Quick Answer:
If your shower temperature slowly drifts colder or hotter during a long shower without touching the handle, the usual causes are the water heater running out of hot water at that flow rate, pressure changes elsewhere in the house, or the shower’s mixing mechanism not keeping up. A simple test is to reduce the flow slightly and see if stability improves, and note how long it takes for the drift to start so you can compare that with your heater’s recovery behavior.
Why This Happens
- Water heater recovery: Tank heaters take time to replace the hot water that leaves the tank. At higher flow rates a heater may not keep up, so the mix gradually shifts to colder water as hot supply drops.
- Pressure-balance or thermostatic valve limits: These valves hold the mix steady but can be overwhelmed by small steady shifts in supply temperature or pressure over a long run time.
- Other fixtures and demand: When appliances or taps elsewhere start, they change cold or hot water pressure and temperature balance. See Temperature changes when other taps used for related situations.
- Sediment or scale in the heater or the shower valve: These reduce capacity and responsiveness, making slow drift more likely.
Step-by-Step What to Do
Step 1 — Time and observe
Start a long shower and use a clock. Note the time (in minutes) when the temperature starts to drift. Keep this number—it’s useful for comparing with the heater’s expected recovery time and for describing the problem if you call a pro.
Step 2 — Reduce flow slightly
Lower the shower flow a small amount (not all the way to a trickle) and repeat the test. Many showers become more stable at a modestly reduced flow because the heater or mixing valve can keep up better. If stability improves, the issue is likely capacity or recovery-related.
Step 3 — Check other water use
- Ask household members not to run washers, dishwashers, or other taps while you test.
- Run the shower again; if the drift only happens when other fixtures run, it’s a pressure-demand issue.
Step 4 — Inspect the water heater
- Check the heater’s set temperature and tank size—both affect how long it will supply hot water at a given flow.
- If you have a tankless heater, compare the time-to-drift with the heater’s rated flow and recovery to see if the unit is undersized.
Step 5 — Test the shower valve
Older pressure-balancing cartridges and thermostatic mixing valves can wear or clog. If the valve is stiff, noisy, or only responds slowly, it may need cleaning or replacement. If you’re comfortable, remove the cartridge following the manufacturer’s instructions; otherwise, call a plumber.
Step 6 — Try temporary fixes
- Stagger hot water use during long showers (avoid starting a dishwasher mid-shower).
- Install a low-flow but not ultra-low-flow showerhead to reduce demand without causing unstable mixing.
What Not to Do
- Do not run extremely low flow trying to conserve hot water; unstable mixing often worsens at low flow.
- Do not attempt complex repairs on gas water heaters or their burners unless you are qualified—turn the unit off and call a professional.
- Do not force or hammer on the valve cartridge; that can break the valve body and make the problem worse.
When to Call a Professional
- The temperature drift happens even at modest flow settings or after you’ve tried the steps above.
- You notice sudden scalding or large, abrupt jumps in temperature.
- Your water heater shows signs of malfunction—strange noises, leaks, or it won’t maintain set temperature.
- You suspect the mixing valve is failing or you’re not comfortable removing the cartridge yourself.
Safety Notes
- Be cautious of sudden hot spikes. If you ever experience a fast jump to very hot water, stop the shower and test with cold water first—this behavior can cause burns and should be treated seriously. See guidance on Shower scalds unexpectedly if sudden scalding occurs.
- Turn off power or gas to a water heater before doing any internal inspection. If you smell gas or see corrosion or leaking, evacuate and call a professional.
Common Homeowner Questions
- Why does the water get colder the longer I shower? Answer: The heater may be unable to supply enough hot water at that flow rate or the mixing valve can’t maintain the set temperature as conditions change.
- Will lowering flow always fix it? Answer: Not always—reducing flow often helps, but very low flow can make mixing unstable, and the root cause may still be the heater or a failing valve.
- Is this dangerous? Answer: It can be if temperatures spike or you get scalded; treat sudden hot surges seriously and call a plumber if you cannot stop them.