Case (cabinet)The wall-hung metal box. Its front cover is off in this drawing.
Exhaust vent (flue)Carries the burned gases from the unit to the outdoors.
Air intakeOn a direct vent unit, brings outdoor air to the burner.
Cold water inletCold supply water enters the heater here.
Hot water outletHeated water leaves here for the faucets.
Gas inletThe gas supply line connects here.
Heat exchanger (primary)Water in its tubes takes up the heat of the flame.
BurnerBurns the gas. Its flame grows or shrinks with demand.
Upper half: vents, heat exchangers, burn 5
Remote controller (temperature controller)Sets the hot water temperature and shows error codes.
Secondary heat exchangerPreheats the cold water with heat left in the exhaust.
Igniter (spark electrode)Makes the spark that lights the burner. There is no pilot.
Flame rod (flame sensor)Tells the control board whether the burner is lit.
Hi-limit switch (high limit switch)Shuts the heater off if the water gets too hot.
Lower half: valves, sensors, connections 13
Isolation valves (service valves)Shut the heater off from the house pipes for flushing.
Pressure relief valveOpens if the water pressure climbs too high.
Combustion fan (fan motor)Blows air into the burner, which pushes exhaust out the vent.
Control board (PCB)The computer that lights the burner and sets gas and water flow.
Gas valveOpens and meters the gas sent to the burner.
Condensate drainLets out the acidic water that forms in a condensing unit.
Flow sensorDetects water moving when a tap opens, which starts the heater.
Inlet water filterA wire mesh at the cold inlet that traps debris.
Gas shut-off valveA hand valve on the gas supply, outside the case.
Water control valve (flow adjustment valve)Adjusts the water flow so the outlet temperature holds steady.
Bypass valveMixes bypassed cold water into the hot to steady its temperature.
Outlet thermistor (temperature sensor)Measures the temperature of the water leaving the heater.
Inlet thermistor (temperature sensor)Measures the temperature of the cold water coming in.
How this list was compiled: Part names and every detail line were checked, for facts only and with nothing copied, against pages read on 2026-09-20. Wikipedia: https://en.wikipedia.org/wiki/Tankless_water_heating (water held only in the heat exchanger coil if no internal buffer tank is fitted; flow sensors activate the heater when water travels through them; a flow sensor, a modulating gas valve and inlet and outlet water temperature sensors on modulated units; a bypass valve and a mixing valve built into the unit; the flow control valve works with thermistors to hold the temperature stable; gas units have an exhaust vent on the top and still need electric power for electronics, sensing and ignition; the flow detector turns on the heating elements or gas burner, and electric units modulate the heating elements; yearly descaling using permanently installed service valves); https://en.wikipedia.org/wiki/Condensing_boiler (a condensing appliance recovers latent heat by condensing water vapour out of the flue gas; the condensate is slightly acidic and needs a drain; a fan is needed to move the cooled flue gas); https://en.wikipedia.org/wiki/Flame_rectification (a flame conducts and rectifies a small current, which flame sensors use to detect the presence of flame). First maker, parts list, two installation manuals and an accessories page: http://takagi.com/media/4012/JR2-Parts-List.pdf (case assembly, front cover, burner assembly, manifold assembly with gas valve assembly, fan motor, flame rod, igniter rod, igniter, gas inlet, heat exchanger, flow adjustment valve and flow sensor, water inlet, inlet water filter, inlet thermistor, outlet thermistor, water outlet, Hi-Limit switch, overheat cut-off fuse, computer board, exhaust port); http://takagi.com/media/3821/T-KJr2-T-K4-T-D2.pdf (the eight steps of normal operation: the flow sensor detects the water flow, the computer starts the fan motor and signals the igniter to create an ignition spark, water circulates through the heat exchanger, the computer uses thermistors and modulates the gas and water valves; the unit has no pilot; the Hi-Limit switch is a built-in shut-off for high temperature; a pressure relief valve must be fitted on the hot water outlet); http://takagi.com/media/12843/T-H2S-DV-OS.pdf (concept diagram naming intake port, exhaust, secondary heat exchanger, primary heat exchanger, burner, thermistors, bypass valve, water control valve, flow sensor, fan motor, gas valve, PCB, condensate drain port, gas, hot water outlet and cold water inlet; the secondary heat exchanger absorbs latent heat from the exhaust gas; a wire mesh filter in the cold inlet traps debris; manual gas shut off valve; the temperature remote controller adjusts the output temperature