How a Tankless Water Heater Works

If you grew up in a home with lots of people taking showers and running appliances, you’ve probably experienced what it feels like when that big tank runs out of hot water. Not too fun, is it? So how can this box, about the size of a suitcase, mounted to your wall keep your showers hot and your tubs full? It’s actually really simple.

  • When the need for hot water arises by turning on a shower, washing machine, dishwasher or faucet, cold water flows into the Rinnai Tankless Water Heater from the inlet pipe at the bottom of the heater.
  • The flame is ignited and heats the two heat exchangers that are included in the Super High Efficiency models. 
  • Water is preheated as it passes through a stainless steel heat exchanger, capturing any extra heat (or latent heat) before it escapes into the vent system. 
  • Water continues to be heated as it passes through the heat exchanger, and exits from the hot water outlet pipe to travel through the pipes of the home to the water fixture where hot water is needed.
  • The tankless water heater will adjust flame output as needed to ensure the temperature set point is maintained.
  • As the hot water fixture shuts off, cold water stops flowing into the tankless water heater and the flame extinguishes. The tankless water heater is now off until the next time hot water is needed!


Recirculation Technology

Rinnai Circ-Logic™ recirculation technology allows homeowners to set recirculation patterns that coincide with their hot water usage patterns. Hot water is available when needed, without the expense of circulating it during times of inactivity. Circ-Logic™ comes standard on most SE, SE+ and HE+ Series models, offering solutions when paired with specific Rinnai accessories.


Rinnai units are gas powered, so you if you’re planning on replacing your tank, certain venting and piping work needs to be done. Your Rinnai Dealer can walk you through the upgrade and help you find the right tankless product for your home and family. You can learn about our vast range of tankless products with our Tankless Product Finder

Tankless vs. Tank for Efficient Water Heating

There are a lot of reasons to consider a Rinnai Tankless Water heater for your home. Be confident in choosing Rinnai tankless technology.


Super High-Efficiency Tankless Advantages

  • Cold water enters inlet pipe only when hot water is required
  • Gas ignites heat exchanger, which heat the water
  • Heated water exits hot water outlet to travel through pipes to where it is needed
  • Heater will adjust flame output as needed to ensure a temperature set point is maintained
  • Product lifespan of a tankless unit is twice as long as a traditional tank*

Standard Tank

  • Large tank requires more installation space and management
  • Water in tank is heated and reheated to maintain desired set temperature regardless of usage or demand
  • During times of heavy use, water can be depleted, causing delays in hot water supply