Your water heater goes cold in the middle of a shower, or the water is lukewarm at every tap. It's frustrating, but in most cases the fix is a $15-60 part and an afternoon of work. Here are the 8 most common causes, in order of likelihood.
Most electric water heaters have a red reset button on the thermostat cover. If the heating element overheats, the high-limit thermostat trips and cuts power. The button pops out and stays out until you press it back in.
Fix: Turn off power at the breaker. Press the red reset button firmly until it clicks. Turn power back on. If it trips again within 24 hours, the problem is a failed thermostat or element — not the button.
Electric water heaters have one or two heating elements that burn out from thermal cycling and mineral buildup. A failed element means no heat on that circuit.
Test: Turn off power. Unplug the element wires. Set a multimeter to ohms. A good element reads 15-40 ohms. An open circuit (OL) means it's burned out.
Fix: Replace the element ($20-40). While you're in there, replace the thermostat if it's older than 5 years.
Gas water heaters need a lit pilot or a working ignition system. If the pilot is out or the gas valve isn't opening, you get cold water.
Fix: Relight the pilot per the manufacturer's instructions. If it won't stay lit, the thermocouple is likely failing ($15-25 part). If the pilot lights but the burner doesn't ignite, the gas valve or igniter is bad ($80-150).
Hard water deposits mineral sediment at the bottom of the tank. Over years, a layer of scale insulates the heating element from the water, so the element burns hot but the water stays cold. In severe cases, sediment overheats and cracks the tank.
Fix: Flush the tank. Attach a garden hose to the drain valve at the bottom. Run cold water into the cold inlet to agitate sediment. Drain until the water runs clear. Annual flushing prevents most sediment issues.
The thermostat controls when the element or burner turns on. If it fails open, the element never gets power. If it fails closed, the element runs continuously and overheats.
Test: Replace the thermostat with a matching model ($15-30). Most thermostats are two-pin or three-pin. Match the wires and the temperature setting.
If the tank is low on water — from a failed dip tube or a drain valve left open — the element can overheat and shut down, or the gas burner can't heat water that isn't there.
Fix: Close all faucets. Check the drain valve is fully closed. Let the tank refill. If the dip tube is broken (you'll see cold water at the hot tap despite the heater being on), replace the dip tube ($20-40).
On gas units, the proportioning valve mixes gas and air for the burner. If it fails, the burner won't ignite or will burn unevenly.
Fix: This is a professional repair. The valve is expensive ($200-400) and requires proper gas line handling.
The sacrificial anode rod attracts mineral deposits instead of the tank. When it's fully consumed, the tank starts corroding and the heater's efficiency drops sharply.
Fix: Replace the anode rod ($20-40). In hard water areas, check it every 1-2 years.
A properly maintained water heater lasts 8-12 years. Flushing the tank annually and replacing the anode rod every 2 years can extend life to 15+ years. If your heater is over 12 years old and failing, replacement is usually more economical than repeated repairs.