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Water heater not working: what to check

Most "no hot water" problems come down to a few causes: no power or gas, a tripped safety device, a failed part, or a tank that's simply worn out. Work through these checks in order, starting with the easy ones.

1. Figure out which type you have

A gas tank heater has a gas line, a control valve/thermostat knob near the bottom, and a flue pipe on top. An electric heater has a power cable going into a junction box on top and usually two access panels on the side. Tankless units are a separate topic; check the error code on the display and the manual.

2. Quick checks for every heater

• Is it all fixtures or just one? If only one faucet or shower runs cold, the issue is likely that fixture (e.g. a shower mixing valve), not the heater.
• Thermostat setting. Make sure it hasn't been turned down or set to "vacation." Around 120°F is a common recommended setting.
• Look for leaks around the base, fittings and relief valve. A leaking tank changes the plan; see "when to call a pro" below.
• Was a lot of hot water just used? A tank can run out after back-to-back showers or laundry, then needs time to recover.

3. Gas water heater checks

• Gas supply. Confirm the gas shutoff valve on the supply line is open (handle parallel to the pipe) and other gas appliances work.
• Pilot light. On older models, look through the sight glass for a small blue flame. If it's out, relight it exactly as the label on the heater describes.
• Pilot won't stay lit. This often points to a dirty or failing thermocouple or flame sensor, a common, inexpensive part.
• Electronic ignition models may need household power (check that it's plugged in) and often show a blinking status light; the blink pattern is decoded on the label or in the manual.
• Flammable vapor sensor / reset. Many newer gas heaters lock out if they detect vapors or overheating. Check for a tripped sensor or reset per the manual, and remove gasoline, paint or solvents stored nearby.
• Venting. A blocked vent can shut the unit down. Don't operate a heater with a damaged or disconnected flue; call a pro.

4. Electric water heater checks

• Breaker. Check the panel for a tripped breaker (usually a double-pole breaker). Reset it once. If it trips again, stop and call an electrician or plumber.
• High-limit reset button. With power off, remove the upper access panel and insulation; press the red reset button above the upper thermostat. If it keeps tripping, a thermostat or element is likely faulty.
• Lukewarm water only. Electric tanks usually have two heating elements. If water gets warm but not hot, or runs out fast, one element or thermostat has often failed. Elements can be tested with a multimeter (power off) and replaced.

5. Not enough hot water (but some)

• Sediment buildup on the tank bottom reduces capacity and efficiency, especially with hard water. Flushing the tank per the manual can help.
• Broken dip tube lets cold incoming water mix at the top of the tank, so hot water runs out quickly.
• Undersized tank for your household's use, especially if you've added people or a large tub.

When to call a pro or replace

• The tank itself is leaking. Tank leaks generally can't be repaired; the heater needs replacing.
• You smell gas, see scorch marks, or the vent is damaged.
• The breaker or reset keeps tripping.
• The heater is near the end of its expected life. Many tank heaters are rated around 8–12 years; check the date code on the label or the manufacturer's site.
• You're not comfortable working with gas or 240-volt electrical parts.