Upgrading to a tankless water heater fueled by propane or natural gas is a clear-cut way for building pros to improve the energy performance of their homes. Because tankless models heat water only when it’s called for, switching from an electric tank water heater to a propane tankless water heater improves a home’s HERS Index score by 5 to 10 points.

The interactive map above shows just how large the improvement can be. The map uses modeling data from energy efficiency consultant Energy Inspectors, Inc., to display the HERS Index and carbon emissions improvements from upgrading to a propane tankless water heater. The map also displays the metropolitan regions that are the top areas for propane use in each climate zone. For more information on the data used in this map, scroll to the end of this article.

HERS Index scores are increasingly relevant in the latest version of the national model energy code. The 2015 International Energy Conservation Code (IECC) includes a new compliance path called the Energy Rating Index that offers builders more flexibility in how to meet the energy code.

The ERI path lets builders “trade off” performance between building elements and mechanical systems. It uses the HERS Index as the standard, requiring a HERS score of 51 to 55, depending on the climate zone. The HERS Index predicts the energy performance of a home versus a “typical new home” benchmarked at a score of 100. Each 1 percent of annual energy savings compared with the benchmark home is associated with a one-point drop in the HERS Index. So a home with a HERS Index of 55 is 45 percent more efficient than the benchmark home.

The Tankless Impact on HERS Index

By clicking to your county on the map, it’s easy to see the impact that upgrading to a propane tankless water can have. “Tankless water heaters only use energy when hot water is being called for in a home, whereas tanks are continually heating the water in the tank even when no one is using hot water,” Vincent Molluzzo says. Molluzzo previously served as director of energy modeling for Energy Inspectors and is now architectural production manager for Las Vegas–based design firm Blue Heron. “The more energy saved, the better the HERS score improvement.”

The improvement varies by region because the weather affects a water heater’s energy usage, Molluzzo adds. “Hotter climates have higher ground temperatures, so the water entering the water heater is warm and takes less [energy] to heat up,” he says. “In colder climates, the water is coming in cold, and therefore more energy is needed to heat the water.”

The HERS score improvement means that propane equipment gives builders more options to meet the code. A builder who includes a propane tankless water heater (as well as other mechanical equipment, such as a high-efficiency furnace) can scale back on window efficiency or avoid costly design changes to add thicker insulation.

What About ROI and Carbon Emissions?

While the HERS Index is increasingly relevant for builders, homeowners are more likely to care about their energy bills and carbon emissions. Clicking your county in the map above will also show you how much upgrading to a propane or gas tankless water heater will save in carbon emissions. These savings will vary depending on how a home’s electricity is produced upstream, but in general, propane tankless systems offer roughly half the CO2 emissions of electric storage tank systems.

Homeowners may also want to know how much a tankless water heater can save on their energy bills — and there’s a tool for that, too.

Using the tool above, you can enter information about your project and energy cost to estimate the energy savings from upgrading to tankless. Here are a few examples using the metropolitan regions with the most propensity for propane installations. Calculations assume a more-efficient 2,400-square-foot house, a 0.98 energy factor (EF) propane tankless water heater, and a 0.90 EF electric storage tank water heater.

Lake Charles, Louisiana

  • Climate Zone: 2
  • Households in top ZIP codes for propane use: 4,378
  • Towns in top areas for propane use: Cameron, Dequincy, Starks, Grand Chenier
  • Annual energy savings from tankless: $68

San Francisco-Oakland-Hayward, California

  • Climate Zone: 3
  • Households in top ZIP codes for propane use: 500,409
  • Towns in top areas for propane use: Burlingame, San Mateo, Redwood City, San Carlos, and more
  • Annual energy savings from tankless: $455

Seattle-Tacoma-Bellevue, Washington

  • Climate Zone: 4
  • Households in top ZIP codes for propane use: 341,035
  • Towns in top areas for propane use: Snohomish, Arlington, Bonney Lake, Issaquah, and more
  • Annual energy savings from tankless: $71

Rochester, New York

  • Climate Zone: 5
  • Households in top ZIP codes for propane use: 105,057
  • Towns in top areas for propane use: Himrod, Keuka Park, Williamson, Hilton, and more
  • Annual energy savings from tankless: $588

Portland-South Portland, Maine

  • Climate Zone: 6
  • Households in top ZIP codes for propane use: 33,802
  • Towns in top areas for propane use: Kennebunkport, Wells, Kennebunk, Woolwich, and more
  • Annual energy savings from tankless: $463

About This Map and Data

Climate zone information comes from the Department of Energy’s Guide to Determining Climate Regions by County. HERS Index and carbon emissions data courtesy of Energy Inspectors, Inc.

The metropolitan regions and ZIP codes with the most potential for propane installations in each climate zone were determined using proprietary data from the Propane Education & Research Council (PERC).