Tankless Gas Water Heaters - Anatomy of a Tankless

Explore tankless gas water heaters to see what makes them special. How gas tankless work. Features to look for and recommended tankless models. How to winterize, maintain, problems and more.

Rinnai tankless gas water heaterRinnai tankless gas water heater

Main components

Heat exchanger

Gas water heaters that are using tank-less technology are not storing hot water in the metal tank as the conventional models, but are heating water on demand, on call.

Heating is done in the heat exchanger that is usually made of copper (stainless steel on advanced models), where water while passing through gets the heat from the gas burner, most of the time located underneath.

The heat exchanger is one of the main components found in tankless gas water heaters. It usually comes with the 12-year warranty that has to be protected from freezing during the winter time and maintain regularly to prevent the limescale build-up.


Tankless gas water heaters are using horizontal or vertical venting to run the flue gases out and provide fresh air for gas combustion. Also, the non-condensing tankless unit must use a dedicated Category III approved stainless steel vent.

Condensing tankless type heaters are more flexible, they can use PVC for example, as the exiting temperature of the flue gases is much lower. Most of today's manufacturer offers the power or direct venting, with the single-wall or concentric venting systems. This applies to indoor models only, while the outdoor models do not require venting pipes.

Safety elements

Every gas-powered tankless water heater is protected by several safety elements such as the flame rod for flame failure, fan sensor, a sensor for overheating protection, thermal fuse, freeze protection and other.

Other components

  • The burner is the component where the natural or propane gas is burning which further heats the water in the heat exchanger
  • The gas valve is computer controlled and supplies the burner with gas
  • Water flow sensor
  • Water filter
  • Electronic board - will modulate the gas supply valve, water flow, monitor the operating parameters, run the diagnostics and show the error codes if there is a problem
  • Fan/blower - provides adequate combustion air and removes the products of combustion out
  • Ignition device with the flame rods - automatically lights the burner
  • Drain valve
  • Pressure relief valve
  • Water and gas line
  • LCD display and remote controls

How does the tankless gas water heaters work

Heaters with the tank-less technology, also called on-demand heaters do not require storage tanks, and they provide hot water on demand. No storage tank means no tank corrosion and rust, also no anode rods and rotten egg odor issue.

The lifespan is two times the lifespan of the tank type. Other advantages are shown in the article about tankless heaters.

Propane or natural gas heaters will start heating only when there is a demand for hot water when the hot water tap is open. To activate the unit, the minimum flow rate must be met. If the flow is below the set value, the burner will not run. This is also a safety feature as, without the minimum flow, the heater will run continuously, and heat exchanger will overheat.

Tankless units are equipped with the powerful burners which heat the water that is running through the heat exchanger instantly - this is why tankless is also called instantaneous.

As the water flows, the flow switch senses the movement and sends the signal to the electric control board where other information (i.e., incoming, desired temperature, fan speed...) are also collected.

If all the safety factors and required parameters are met, the tankless gas hot water heater will run the fan first and purge the combustion chamber, and the electrode will ignite the gas burner which gets the gas from the gas burner.

A gas valve is usually designed with to modulate so based on the temperature of the incoming and exiting water, demand and set parameters, it increases or lowers the flame or heating power while working between the designed minimum and maximum BTU input ratings. This is how one tankless gas water heater keeps the temperature even while it uses the exact amount of gas needed for heating.

This is especially important when someone starts using hot water while you are showering. As the pressure changes in your home plumbing system and the tankless unit, modulation prevents the temperature fluctuation.

When you turn the hot water tap OFF the tankless unit senses no flow, so it shuts down.

Keep in mind that tankless gas water heaters heat the water very fast but will not deliver it instantly. You will have a constant flow of hot water at the desired temperature, but it still takes some time to get it (cold water inside the pipes has to be pushed out before you get any hot). The only way to get it instant is if you install the recirculation pump, or buy a model with the recirculation built-in. A good example is Rinnai RUR98 or RUC98.

Almost all gas tankless water heaters require electricity (Bosch Aquastar 1600 does not), gas and water supply and proper venting.


The best gas tankless water heaters can be found among the manufacturers shown below in the "related" section. They are the best because of the quality of the products, a variety of available models, the most innovative features, advanced technology, and high efficient heating. Top gas units also include the most advanced condensing models which are Energy Star approved.


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