Quick Answer:
Temperature drift partway through a long shower is usually caused by limited hot-water capacity, mixing-valve behavior, or a sudden change in household water demand. Time the drift, compare with other hot uses, and try slightly reducing the shower flow to see if the water stays more stable. If the problem continues despite these checks, call a plumber.
Why This Happens
- Hot-water supply limits: With tank heaters you can exhaust the stored hot water; with some tankless units the flow may exceed the heater’s recovery capacity after a period of continuous use.
- Mixing valves and pressure balance: Many shower valves keep a set ratio of hot and cold based on pressure. When other taps or appliances run, the balance shifts and temperature can change.
- Appliance cycling or scheduled draws: Dishwashers, washing machines, or other showers starting on a timer or mid-cycle can cause a sudden drop or spike.
- Thermostat or element issues: Less common but possible—elements, burners, or thermostats not working as intended can reduce available hot water over time.
- If you experience sudden hotter water after a cold burst, consider the guidance on Shower scalds unexpectedly to learn more about causes and immediate steps.
- If the drop happens when another tap runs, read the note about Temperature changes when other taps used for related checks.
Step-by-Step What to Do
1. Time how long until the drift starts
Start the shower and note (with a clock or phone) the minute mark when the water first noticeably shifts colder or hotter. Record that time. This gives you an objective measure to compare with other events.
2. Compare with other hot uses
Ask household members or watch appliances to see if any other hot-water draw happens near that minute mark. Do this:
- Turn off other possible draws (dishwasher, washing machine) and repeat the shower test.
- If the drift disappears when other draws are off, those appliances are likely causing pressure or temperature changes.
3. Test another faucet or shower
Run a nearby hot faucet during the same timeframe. If both show the same timed change, it points to water heater capacity or distribution issues rather than a single valve.
4. Reduce flow slightly and observe
Turn the shower down a notch (not to an extremely low trickle) and repeat the timed test. Lower flow can reduce how quickly the hot supply is used and can improve valve stability. Note if the temperature stays steadier with reduced flow.
5. Check heater type and settings
Note whether you have a tank or tankless heater and the set temperature on the thermostat. For tank systems, a very long shower can simply exceed stored capacity; for tankless, high continuous flow can overrun the unit’s output. Don’t attempt repairs on gas or electric heaters yourself unless you are qualified.
6. Look for patterns and document
Keep a short log: time-to-drift, whether another appliance was running, and whether reduced flow helped. If the drift always starts near the same minute mark, that pattern is useful to share with a professional.
What Not to Do
- Don’t run extremely low flow to “stretch” hot water—some valves become unstable at very low flow and can make temperature worse.
- Don’t ignore persistent, repeatable fluctuations; if long-shower fluctuations persist, a plumber is appropriate to diagnose mixing valves, pressure issues, or heater performance.
- Don’t attempt major water-heater repairs (gas burner work, element replacement, or pressure-relief valve replacement) unless you are properly trained—these are safety risks.
When to Call a Professional
- Temperature changes continue despite turning off other draws, timing the drift, and testing reduced flow.
- You notice repeated scalding risk (sudden very hot bursts) or complete loss of hot water mid-shower.
- There are signs of the heater failing: strange noises, leaks, or inability to maintain set temperature for any short draw.
A plumber can test system pressure, inspect mixing and pressure-balance valves, and evaluate the water heater’s capacity and performance.
Safety Notes
- Scalding is a real risk. If water becomes uncomfortably hot, turn it off and test with caution.
- Avoid full DIY work on gas or electric water heaters unless you are certified. Faulty repairs can cause leaks, fires, or carbon monoxide hazards.
- If you reduce flow, don’t reduce it to a near-trickle—this can cause valve instability and make temperature control worse.
Common Homeowner Questions
- Why does it only happen after several minutes? Often because stored hot water is being depleted or a tankless unit is reaching its continuous output limit; mixing behavior can also take time to shift.
- Will lowering flow always fix it? Not always. Lowering flow can help in many cases but should be moderate—very low flow can cause valve instability.
- Is this a sign my heater is broken? Not necessarily. It may be capacity, household demand timing, or valve behavior. Persistent or severe problems warrant a professional check.