FHB Logo Facebook LinkedIn Email Pinterest Twitter Instagram Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe

In every issue you'll find...

  • Expert insights on techniques and principles
  • Unbiased tool reviews
  • Step-by-step details to master the job
  • Field-tested advice and know-how
Subscribe Now!
Subscribe
  • How-To
  • Design
  • Tools & Materials
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
  • Join
  • Log In
Subscribe

Fine Homebuilding Project Guides

Plumbing

Guide Home
Chapter
  • Plumbing Basics
  • Bathrooms
  • Kitchens
  • Roughing In
  • Water Heaters, Washers, and More
  • Maintenance and Repair
How it Works

How a Tankless Water Heater Works

Rob Yagid explores the inside of a tankless heater and describes what happens at each step of the heating process.

By Rob Yagid Issue 209

Synopsis: Tankless water heaters, which produce hot water on demand rather than keeping a tankful of water constantly hot, are becoming increasingly popular as builders and homeowners realize how much energy they can save over a conventional tank-style heater. In this “How It Works” article, associate editor Rob Yagid explores the inside of a tankless heater and describes what happens at each step of the heating process.

Unlike tank-style water heaters that keep a barrel of hot water ready to go 24 hours a day, tankless water heaters can help you to save energy and money by heating only the amount of water you need, and only when you need it. Manufacturers say that by heating water on-demand, tankless water heaters provide a 30% savings in water-heating costs and a 50% decrease in energy consumption.

Electrician Brian Walo and plumber Shannon Neff show how to install a gas-fired Rinnai tankless water heater. However, several manufacturers, such as Bosch and Takagi, make tankless water heaters that work in similar ways. Regardless of the manufacturer, a tankless water heater is one of the smarter pieces of equipment that you can install in your home.

Here’s how it works

1. A water-flow sensor triggers the heating sequence. The water heater fires when it detects a flow rate of approximately 0.6 gallons per minute (GPM), which is just a trickle of water at the hot-water tap.

2. The combustion fan forces air into the burner. In this particular unit, the air is drawn from outside through a concentric venting system, which provides sealed combustion.

3. The igniter sparks until the flame rod—which is really just a sensor—detects proper combustion. Then it shuts off.

4. The modulating gas valve supplies fuel to the burner. When a flame is detected, the valve opens and closes based on the amount of heat required.

5. The water supply line passes through the copper heat exchanger multiple times. Metal fins in the heat exchanger help to concentrate heat around the pipes, transferring heat from the burner into the water quickly.

6. A bypass introduces cold water to the hot water, which can be much hotter than it needs to be (upwards of 140°F). This brings the water down to the set-point temperature, which is controlled by the thermostat on the front of the unit, before being supplied to the tap. This allows the unit to heat a smaller amount of water while still delivering a high flow rate.

7. A temperature sensor also called a thermistor, and the water-control valve (which is located under the blower near the water-flow sensor) work in sync to ensure that the water leaving the unit is at the temperature being requested.

8. The electrical demands of the unit are met with a standard 120v power plug. The power consumption for this particular tankless unit is less than 100w under normal operation.

9. The printed circuit board (PCB) is the brain of the unit. It monitors the flame rod, the fan-motor frequency, the outlet water temperature, the set-point temperature, and the water flow rate. When the water-flow sensor and the PCB sense a water flow of less than 0.5 GPM, the unit enters standby mode. The PCB monitors all the components and fires the unit as needed to prevent water from freezing inside the heater, which can be an issue with outdoor models or with models installed in unheated areas of a house.

10. The hot-water supply flows to the tap at a capacity of roughly 7.5 GPM, depending on the tankless-heater model and the incoming water temperature. Tankless water heaters tied to colder incoming water supplies demand a longer heating time, which can reduce a unit’s flow rate.

Tankless Water Heater Diagram

To explore our graphic on tankless hot water heaters, click the View PDF button below. 

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters
View PDF
Previous: Connecting and Insulating a Water Heater Next: How to Install a Tankless Water Heater

Guide

Plumbing

Chapter

Water Heaters, Washers, and More

View Comments

  1. User avater
    RandallJLy | Apr 19, 2018 06:23am | #1

    Thanks for sharing the produce of Tankless Water Heater.

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Plumbing

Plumbing

Trusted, comprehensive guidance from the pros for tackling any plumbing project

View Project Guide

View All Project Guides »

Become a member and get unlimited site access, including the Plumbing Project Guide.

Start Free Trial

Plumbing Basics
  • Plumbing Systems
  • Materials
  • Plumbing Tools
Bathrooms
  • Sinks
  • Showers & Tubs
  • Toilets
Kitchens
  • Kitchen Sinks
  • Water Filtration
  • Appliances
Roughing In
  • Layout
  • Installation
  • Copper Pipe
  • PEX Tubing
  • PVC Drainpipe
Water Heaters, Washers, and More
  • Water Heaters
  • Whole-House Filters
  • Sump Pumps
  • Washers
  • Sprinkler Systems
Maintenance and Repair
  • Sinks
  • Toilets
  • Showers & Tubs
  • Systems

Fine Homebuilding

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences

Taunton Network

  • Green Building Advisor
  • Fine Woodworking
  • Fine Gardening
  • Threads
  • About
  • Contact
  • Advertise
  • Careers
  • Copyright
  • Terms of Use
  • Site Map
  • Do not sell my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2022 The Taunton Press, Inc. All rights reserved.

X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Account

  • Log In
  • Join

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Shop the Store

  • Books
  • DVDs
  • Taunton Workshops

More

  • FHB Ambassadors
  • Reader Projects
  • Podcast
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in