A water heater holds hot water under pressure and runs on gas or electricity. A handful of parts keep that in check. A thermocouple stops the gas if the pilot goes out, the vent takes exhaust outdoors, the relief valve and expansion tank handle pressure, thermostats run the electric elements, and a drain pan and straps limit what a leak or an earthquake can do. This guide takes those parts in order and points to the page that covers each in full.
Those pages take every step and figure from the maker’s own manual, a federal agency or a model code. Your heater’s label and manual win wherever they differ.
General information, not professional advice. Gas, 240-volt power, scalding water and a pressurised tank can hurt you. Follow the label and manual on your own heater; where they say to stop and call a qualified technician, stop. Codes and licensing rules are set locally.
Gas smell or a CO alarm: this comes first
- If you smell gas, leave. PHMSA, the federal pipeline safety agency, says not to start an engine, strike a match, use a phone or turn any light or electrical switch on or off. Call 911 from a neighbor’s house or somewhere well away from the smell. Rheem’s manual adds: don’t go back in until the gas supplier or fire department says you can.
- If a carbon monoxide (CO) alarm sounds, or people feel ill, get everyone outside into fresh air, then call 911 (CPSC). Headache, dizziness, weakness, nausea, vomiting and confusion are common signs.
- A heater that has been under water is not relit or repaired by its owner: A. O. Smith and Rheem say to have it replaced.
The full steps, the warning signs you can see on a gas heater, and where CPSC says CO alarms go are on gas smell or carbon monoxide near a water heater. CDC also advises having gas water heaters serviced by a qualified technician every year.
Know your shut-offs before you need them
The makers’ manuals start with the same moves. On a gas heater: the gas control knob to OFF by hand, then the manual gas valve on the supply pipe beside the heater closed. On an electric or heat pump heater, or a gas heater with a fan or display: the circuit breaker off. If the tank is leaking: the cold water valve to the heater closed so it stops refilling. Find them on your own heater on an ordinary day; Rheem’s manual asks owners to have the installer show them the gas shut-off valve.
Never switch the power or gas back on before the tank is completely full again: on an electric heater, power on an empty tank burns out the upper element. Before relighting, wait as long as your lighting label says.
Gas heaters: the flame and the vent
The thermocouple, thermopile or flame sensor
On a standing-pilot heater, the pilot heats a thin copper lead, the thermocouple, which makes a tiny voltage that holds the gas valve open. If the pilot goes out, the gas stops. A pilot that lights while you hold the knob but goes out when you let go is the classic sign of a weak one, though other faults look the same. Many newer heaters use a thermopile instead, and powered heaters usually a flame sensor. The water heater thermocouple shows which part your heater has, the checks the manuals give owners, and Bradford White’s replacement procedure. Every maker whose documents we read reserves the replacement for a qualified technician.
How the exhaust gets out
An atmospheric heater lets hot exhaust rise through a metal vent with no fan. A power vent heater pushes it through a plastic pipe with a blower. A direct vent heater is sealed and takes its burner air from outdoors; a power direct vent heater adds a blower. The type decides where the heater can go, whether it needs electricity and what you check each year. EPA lists back-drafting from gas water heaters among the sources of carbon monoxide in homes. Power vent, direct vent and atmospheric gas water heaters compares them and gives the yearly checks.
Pressure: the relief valve and the expansion tank
Water grows as it heats. If a check valve, backflow preventer or pressure-reducing valve stops it pushing back toward the street, the pressure climbs each time the heater runs. A relief valve that drips now and then is the sign A. O. Smith, Bradford White and Rheem all name. The usual answer in their manuals is an expansion tank. A 50-gallon tank heated from 40°F to 120°F pushes out about half a gallon of water. The expansion tank calculator runs that arithmetic for your heater and gives an estimate to compare with the tank maker’s own chart.
The temperature and pressure (T&P) relief valve opens to let water out when the tank gets too hot or the pressure too high. It is never capped, plugged or removed. If it is releasing a lot of very hot water, keep everyone away from the end of its pipe, turn the gas or power off and call a qualified person. The T&P relief valve covers what it does and how the makers say to test it.
Electric heaters: elements and thermostats
Most electric tanks have two heating elements, an upper and a lower, each switched by a thermostat clamped against the tank above it. A. O. Smith’s guide says to switch the breaker off, then confirm with a non-contact tester that the power is off, before opening a panel.
Elements mostly fail in three ways: they burn out in seconds when power comes on before the tank is full, they short to the tank, or they lose output under lime and sediment. Water heater elements explains each and the multimeter test. A. O. Smith’s owner’s guide gives owners the test and replacement steps; Rheem and Bradford White tell owners to call a qualified technician.
Setting the thermostats is an owner step. With the breaker off, CPSC says to set both to the same temperature and suggests 120°F. Every owner’s manual we read sends testing to a qualified person. Electric water heater thermostats sets out which maker allows what. For the temperature itself and CPSC’s burn times, see the water heater temperature setting.
Around the tank: leaks and earthquakes
A drain pan catches drips and carries them through a pipe to a drain. The 2024 International Residential Code, a model code that states and cities adopt with changes, asks for one where a leak would cause damage, such as in an attic or above finished rooms. A leak sensor sounds an alarm, and some models also shut off the water. Neither stops a tank from failing. Water heater drain pans and leak sensors covers where the codes and manuals disagree and what to do when you find water in the pan.
In earthquake country, straps keep a tank from tipping and pulling apart its gas, water and vent connections. The 2024 IRC requires them in Seismic Design Categories D0, D1 and D2, and for townhouses in Category C. California’s law covers residential heaters up to 120 gallons, including ones already installed. Water heater straps and earthquake bracing covers each rule, checking existing straps and what to look at after a quake. Your building department decides which rules apply.
Hot water sooner: recirculation pumps
A recirculation pump moves cooled water out of the hot pipes so hot water reaches a far tap sooner. The control matters more than the pump. ENERGY STAR warns that systems running all the time “have the potential to use more energy” than they save, and the model energy code wants controls that turn the pump off when no one needs hot water. A pump is not an anti-scald device either: it brings water at the heater’s setting to the tap faster. Hot water recirculation pumps covers the layouts, the controls and the tankless differences.
Where to start
- A gas smell, a CO alarm, soot or a loose vent: the gas and CO page in the box above, first.
- The pilot won’t stay lit: the thermocouple page, then why a pilot keeps going out for the causes a new part won’t fix.
- The relief valve drips: the expansion tank calculator and the T&P valve page.
- No hot water from an electric tank: the elements page, then the thermostats page.
- Water on the floor: the drain pan page, then what a leak’s location tells you.
Your next step
Go and look at your own heater. Read the rating plate for its fuel and model number, find the lighting label or breaker and both valves, then get the owner’s manual for that model from the maker’s own site. Some states and cities require a licensed gas fitter or a permit for gas work: ask your building department before any part is replaced.
Figures here come from the linked guides, which cite their sources. Sources checked on 9 November 2026.