and gives an error code); http://www.takagi.com/products/accessories/isolation-valves-pressure-relief-valve (isolation valves isolate the unit for service; the pressure relief valve opens if pressure passes its rating). Second maker, installation manual: https://www.navieninc.com/downloads/npe-installation-manual-en/download (section 2.5 Components: exhaust duct, intake air duct, fan motor, ignition transformer, burner, primary heat exchanger, secondary heat exchanger, gas pipe, flow sensor, front panel, condensate trap, gas valve, PCB box, water adjustment valve, mixing valve, gas inlet fitting, water inlet filter, hot water supply fitting, condensate drain fitting; safety devices: flame rod and water temperature high limit switch; front cover screwed to the case; gas shutoff valve, pressure relief valve and remote controller named in the installation chapters; direct venting takes combustion air from outside the building). Third maker: https://www.rinnai.us/tankless-101 (water is preheated in the secondary heat exchanger and brought to temperature in the primary heat exchanger; the flame modulates) and https://www.rinnai.us/residential/flushing-my-rinnai-tankless-water-heater (a cold water valve and a hot water valve with purge port valves and caps). Fourth maker: https://noritz.com/how-tankless-works and https://noritz.com/post/how-to-resolve-the-seven-most-common-noritz-tankless-error-codes (sensors, internal computer, burners, heat exchangers, hot water outlet; ignition plug, gas valve, exhaust vent, condensate drain line, circuit board, remote control, fan motor). The plan names the US Department of Energy Energy Saver page https://www.energy.gov/energysaver/tankless-or-demand-type-water-heaters as a source. It returned 404 on 2026-09-20, as did every Energy Saver address tried, so nothing here rests on it. No maker is named on the page. Re-checked on 2026-09-30: all 26 names found again in two or more of Wikipedia (Tankless water heating, fetched as wikitext), the first maker's three documents listed above, a second maker's installation and operation manual (https://archive.org/details/manualzilla-id-7388842) and a third maker's installation manual (https://www.navieninc.com/downloads/npe-installation-manual-en/download); secondary heat exchanger and hi-limit switch rest on the first and third makers. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; own work, nothing traced or copied. Licence: own work
How it works
The parts of a tankless water heater sit on two paths that meet at the heat exchanger: water from the cold inlet to the hot outlet, and gas and air to the burner. This drawing takes the front cover off a wall-hung gas unit.
Read the full guide
How the parts work, from open tap to hot water
Water enters at the cold water inlet and passes the inlet water filter. The flow sensor detects it. The control board starts the combustion fan and signals the igniter, the gas valve opens and the burner lights. The water is preheated in the secondary heat exchanger, brought to temperature in the primary one and leaves at the hot water outlet. Thermistors report temperatures, and the board trims the gas valve and the water control valve to hold the setting.
What an electric tankless water heater has instead
An electric unit heats the water with heating elements, so it has no burner, gas valve, combustion fan or exhaust vent. A flow sensor still turns it on.
Start here: 8 main parts covers the case, the two pipes on top, the three connections underneath, the heat exchanger and the burner.
A non-condensing unit has one heat exchanger and no condensate drain, so parts 15 and 16 will be missing on it.
Questions and answers
What are the main parts of a tankless water heater?
A gas unit has a heat exchanger, a burner, a gas valve, a combustion fan, a flow sensor, a control board and thermistors. They sit in a case with a cold water inlet, a hot water outlet, a gas inlet and an exhaust vent.
Does a tankless water heater have a pilot light?
No. A gas tankless unit uses electronic ignition. When the flow sensor detects water moving, the control board signals the igniter to spark and the burner lights. The flame rod tells the board that the burner is lit.
What is the secondary heat exchanger in a tankless water heater?
It is the second heat exchanger of a condensing unit. Cold water passes through it first and is preheated by heat left in the exhaust. Water vapor in the exhaust condenses there, and that acidic water leaves by the condensate drain.
What are the valves under a tankless water heater?
Usually a pair of isolation valves, also called service valves, one on the cold line and one on the hot, each with a capped port for flushing. A pressure relief valve sits on the hot side, and a gas shut-off valve on the gas line.