What it is

This is the troubleshooting tree for the most common gas-water-heater call: "I have no hot water." The goal is to find where the heating chain broke — gas supply, pilot/ignition, the gas control valve, or the water side (dip tube, sediment) — without throwing parts. As with a furnace, the trick is to figure out exactly where in the sequence it quit, because that step IS the diagnosis.

How it works

A gas tank only makes hot water if four things are true: gas is getting to it, the pilot/ignition source is working, the gas control's thermostat is calling and opening the main burner, and the burner is actually firing and drafting. Break any link and you're cold. "No hot water at all" and "lukewarm / runs out fast" point at very different parts of that chain, so the first job is to separate them.

In the field

Work it in order:

  1. Confirm the complaint. Stone-cold at every tap, or lukewarm/short supply? Is it only hot water, or is there no water pressure at all (different problem)? Check the gas control dial isn't set to "Pilot" or "Off" and the setpoint isn't turned way down. Confirm the gas is on (other gas appliances working).
  1. Check the burner/pilot status. Most modern units have a status LED that blinks a code through a sight window — read it; it'll often point straight at "pilot not proven," "no thermocouple signal," etc. Look through the sight glass: is the pilot lit? Is the main burner firing on a call?
  1. If the pilot is OUT: try to relight per the unit's procedure.
  • Won't light at all: no gas to the pilot (supply, control valve closed, plugged pilot orifice), or on an electronic-ignition unit, a dead igniter/no power (check batteries on battery-spark models).
  • Lights but dies when you release the button: classic thermocouple/thermopile failure (or it's not in the flame, or a failing gas-control magnet). The thermocouple isn't making enough millivolts to hold the safety open.
  • Lights, holds, but main burner never fires on a call: the gas control's thermostat isn't opening the main — bad gas control, or the tank's already at setpoint (it isn't actually cold).
  1. If the pilot is LIT and burner fires but you're still cold-ish: now it's the water side or capacity.
  • Lukewarm / runs out fast: broken/shortened dip tube (cold mixes at the top), heavy sediment insulating the burner (rumbling, slow recovery), undersized for demand, or crossover (a bad mixing valve/fixture letting cold into the hot side).
  1. If the burner won't stay lit / FVIR lockout: a lazy yellow flame or a unit that lights then drops out can be a starved FVIR flame-arrestor screen (linted up), a blocked flue, or a combustion-air problem. Check and clean the arrestor; verify draft.
  1. Verify draft and CO before you leave any gas unit you've been into — confirm it drafts at the hood and check CO.

Normal values & targets

  • Thermocouple output: ~25–30 mV open-circuit healthy; gas control drops out below ~8–10 mV.
  • Thermopile (millivolt units): ~500–750 mV.
  • Setpoint: typ. 120°F; confirm the dial and that the tank is actually below it.
  • Recovery: a 40K BTU/h unit ≈ 40+ GPH at a 90°F rise — "runs out" can just be demand outrunning recovery, not a fault.
  • Status LED: blink-code through the sight window points to the failed step — read it first.

Common faults & what they mean

  • Pilot dies on button release → thermocouple/thermopile or gas-control magnet. Most common gas no-hot-water cause.
  • Pilot won't light at all → no gas (supply/valve), plugged pilot orifice, or dead electronic igniter/batteries.
  • Pilot fine, main never fires → gas control thermostat not calling/opening (bad control), or tank already hot.
  • Lukewarm / runs out fast → dip tube broken, sediment, undersized, or a crossover letting cold into the hot side.
  • Lights then drops out / yellow lazy flame → FVIR arrestor clogged, blocked flue, or combustion-air starvation. CO concern.
  • Rotten-egg smell but it heats fine → anode reaction, not a heating fault (different article).

Tech tips & gotchas

  • Read the status LED first. Modern gas controls blink a diagnostic code through the sight window. It saves you the guesswork on which step failed.
  • The "dies on release" test is gold. If the pilot only stays lit while you hold the button and drops the instant you let go, it's the thermocouple/magnet circuit almost every time — not the gas valve body.
  • Lukewarm ≠ no hot water. Splitting "stone cold" from "lukewarm/short" up front keeps you from chasing a dip tube when the pilot's out, or a thermocouple when it's really a worn dip tube.
  • Don't overlook the FVIR screen. A modern atmospheric heater that lights then goes lazy/yellow and trips is often just starved by a lint-clogged arrestor at the base — clean it before condemning the gas control.
  • Confirm it's actually cold. Sometimes "no hot water" is a setpoint turned down, the dial left on Pilot after someone messed with it, or another appliance bleeding the supply. Check the easy stuff first.

Safety / code notes

  • Any time you work on a gas appliance, verify draft and check for CO before returning it to service — a unit that lights then spills or drafts poorly is a carbon-monoxide hazard. Recommend working CO alarms.
  • Never bypass the thermocouple/pilot safety — it's what shuts the gas off when the pilot is out. If it won't hold, replace it; don't defeat it.
  • Confirm the T&P valve and discharge are correct and unobstructed on every unit you service (IPC water-heater/relief provisions).
  • Venting and combustion air follow the appliance listing and fuel-gas code venting/combustion-air sections (IFGC §503 and the combustion-air provisions) — a blocked flue is both a no-heat and a CO problem.