Short definition

No hot water at every tap points to the water heater itself; cold at one tap with hot at others points to that fixture. For electric heaters, start at the breaker and the upper thermostat reset button. For gas, check the pilot. For tankless, check the inlet temperature and gas pressure. Most causes are diagnosable in five minutes.

What it is

“No hot water” is the umbrella diagnostic for any failure of the hot side of your plumbing system. The first decision is whether the problem is system-wide or local — and that one observation routes the rest of the diagnosis.

All fixtures cold = water heater. Walk through this in order:

  • Electric tank:
    1. Check the breaker. Most common cause; often trips during a power blink.
    2. Press the red ECO reset button on the upper thermostat (behind the upper access panel).
    3. Confirm thermostat setpoint of 120°F (UPC standard for scald control with anti-Legionella margin).
    4. Listen for element activity. If silent, an element or thermostat has failed — multimeter test or service call.
  • Gas tank:
    1. Check that the pilot is lit. Relight per manufacturer instructions.
    2. If the pilot won’t stay lit, the thermocouple is most likely failed — replaceable, but a service call for most homeowners.
    3. Check that the gas valve is on at the meter and at the heater.
    4. If the burner runs but water stays cold, the dip tube or thermostat may have failed.
  • Heat-pump water heater (HPWH):
    1. Check operating mode. In a cold WA garage in winter, “heat-pump only” mode may not deliver — switch to hybrid or resistance backup.
    2. Confirm the air filter is clean.
    3. Compressor failures are uncommon but service-call territory.
  • Tankless:
    1. Confirm gas supply and pressure.
    2. Check the flame-rod and igniter (manufacturer-specific).
    3. PNW winter inlet water at 45°F drops a tankless’s effective gallons-per-minute capacity — if you only get cold during heavy demand, the unit may be undersized for the inlet temperature.

One fixture cold = local issue:

  • Worn cartridge in a single-handle faucet or shower valve.
  • Failed thermostatic mixing valve or anti-scald valve.
  • Cross-connected supply lines (rare; usually only after recent work).

Why it matters to a homeowner

A cold shower at 6:30 AM is one of the most common emergency plumbing calls. About half of “no hot water” calls in the field turn out to be a tripped breaker or an out-of-pilot — five-minute fixes that don’t need a service truck. The other half need parts and tools you probably don’t have, but knowing which half you’re in saves the cost of an unnecessary visit.

When a plumber says they replaced “the upper thermostat with the ECO” or “the thermocouple,” they’re talking about exactly the parts in the diagnostic above. If you’ve already checked the breaker and the pilot before they arrive, you’re paying for the fix, not the diagnosis.

Common failure modes

  • Tripped breaker (electric) — most common; check first.
  • Pilot out (gas) — relight; if it won’t stay, thermocouple.
  • Failed upper element (electric) — produces “warm but no hot” pattern, see sister entry.
  • Failed thermostat — often paired with reset-button trip.
  • Cold-mode HPWH in winter garage — switch operating mode.
  • Tankless gas pressure or flow-sensor issue — service call.
  • Single fixture cross-connection — cartridge or mixing valve.

Common variants

  • No hot water vs. warm but no hot. Full failure (no hot at all) vs. partial (lukewarm only) — different elements or thermostats fail.
  • No hot water vs. hot water runs out fast. Capacity issue (sediment, undersized heater) vs. heating failure.
  • Tankless “no hot” vs. “not enough hot.” Gas/igniter issue vs. capacity limited by cold inlet temp.

Washington note

Two WA-specific patterns to know about. First, heat-pump water heaters in detached or unheated garages can underperform in January and February — a HPWH is essentially a refrigerator running in reverse, and below about 40°F ambient it leans heavily on the resistance backup. If yours is in cold space, set it to hybrid mode for winter.

Second, tankless heaters during the January 2024 PNW freeze quit on hundreds of WA homes — incoming water below the unit’s minimum inlet temperature, frozen condensate drains on condensing models, or simply maxed-out capacity at 38°F inlet. If your tankless is sized to a “warm climate” spec, undersizing reveals itself only during the worst week of the year.