Why is my water heater not making hot water?

By Dana Whitlock. Updated October 2026. Repair or replace?

Every tap running cold usually means the heater has lost its heat source. No gas is reaching the burner, no electricity is reaching the elements, or a safety control shut the unit down. You can confirm a few of those from the outside in five minutes. The rest sit behind sealed covers and belong to a licensed plumber. Start with the checks below. And if you smell gas at any point, stop reading and get outside.

Likely causes

The breaker or the gas supply is off Safe to check

Electric models need a dedicated 240-volt circuit, and a tripped breaker leaves the tank cooling for hours before anyone notices. Gas models need the supply valve on the pipe feeding the heater to be open. Someone working nearby may have closed it. Both are things you can see without opening anything.

The temperature dial was turned down Safe to check

A bumped or reset control knob can leave the setpoint at its lowest mark or on a vacation setting. A. O. Smith ships its gas heaters at the lowest setting to cut scald risk, so a new or recently serviced unit may simply be set too low.

The pilot or ignition system shut the burner down Pro job

On a gas tank, the pilot or igniter and the flame-proving safety device will stop gas flow if they cannot confirm a flame. Some models also lock out after repeated failed starts. Diagnosing this means working on the gas control and burner assembly, which is a job for a licensed plumber.

A failed heating element or thermostat (electric) Pro job

Electric tanks heat with one or two elements controlled by thermostats behind covered access panels. Those panels expose live 240-volt terminals. Testing and swapping these parts is work for a licensed plumber or electrician.

A tripped high-limit switch Pro job

Electric heaters have a reset button behind the upper panel that trips when water overheats. It often trips because something else failed, such as a stuck thermostat. Since reaching it means opening a live panel, leave it to a pro.

Safe things to check first

• Look at the electrical panel. If the water heater breaker is tripped, reset it once. If it trips again, leave it off.
• On a gas unit, find the supply valve on the gas pipe near the heater and confirm the handle runs parallel to the pipe (open). Look only. Do not touch the appliance's own gas control.
• Check the temperature dial or display and set it back to the normal mark (about 120 degrees F on many models).
• If the label says the pilot can be relit, do so only per the instructions printed on the unit's label. If it will not stay lit, stop.
• Check whether another fixture runs hot. If one shower is cold but the kitchen is fine, the problem may be that fixture's mixing valve.

When to stop and call

Call a licensed plumber if the breaker trips a second time, if the pilot will not light following the label, or if the unit shows an error code you cannot clear by restoring power. Smell gas near the heater? Do not flip switches or use the phone inside. Leave the house, then call your gas utility or 911 from outside or from a neighbor's home, and stay out until they say it is safe.

Repair or replace?

Start with its age. Type the serial from the label:

A lost heat source is often a single-part repair, so age decides it. DOE's analysis puts the average gas tank heater at about 14.5 years and the average electric tank at about 15.1 years. Date yours with the age decoder above. Within a few years of those figures, or near the 6 to 12 years some inspector charts list for a conventional heater, a repair bill buys limited time, and the replacement range above is the better number to study. A young heater with a failed igniter or element is usually worth fixing.