Quick Answer:
If the water goes cold suddenly while you’re mid-shower, don’t panic. First check a nearby sink to see if its hot water stays hot at the same time. If other fixtures also drop to cold, the problem is likely with the water heater or the hot supply. If only the shower goes cold, the shower valve or cartridge is the most likely cause.
Why This Happens
There are two common groups of causes:
- Supply or heater issues: The hot water source may be temporarily unable to keep up with demand, a tankless unit may cycle off, a breaker or pilot light could have failed, or a recirculation pump or control may have stopped.
- Shower-specific problems: The shower valve, mixing valve or cartridge can fail or get clogged. Pressure-balancing valves or thermostatic cartridges can stick, cutting off hot water to the shower even though the house still has hot water.
If you want to learn about running hot water during long showers or multi-fixture use, check the bolded notes below for related reading on common patterns like Temperature fluctuates during long showers and Temperature changes when other taps used.
Step-by-Step What to Do
1. Stay safe and finish the shower carefully
- If the water turns cold, step out of the direct spray if it’s uncomfortable. You can quickly rinse off using a cup or handheld if available, or turn the shower off and towel-dry until you can inspect.
2. Test another hot tap right away
- Run the hot water at a nearby sink for 10–20 seconds and note whether it stays hot or also goes cold. This one check separates shower-only problems from whole-house supply problems.
3. If other fixtures also go cold, check the heater and supply
- Look at the water heater: is the pilot light on (gas)? Is the breaker tripped (electric)? Any obvious error lights on tankless units?
- Consider recent heavy use: has someone run laundry or a dishwasher at the same time? A tank-type heater can run out of hot water; tankless units can be overwhelmed by very low flow or simultaneous high demand.
- If basic checks (pilot, breaker, recent heavy use) don’t explain it, the heater or supply-side equipment may need servicing.
4. If only the shower goes cold, suspect the valve or cartridge
- Shower valves and cartridges are the parts that mix hot and cold. They wear, seize, or clog with mineral deposits. A failing cartridge often causes sudden cold water in the shower while sinks remain hot.
- If you’re comfortable doing a quick inspection and your trim allows, you can turn off the shower water supply, remove the handle and trim, and look for obvious damage or blockage. Otherwise, call a plumber—replacing a cartridge is usually the fix.
5. Temporary workarounds while you arrange a fix
- Use the hot water at a sink for washing until the underlying issue is fixed.
- Avoid forcing the shower handle or repeatedly cranking the temperature control; that can make internal valve damage worse.
What Not to Do
- Don’t assume the water heater is bad if only the shower goes cold; if shower-only, a valve/cartridge issue is likely and a plumber is appropriate.
- Don’t try to remove or force valve parts beyond simple trim removal unless you have plumbing experience—internal parts can be delicate and water can be hard to stop without the right shutoffs.
- Don’t leave children or vulnerable people unattended in a shower if the water temperature is unpredictable.
When to Call a Professional
- Call a plumber if only the shower is affected and basic troubleshooting (testing other taps, checking obvious trim issues) points to a valve or cartridge problem.
- Call a water-heater technician or plumber if multiple fixtures lose hot water and you’ve checked the heater pilot, breaker, and obvious supply problems without a fix.
- Call immediately if you smell gas, see leaks, or if the heater shows error lights you don’t understand.
Safety Notes
- Hot water can cause scalding. If temperature swings also include sudden hot surges, keep children and elders out of the shower until repaired.
- Turn off electrical power to an electric water heater at the breaker before touching any electrical components. Don’t relight a gas pilot unless you’re comfortable doing so and there’s no gas smell.
- If you detect gas odor or a major leak, evacuate and call emergency services or the gas company as appropriate.
Common Homeowner Questions
- Why did it happen only in my shower? A faulty shower valve or cartridge is the most common cause when other fixtures stay hot.
- Could mineral buildup cause this? Yes — scale can seize a cartridge or block passages and cause sudden loss of hot water to the shower.
- Is this an emergency? Not usually, but don’t ignore repeated or severe temperature swings; call a pro if basic checks don’t resolve it.