Tankless Water Heater Parts

What you need to know about gas burner, pilot light, heat exchanger, venting

Reviews  | Submit a Review

The article about tankless water heater parts explains the main components on electric and gas water heating appliances that heat water on demand. We will cover the top manufacturers of on demand models, including AO Smith, Rheem, Paloma, Rinnai, Bosch, Noritz and others.

Article about tankless water heater parts is to help you in your home DIY project, troubleshooting, servicing, repairing and when replacing the broken parts. As described before, in the article about tankless water heaters, tankless technology allows water to be heated on demand and delivered instantly.

This new and innovative technology works different from storage tank heaters. The great advantage over tank type is that majority of the parts are replaceable and recyclable. The main tankless water heater parts are:

  • Heat exchanger

  • Gas valve

  • Burner

  • Ignition system

  • Combustion chamber

  • Combustion Fan

  • Temperature sensors

  • Safety elements

  • Venting

  • Electronic system

  • Water valve

  • Remote control

Keep in mind that above tankless water heater parts are not present all on different brands and models; our goal is to cover majority of the important elements found among different manufacturers.

Heat exchanger

The heat exchanger is a device, made of copper tube, or stainless steel, where small quantity of water is stored and used to transfer the heat to the water that is flowing through. The heat is generated from the gas on the gas burner that is burning below the heat exchanger. In a case of the copper heat exchanger, the copper tube is combined with the copper fines, to increase the overall surface and increase the heat transfer.

In electric water heaters, water is heated by a heating element (i.e. electric coils) which is turned on by a flow activated switch.

tankless water heater partsBetter quality tankless heaters like Rinnai have a commercial grade heat exchanger. Noritz has a super heat exchanger with a thicker piping for longer life and increased thermal efficiency. Condensing water heaters are equipped with the stainless steel heat exchanger.

In winter, when heat exchanger is not used, cold water inside might freeze, especially when the cold air comes down through the vent pipe. This is why a minimum vent pipe and the amount of combustion air is required.

Due to the high mineral content in water, the heat exchanger may be scaled internally so the proper maintenance is necessary.

Most of the manufacturer will provide a warranty on heat exchanger, which can be decent 12 years while on other tankless water heater parts usually one year.

Combustion elements

Combustion chamber is where the gas burns. Combustion chamber is designed as room sealed chamber (direct water heaters) and open (on atmospheric models).

Gas valves can be multi-stage and single-stage device, or with the modulation and ON/OFF option.

Gas valves with the modulation are adjusting its power based on the water flow, demand and changes in any incoming and outgoing water temperatures; ON/OFF works with the maximum gas power only.

Combustion fan is positioned below the burner so the air can be rapidly mixed with burning gas resulting in an increased efficiency.

Burner elements

The gas burner is usually installed below the heat exchanger, and on some commercial and condensing water heater models is made of high quality, stainless steel material, so it can work with higher heat outputs. Some manufacturers are designing high efficiency and high power burners like Ceramat burner from Bosch and Aquastar units.

Noritz water heaters are using a dual-flame technology to maximize the output, provide temperature stability and keep the gas emissions low.

With other manufacturer you can see a huge progress toward environmentally friendly burners, burners designed to reduce NOx emission.

The burner flames that are generated on the burner should have a blue color, while yellow flames are sign of a wrong orifice size, dirty burner or blockage of the heat exchanger fins and improper combustion.

It is recommended to check the shape, pattern and color of the burner flame through the provided sight window located at the front of the heater's cover.

emergency plumber

Ignition system

Ignition system is using either a pilot light with thermocouple or electronic ignition device, usually with one or two electrodes. Both systems are used to light the fire on the main gas burner and inform the control device that the flame is either on or off.

Bosch has a great innovative design where the water heaters are equipped with hydro-generated ignition system that allows this tankless water heater to operate without a pilot light, electricity, or a battery.

Safety features

Every tankless water heater is equipped with the safety and control elements that are working together with the PC board for better protection, safety and performance.

  • Flame sensor or ionization rod is used to ensure that the flame is present, which secures the proper water heater work. When the flame is extinguished it sends the signal to stop the gas flow and the water heater will shut down. Thermocouple is used on models with the pilot light.

  • Overheat sensor will trip due to overheating shutting the unit down. As the hot water temperature had exceeded predetermined value, the unit must cool down before resuming the operation.

  • Temperature limiter and frost sensor protects the water heater from freezing.

  • Fan speed monitoring is used to ensure the proper fan operation.

  • Water flow sensor protects the system from minimum flow so heat exchanger can get enough water for normal operation.

  • Pressure relief valve is usually not supplied with the unit and is used to protect the heating system from the excessive pressure in plumbing.

Control panel

PC electronic board is the most sophisticate among tankless water heater parts and is the heart of the water heater. It is used to read and digitally display the temperature and error codes on the LCD display and using an on-board memory chip to store the code for easy maintenance and troubleshooting.

This self diagnostic computer system and intelligent electronic controls are also designed to increase energy efficiency and safety, also working toward reducing the NOx gases.


Tankless water heaters using gas are designed to vent the gases out and bring the air in using one of the three options:

  • Direct Vent

  • Power Vent

  • Atmospheric Vent

Note: The installation of venting must comply with the national and local codes and per manufacturer's instructions (to meet minimum and maximum lengths, location, recommended distances).

Venting pipes can be run horizontally or vertically, with the two-pipe system or concentric venting.

Condensate collector must be used on all vertical installations, while on horizontal runs downward slope can be used instead.

The air intake pipe can use aluminum, ABS, CPVC or PVC pipe, while for combustibles, UL approved, Category III, stainless steel vent material must be used.


For placing a tankless water heater parts order contact a store where the water heater was purchased or the distributor following the next steps:

  • Include model and serial number of your tankless water heater found on the rating plate.

  • Include type of gas (natural gas or propane gas (LP)).

  • Include part description and number of parts.

(Photo credit to Amazon)