Quick Answer:
If your shower starts hot but cools down quickly, first time how fast the temperature drops. An immediate fade (within a minute or two) usually points to the shower valve or mixing device. A gradual fade over several minutes usually points to the water heater’s capacity or recovery. Compare the shower against a sink’s hot-water time to narrow it down before assuming a major heater problem.
Why This Happens
- Mixing valve or anti-scald cartridge: These parts control the mix of hot and cold water. If they fail, cold can be introduced quickly and the shower feels like it goes cold immediately.
- Water heater capacity or recovery rate: A tank that’s too small for the household or one that can’t recover fast enough will let water run hot for a few minutes and then cool as the stored hot water is used up.
- Broken dip tube (tank heaters): A damaged dip tube can let incoming cold water mix at the top of the tank, so you get only a small amount of hot water before it drops off.
- Flow restrictions or partial blockages: Mineral buildup or a restrictive shower cartridge can reduce hot flow and change how long hot water lasts.
- Plumbing layout: Long runs or a single mixing valve serving multiple fixtures can affect how quickly cold water reaches the shower.
Step-by-Step What to Do
1. Time the fade
Start the shower fully hot and note how long it stays hot:
- Immediate fade (seconds to a minute): points to a valve or mixing issue.
- Gradual fade (several minutes): points to heater capacity or recovery limits.
2. Compare with a sink
Run hot water at a nearby sink and time how long hot water lasts. If the sink stays hot much longer than the shower, the problem is likely the shower valve or head, not the heater. If both fade similarly, the heater or supply is more likely the cause.
3. Check flow and shower head
- Turn the shower to full hot and remove the shower head or use a bucket to feel flow and temperature. Reduced flow from mineral buildup can change mixing behavior.
- Clean or replace a clogged shower head or aerator-style flow restrictor to rule out a simple obstruction.
4. Test other fixtures and times
- Run a sink and then immediately run the shower. Do patterns change when other fixtures are used? This helps identify shared valve or supply issues.
- Note whether the issue happens at all times or only after heavy use (morning rush vs quiet times). Heater recovery shows up after repeated or long hot-water usage.
5. Check the water heater basics
- Look at the thermostat setting — 120°F is a safe normal setting for households. Lower settings can feel like the heater runs out sooner.
- Take note of the tank size and household hot-water demand. A small tank with multiple users or long showers will run out faster.
- For electric units: if both elements aren’t working, capacity drops. For gas units: weak burner or pilot issues can limit recovery. If you suspect these, a plumber or qualified tech should check them.
6. Inspect the shower valve and cartridge
- An aging mixing cartridge or anti-scald device often causes an immediate cold surge. If you’re comfortable, shut off water and remove the cartridge to inspect for wear or mineral buildup. If not comfortable, call a pro.
- Replacing a cartridge is a common DIY for many homeowners who follow the manufacturer’s steps and shut off water first, but it can be tricky if the valve is corroded or if the trim needs special tools.
7. When the dip tube is suspected
If you have an older tank and the heater gives only a small amount of hot water before cooling, a broken dip tube is possible. Inspecting or replacing the dip tube requires draining the tank and working on the heater — this is often best left to a professional unless you have plumbing experience.
What Not to Do
- Don’t assume a quick fade always means the tank is too small — compare with sink duration before blaming the heater.
- Don’t open or attempt major repairs on a gas water heater (burner area or gas lines) unless you are qualified. That can be dangerous.
- Don’t leave the heater set above 120°F to “fix” short hot-water duration — higher settings increase scald risk and energy use.
- Don’t hesitate to call a plumber if the cause is unclear or if the work requires gas, wiring, or draining the tank; a qualified pro can diagnose dip tubes, elements, and valves safely.
When to Call a Professional
- Immediate cold surges after a fresh hot start and simple cartridge cleaning didn’t help — this often needs a valve specialist.
- Signs of a failing dip tube, corroded tank, or electrical/gas issues on the water heater.
- If you’re uncomfortable shutting off water, draining the tank, or working with gas or the heater’s wiring — call a licensed plumber or service tech.
- Persistent problems after the basic checks above. If the diagnosis is unclear, professional testing will save time and prevent mistakes.
If the issue looks specific to the bathroom fixtures, see the bolded note below about similar problems: Hot water fine at sinks but not shower. If the shower has no hot water at all while other fixtures are hot, read No hot water at shower only.
Safety Notes
- Risk of scalding: Keep thermostat at a safe level (around 120°F). Test water before stepping in.
- Turn off power at the breaker before working on electric heaters. For gas units, do not work on burners or gas lines yourself unless qualified.
- Draining a tank can be heavy and messy — follow manufacturer instructions or hire a pro if unsure.
- If you smell gas or suspect a leak, leave the house immediately and call your gas company or emergency services.
Common Homeowner Questions
- Why does it start hot then go cold? Immediate cold usually means a shower valve or cartridge is mixing in cold; gradual cooling usually means heater capacity or recovery limits.
- Could the dip tube be the cause? Yes — a broken dip tube often causes only a small amount of hot water; diagnosing it requires inspecting the tank.
- Can I replace the cartridge myself? Many homeowners can if they shut off water and follow instructions, but call a plumber if the valve is corroded or you’re not confident.
For more related articles, see the No Hot Water at Shower but Everywhere Else hub.