider a larger tank, a higher-recovery element, a properly sized tankless unit, or point-of-use heaters for high-demand bathrooms.
What Not to Do
- Don’t keep running multiple showers to “test” while they are cold — that wastes energy and is uncomfortable and unsafe.
- Don’t assume raising the water heater thermostat will fix capacity problems; higher temperature increases scald risk and only briefly increases usable hot volume.
- Don’t try complex repairs on gas burners, sealed controls, or electrical elements if you aren’t qualified — call a licensed pro.
- If capacity or complex valve issues look likely, don’t delay contacting a plumber for a proper diagnosis and safe recommendations.
When to Call a Professional
- If the heater won’t produce enough hot water even with a single shower running, or the unit shows mechanical or ignition faults.
- If several fixtures lose hot water at once and you suspect sizing, a damaged dip tube, failed heating element, or a tankless flow limiter.
- If the problem seems isolated to a shower valve and you’re not comfortable disassembling or replacing cartridges and valves yourself.
- If you smell gas or see electrical issues — leave the area and call the appropriate emergency service.
Safety Notes
- Always check water temperature before stepping into a shower—sudden hot or cold changes can cause falls or scalds.
- Keep water heater thermostat at a safe setting (commonly around 120°F / 49°C) unless advised otherwise by a professional.
- Do not work on gas lines or high-voltage electrical components unless you are licensed; call a qualified technician.
Common Homeowner Questions
- Why does one shower go cold but others are okay? Often a pressure-balancing valve, a clogged cartridge, or restricted piping to that shower is to blame.
- Can my water heater really not keep up with two showers? Yes—if its tank size or recovery rate (or a tankless unit’s flow limit) is too low for simultaneous demand.
- Is it safe to just raise the heater temperature to fix this? No—raising temperature increases scald risk and only temporarily helps usable hot volume; resizing or adding capacity is a safer long-term fix.
For more related articles, see the No Hot Water at Shower but Everywhere Else hub.