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RED Calc Free Help DHW Volume per Use
DHW Volume per Use Tool User Guide

DHW Volume per Use
Red Calc Tool User Guide

What this tool can do for you

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.

Storage electric water heater
Source: U.S. Department of Energy

Calculated values

  • Hot water used in gallons or liters.
  • Mixed water volume.
  • Percentage of hot water and cold water (displayed in additive bar chart).
  • Hot water flow rate.


  • 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 amount of mixed water can be specified by volume directly, or from a flow rate and duration. Specify the method using the drop-down selector.
  • The hot water temperature entered in the tool must be greater than the cold water temperature.
  • The "Mixed water temperature" entered must be between the hot and cold water temperatures.
  • The percentage of hot water and cold water for the volume of water is displayed as an additive bar chart under the "Mixed water temperature" input.

Inputs and field measurements

  • Mixed water volume determined by "Direct measurement" or "Flow rate & duration" (selected from the drop-down).
  • Mixed water flow rate.
  • Draw duration - elapsed time of the draw of mixed water. Include any time spent waiting for hot water to arrive at the usage point.
  • Hot water temperature.
  • Cold water temperature.
  • Mixed water temperature.

Related tools

  • DHW Systems Comparison: 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.
  • DHW Average 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.
  • 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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