Quick Answer:
If a shower suddenly scalds with no warning, get out of the shower immediately, stop using that shower, and arrange an inspection. If you can safely do so, lower the water heater setpoint slightly to reduce the maximum hot temperature until the shower’s valve or mixing device has been checked.
Why This Happens
- Most modern showers have a pressure-balancing or thermostatic mixing valve that blends hot and cold water. If that valve fails, hot water can flood the shower without warning.
- Sudden pressure changes elsewhere in the house (toilet flushes, dishwasher, washing machine) can upset a failing valve and cause dangerous spikes.
- Mineral buildup, corrosion, or internal wear can cause a valve to stick or stop responding.
- Issues in the water heater or distribution piping (blockages, thermostat failure) can make hot water hotter than expected. If this seems to affect only the upstairs, see **Temperature unstable only upstairs** for related checks.
- If you also feel brief cold surges while the water is hot, that pattern can point to a different failure—read about **Cold bursts during hot shower** for details that match that symptom.
Step-by-Step What to Do
1. Get out and stop using the shower
- Turn the water off and leave the shower immediately. Do not try to tough it out—exposure to scalding water can cause serious burns.
- Keep the shower out of use until a competent inspection is done.
2. Make a quick check of other fixtures
- Carefully test hot water at a kitchen sink or another bathroom tap (briefly and with your hand ready to pull away). Note whether those fixtures also get dangerously hot.
- Do not run long tests that could cause burns; this is only to help determine whether the problem is isolated.
3. Lower the water heater setpoint slightly if safe
- If you are comfortable and know how your heater controls work, reduce the thermostat by a few degrees (or to about 120°F / 49°C) to limit maximum hot-water temperature. Only do this if you can do it safely and without accessing electrical or gas components you’re unsure about.
- If you have a tankless heater or are unsure, skip adjustments and proceed to the next step—better to wait for a pro than create a hazard.
4. Stop using the shower until the valve is inspected
- Do not use the affected shower. Arrange an inspection of the shower valve or mixing device by a plumber experienced with thermostatic and pressure-balancing valves.
- If possible, take notes: when the spike happened, whether other appliances were running, and any visible leaks or drips—this helps the technician diagnose the cause.
5. If the problem is urgent
- If the spike is frequent, unpredictable, or you’ve already had a burn, call a licensed plumber right away. They may recommend replacing the mixing valve or making repairs to the water heater or plumbing.
What Not to Do
- Do not continue using a shower that scalds unexpectedly; stop use and have it inspected.
- Do not try to disassemble an internal thermostatic or pressure-balancing valve unless you have the correct parts and experience—incorrect reassembly can make the problem worse.
- Do not ignore the issue hoping it will go away; valve failures can lead to serious injury, especially for children or older adults.
When to Call a Professional
- Call a plumber if the scalding spike happens more than once, if lowering the heater setpoint does not prevent high temperatures, or if other fixtures show abnormal hot temperatures.
- Call immediately if anyone is burned; seek medical attention for burns and contact a plumber to secure the plumbing so no further harm occurs.
- Ask for a technician who can inspect and, if needed, replace the shower’s mixing valve or thermostatic cartridge.
Safety Notes
- Scalding can happen quickly; water at 140°F (60°C) can cause third-degree burns in seconds. Keeping a water heater at 120°F (49°C) reduces risk for most homes while still supplying hot water for cleaning.
- Lowering the water heater setpoint slightly is a temporary safety step, not a fix for a faulty valve.
- Children, older adults, and people with limited mobility are at higher risk—restrict their access to the affected shower until it’s repaired.
Common Homeowner Questions
- Q: Can I fix this myself?
A: Not usually—valve repairs require parts and experience; call a licensed plumber. - Q: Could the water heater alone cause this?
A: Sometimes—thermostat or dip tube issues can raise tank temperature, but sudden spikes are often a valve problem. - Q: How fast should a plumber respond?
A: If there is a burn or ongoing risk, request an emergency visit; otherwise schedule a prompt inspection within a day or two.