When hot water stops working, the most common causes are a failed heating element or burner, a tripped circuit breaker or extinguished pilot light, a faulty thermostat on the water heater, or — in severe cases — a failed water heater that needs replacement. The right diagnosis depends on whether you have a gas or electric unit, whether you’ve lost hot water completely or just have reduced temperature or volume, and how old your equipment is.

Electric Water Heater: Check the Breaker First

Electric water heaters draw significant amperage and can trip their dedicated circuit breaker, especially during high-demand periods or if there’s a voltage spike. Go to your electrical panel and look for a tripped breaker in the 30-amp double-pole position typically associated with the water heater. Reset it and wait 30–60 minutes to see if hot water returns. If the breaker trips again immediately or shortly after resetting, there’s an underlying electrical problem — a shorted heating element, a wiring fault, or a ground fault — that needs a plumber or electrician to diagnose safely.

Gas Water Heater: Pilot Light and Thermocouple

Older gas water heaters with standing pilots can lose the pilot flame — a draft, a momentary gas supply interruption, or a thermocouple that’s failing can all extinguish it. Check the viewing window on the burner compartment; if the pilot is out, the manufacturer’s relighting instructions are typically on a label on the unit. If the pilot won’t stay lit when you hold the reset button and then release it, the thermocouple (the safety sensor that keeps the gas valve open when the pilot is burning) has likely failed and needs replacement. This is an inexpensive repair a plumber can handle quickly.

Failed Heating Elements or Thermostats

Electric water heaters have two heating elements — upper and lower — each controlled by its own thermostat. If the upper element or upper thermostat fails, you may get some warm water but it runs out quickly. If the lower element fails, the water may never get fully hot. On gas units, thermostat failure results in the burner either not firing at all or running continuously without cycling off. A plumber can test elements and thermostats with a multimeter and replace the failed component — on a unit that’s under 8 to 10 years old, this repair is usually cost-effective.

Sediment Buildup Reducing Efficiency

In Denver’s hard water environment, mineral sediment accumulates at the bottom of tank water heaters over time. A thick sediment layer insulates the water from the burner (gas) or coats the lower heating element (electric), dramatically reducing the unit’s ability to heat water efficiently. You’ll typically hear this as a rumbling or popping sound during the heating cycle. Heavy sediment buildup can make it feel like you’ve lost hot water when the system is actually just severely inefficient. A tank flush may improve output, but at a certain point sediment damage is permanent and replacement is the better call.

Tankless Water Heater Issues

Tankless units have their own failure modes. They require minimum flow rate to activate — if a fixture’s aerator is heavily clogged, the unit may not turn on. Error codes on the unit’s display often point directly to the problem: ignition failure, scale buildup on the heat exchanger, a venting blockage, or a flow sensor issue. Tankless heaters in Denver’s hard water benefit from annual descaling; a severely scaled heat exchanger will cause the unit to shut down on overheat protection.

Quick Diagnosis Checklist

  • Electric unit: check and reset the breaker
  • Gas unit with standing pilot: check and relight the pilot; replace thermocouple if it won’t stay lit
  • Gas unit with electronic ignition: listen for ignition click when demand is present; check gas supply
  • Any unit over 10 years old showing multiple symptoms: consider replacement
  • Tankless: check the error code display and verify flow rate at fixtures

Done! diagnoses and repairs all types of water heater problems — and can replace your unit the same day if needed. Visit our water heater repair page to schedule service, or if you’ve been thinking about upgrading, our tankless water heater page explains your options.