Water heater parts explained: what each part does and costs

Safety first

If you smell gas, do not touch the heater. Leave the building, do not use switches or phones inside, and call the gas utility or 911 from outside^[7]. Never cap or plug the relief valve or its discharge pipe.

The parts at a glance

Part	What it does	Signs it is failing	Typical repair cost	Who should replace it
Anode rod	Corrodes in place of the steel tank	Rotten egg smell, rusty water, an old tank	$150 to $350[1]	Licensed plumber
Heating element	Heats water in electric tanks (usually two)	No hot water, or hot water runs out fast	$100 to $350[1]	Licensed plumber or electrician
Thermostat	Controls element or burner temperature	Water too hot, too cold or uneven	$100 to $350[1]	Licensed plumber
Thermocouple	Proves the pilot is lit on standing-pilot gas units	Pilot will not stay lit	$100 to $325[1]	Licensed plumber
Flame sensor and igniter	Lights and proves the burner on electronic ignition units	Ignition failure codes	Igniter $75 to $350[1]	Licensed plumber
Gas control valve	Meters gas to the pilot and burner	No heat, status light codes	$150 to $550[1]	Licensed plumber only
Temperature and pressure relief valve	Opens if tank pressure or temperature gets too high	Dripping, discharging	$50 to $300[1]	Licensed plumber only
Expansion tank	Absorbs water that expands as it heats	Relief valve drips after heating	$300 to $800 to replace[1]	Licensed plumber
Dip tube	Sends cold water to the tank bottom	Lukewarm water, plastic bits in aerators	$100 to $300[1]	Licensed plumber
Drain valve	Lets you drain and flush the tank	Drips at the valve	See guide	Plumber, or careful owner per manual
Drain pan	Catches leaks under the tank	n/a	See guide	Installed with the heater
Vent pipe and draft hood	Carries exhaust outside on gas units	Rust, gaps, scorch marks, venting codes	Flue pipe $100 to $350[1]	Licensed plumber only
Condensate neutralizer	Treats acidic condensate on condensing units	Service code, drain problems	See guide	Licensed plumber
Mixing (tempering) valve	Blends hot and cold to a safe outlet temperature	Water too hot or not hot enough	See guide	Licensed plumber
Flex connectors and shutoff valves	Connect the heater to water and gas lines	Drips, corrosion, a valve that will not turn	See guide	Licensed plumber

Costs are HomeGuide's national installed ranges from August 2025^[1]. A service call fee of $50 to $200 may be added^[1].

The parts that protect the tank

The anode rod is a metal rod that corrodes so the tank lining does not. When it is used up, the tank starts to rust from the inside. In some water, a magnesium anode reacts with bacteria and causes a rotten egg smell. Bradford White warns that removing the anode entirely voids its warranty and shortens tank life^[4]. See anode rod.

The dip tube carries incoming cold water to the bottom of the tank so it does not mix with the hot water at the top. A broken dip tube gives lukewarm water fast.

The parts that protect you

The temperature and pressure relief valve is the most important safety part on a tank. The model code sets strict rules for its discharge pipe: full size, no valves or tees, no threaded end, draining by gravity, and ending within 6 inches of the floor or a receptor^[3]. A dripping valve is a warning, not a nuisance. See relief valve dripping.

The expansion tank gives heated water room to expand. The residential code requires a thermal expansion control device where a check valve, backflow preventer or pressure-reducing valve closes the system^[2]. Without one, pressure spikes after each heating cycle.

The mixing valve blends hot and cold water so taps cannot deliver scalding water. The CPSC recommends 120 F at the heater to reduce scald risk^[6].

Gas, electric and tankless parts

Gas tanks rely on a gas control valve, a pilot with a thermocouple or an electronic igniter with a flame sensor, a burner, and a vent. Electric tanks use two elements and two thermostats, plus a high-limit switch. Tankless and condensing units add sensors, a fan, a circuit board, and on condensing models a condensate neutralizer. Rheem's code 92, for example, warns that the neutralizer needs service^[5].

Every part guide

• Anode rod
• Heating element
• Thermostat
• Thermocouple
• Flame sensor and igniter
• Gas control valve
• Temperature and pressure relief valve
• Expansion tank
• Dip tube
• Drain valve
• Drain pan
• Vent pipe and draft hood
• Condensate neutralizer
• Mixing (tempering) valve
• Flex connectors and shutoff valves

When to call a pro

• Any work on gas valves, gas lines, burners, igniters or venting.
• Any work on elements, thermostats or wiring.
• A relief valve that drips, discharges or has never been tested.
• An anode change on a tank still under warranty: check the warranty terms first.