What it is

A gas storage water heater is a steel tank lined with glass (porcelain enamel), wrapped in foam insulation, with a gas burner sitting underneath it in a sealed combustion chamber. Cold water comes in, the burner heats it, hot water leaves out the top. Simple in principle, but there's a stack of safety devices between "thermostat is satisfied" and "burner is firing," and if you know the order, you can diagnose a dead heater fast.

This covers the standard atmospheric (natural-draft) tank — the kind that drafts up a flue through the center of the tank and out a draft hood into a B-vent. Power-vent and condensing units add a blower and electronics; the core water-handling parts are the same.

How it works

Cold water enters through a dip tube that carries it down to the bottom of the tank, near the burner, so the coldest water gets heated first and you don't dilute the hot water at the top. Heat rises, so the hottest water stratifies at the top, which is exactly where the hot outlet pulls from. A flue tube runs up through the center of the tank; combustion gases travel up that flue and transfer heat into the surrounding water before exiting the draft hood at the top. That center flue usually has a twisted metal baffle in it to slow the gases down and pull more heat out.

The gas control valve (the round dial bolted to the side near the bottom) is the brain. It contains the temperature-sensing thermostat — a probe that reaches into the tank water — plus the pilot safety and the main gas shutoff. On a standing-pilot unit, a thermocouple or thermopile sits in the pilot flame and generates a tiny voltage that holds the pilot gas valve open; lose the pilot, the voltage drops, and the safety closes the gas. Newer units use an electronic igniter and a flame sensor instead of a standing pilot.

In the field

Sequence on a standard standing-pilot atmospheric unit, call for heat:

  1. Tank temperature drops below the gas control's setpoint. The thermostat inside the gas control valve senses it through its probe in the water.
  1. Thermostat calls. The gas control opens the main gas port to the burner. (The pilot is already lit and being proven by the thermocouple — that never stopped.)
  1. Main burner lights off the standing pilot flame. You'll hear the soft "whump" and see the burner ring light.
  1. Combustion drafts up the flue. Hot gas rises through the center flue, heats the water, and exits the draft hood into the vent. The draft hood lets room air dilute the flue gas and provides a break so a downdraft spills at the hood instead of snuffing the pilot.
  1. Water heats, stratifies. Hot water rises to the top; the dip tube keeps feeding cold to the bottom.
  1. Thermostat satisfied. Gas control closes the main burner. Pilot stays lit, held by the thermocouple, ready for the next call.

On an electronic-ignition unit, swap steps: the call energizes an igniter (hot surface or spark), the gas valve opens, a flame sensor proves the burner, and the igniter drops out — same idea as a furnace, just heating water.

Normal values & targets

  • Setpoint: factory default is usually around 120°F. Code and scald guidance push toward 120°F at the tap; some applications run hotter for sanitization or demand and temper down with a mixing valve.
  • Recovery: a 40-gal 40,000 BTU/h unit recovers roughly 40 gallons in an hour at a 90°F rise — recovery scales with input BTU and inversely with the temperature rise you're asking for.
  • Thermocouple output: roughly 25–30 millivolts open-circuit on a healthy one; the gas control drops out when it falls under about 8–10 mV.
  • Thermopile (millivolt systems): generates on the order of 500–750 mV — enough to run the whole control without external power.
  • Flue gas temperature: warm but not glowing; an over-temp flue or roaring sound suggests a sooted/restricted flue or overfiring.

Common faults & what they mean

  • No hot water, pilot won't stay lit. Thermocouple weak or not in the flame, dirty pilot orifice, or a failing gas control magnet. Hold the button — if it dies the second you release, that's the thermocouple/magnet circuit.
  • Pilot lights, burner won't come on with a call. Gas control thermostat not opening the main, or the setpoint is below tank temp. Confirm the dial is calling and the tank is actually cool.
  • Runs but not enough hot water / runs out fast. Broken or shortened dip tube (cold mixes at the top), or undersized for the demand. A cracked dip tube is classic on units where cold and hot suddenly mix.
  • Sooting, yellow flame, rollout at the draft hood. Combustion-air problem, blocked flue, or a dirty burner/FVIR screen. This is a CO concern — don't hand it back without fixing the draft.
  • Rotten-egg smell from hot water (not gas). Anode reacting with sulfate in the water making hydrogen sulfide — different problem, covered under anode rods.

Tech tips & gotchas

  • Most modern atmospheric gas heaters are FVIR (flammable-vapor ignition resistant): a sealed combustion chamber with a flame-arrestor screen on the bottom that keeps the burner from igniting gasoline vapors rolling across the floor. If that screen lints up, the unit starves for air, the flame goes lazy/yellow, and it may trip a one-time thermal cutoff. Check and clean the arrestor before condemning the gas control.
  • A thermocouple that "tests good" cold can still drop out hot. If the pilot holds when you light it but drops after the main burner has run a while, suspect a marginal thermocouple or a gas control losing its magnet hold.
  • The draft hood is supposed to spill a little on startup for a second — but persistent spillage (you feel exhaust at the hood with a match/smoke) means the vent isn't drafting. Check the flue, the vent connector pitch, and combustion air before anything else.
  • Sediment in the bottom of a gas tank insulates the water from the burner — you get rumbling/percolating sounds, slow recovery, and premature tank failure. Flush it.

Safety / code notes

  • A standard atmospheric gas heater is a fuel-burning appliance — venting follows the appliance listing and the fuel-gas code venting provisions (IFGC §503), and it needs adequate combustion air per the combustion-air sections of the fuel-gas/mechanical code.
  • Every storage water heater requires a properly rated temperature-and-pressure (T&P) relief valve with a full-size drain line — required by the plumbing code (IPC water-heater provisions) and the appliance listing. Never cap it or reduce the discharge pipe.
  • Persistent flue-gas spillage at the draft hood is a carbon-monoxide hazard. Confirm draft with a smoke source or analyzer and verify CO levels before returning the unit to service; recommend working CO alarms in the home.