Sunday, September 22, 2019
RED Calc Free Help DHW Systems Comparison
DHW Systems Comparison Tool User Guide

DHW Systems Comparison
Red Calc Tool User Guide

What this tool can do for you

This tool allows you to calculate the annual cost of heating domestic hot water. In addition, you may compare the annual costs of up to three systems to a base system. Four water heating fuel types are available; electricity, natural gas, LPG, and #2 oil. You can use this tool for any system for which you have an energy factor (EF), including electric resistance and heat pump systems, and various types of oil- and gas-fired systems. Note that if a heat pump water heater is installed in a condition or semi-conditioned space, some or all of the heat transferred to the water during the heating season will need to be replaced by the heating system. This "heating penalty" and the corresponding "cooling benefit" during the cooling season, are not accounted for by this tool.

An interactive chart displays the comparison of water heating systems, reporting the annual energy usage and annual fuel cost for each. The chart can be printed or saved in various formats, making it easy to include it in your reports or presentations.

Source: U.S. Department of Energy
Source: U.S. Department of Energy

Calculated values

  • Annual hot water usage.
  • Annual fuel used.
  • Annual fuel cost.
  • Annual energy savings compared to base system.
  • Annual cost savings compared to base system.
  • Annual fuel usage.


  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs). Read more.
  • The daily how water usage for a household in North America is from 45 to 70 gallons (170 to 265 liters), the average being 60 gallons (228 liters). Per person hot water usage, is from 8 to 24 gallons (30 to 90 liters), the average being 16 gallons (60 liters).
  • The units selected for the prices in the Fuel Prices table are used to calculate the results in the Systems Comparison table. For example, if under Nat Gas in the Fuel Prices table, "$/therm" is selected and $1.20 is the price input, any natural gas system included in the Systems Comparison table will reflect the unit price of $1.20/therm.
  • The units selected under "Savings" in the second to last column of the Systems Comparison table also sets the units used/displayed for the "Annual energy usage" (left vertical axis) in the chart below this table.
  • In the System Comparisons table, enter the data for the base water heating system you wish to use. You may then enter data for up to three alternate water heating systems that will be compared with the fuel used and the fuel cost of the base system. In the table, each of the alternate systems will show annual savings in energy units and cost compared with the base system.
  • Check "Show chart" to display the interactive chart. The chart values change as tool inputs above it are changed. Please refer to the Interactive Chart Use section below for more information.
  • This tool can be used to determine the savings from lowering the water heater temperature setting. For example, first enter the "Temperature rise" of the higher temperature setting, say 90°F (140 hot - 50 cold). With the use of the "Systems Comparison" table, find the "Fuel used" and the "Fuel $". Now lower the "Temperature rise" to 70°F (120 hot - 50 cold) and use the "Systems Comparison" table to find the "Fuel used" and "Fuel $" for the lower water temperature. Subtract these lower use values from the higher use values to find the fuel and dollar savings for lowering the temperature.

Interactive Chart Use

This tool includes a powerful interactive chart that can be used to display results; it may also be used for educational, sales, and marketing purposes. For example, you can save a chart in one of four different file formats, embed it into a customer report, or print it separately.

Tips for using the interactive chart:

  • Click/touch the labels "Energy usage" or "Fuel cost" in the legend below the chart to turn the corresponding chart bars on or off.
  • Hover/touch any of the bars in the chart to make a box appear that displays the numerical value of the bar.
  • Click/touch the menu icon in the upper right corner of the chart to show the choices for printing the chart or downloading it as an image file in PNG, JPEG, PDF, or SVG format. You are free to use the downloaded image in any way, including reports, presentations, websites, as long as the attribution for "Residential Energy Dynamics, LLC" is included.
    • If you save the chart as an SVG file, you can change the title or any other chart element with the use of third-party software. Examples of third-party software include Inkscape (a free version is available) and Adobe Illustrator.

Inputs and field measurements

In order to use this tool, you must enter the energy factor (EF) for each water heating system. If a system you are interested in appraising has a published energy factor and used one of the four fuel sources included in the tool, you can analyze it. Using the appropriate energy factor is very important because it can significantly affect the results.

The current and future minimum U.S. energy factors are listed in Tables 1 and 2, below. The minimum required U.S. energy factors in Table 2 take effect on April 16, 2015.

The best source for energy factors for individual water heaters is the AHRI Directory of Certified Product Performance (Please see the reference below). For existing water heaters, you will have to estimate the energy factor if it is not listed on the water heater label. The values in Table 3 are intended for guidance if you are not able to find an energy factor for a specific unit.

Source: U.S. Department of Energy

Source: U.S. Department of Energy

 Table 3. Suggested Energy Factors for Water Heating Systems 

Water Heating System  Suggested Energy Factor (EF) 
 Conventional gas storage  0.60 
 High-efficiency gas storage  0.65 
 Condensing gas storage  0.86 
 Demand gas with no pilot 0.82
 Oil-fired storage  0.55 
 Low-efficiency electric storage  0.90
 High-efficiency electric storage  0.95 
 Electric heat pump storage   2.20 
 Solar with electric back-up  1.20
Source: ACEEE (See "Related external resources" below)

Related tools

  • DHW Average Daily Usage: This tool calculates average daily hot water use based on the amount of hot water used by various appliances and use incidents. Default usage values are included.
  • DHW First Hour Rating: This tool helps you determine the First Hour Rating for the purpose of sizing a storage water heater. This method is based on guidance from the Air-Conditioning, Heating and Refrigeration Institute (AHRI), the industry standard for sizing storage water heaters.
  • DHW Instantaneous Sizing: This "solve-all" tool is used for sizing an instantaneous (tankless) hot water heater. It can also be used to determine the maximum hot water flow rate for a given system and temperature rise.
  • DHW Volume per Use: This tool determines the volume of hot water used given an amount of mixed (hot + cold) water used and the temperatures of the hot, cold, and mixed water. The amount of mixed water can be specified by volume directly, or from a flow rate and duration.
  • Water Flow Rate: This is a "solve-all" tool that relates the volume of water drawn, the duration of the water draw, and the flow rate. The primary expected use case is determining flow rate using a test container with known volume and a stop watch.

Related external resources

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