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RED Calc Free Help DHW First Hour Rating
DHW First Hour Rating Tool User Guide

DHW First Hour Rating
Red Calc Tool User Guide

What this tool can do for you

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.

Source: EPA EnergyStar

Calculated values

  • Peak hour demand (1st hour rating).


  • 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.
  • Because this procedure is for sizing storage water heaters, it is recommended that high-end "Hot water/use" values be used for the listed appliances/uses, rather than using low-flow values. This will ensure that the selected water heater will serve the needs of all occupants during the life (average of 13 years) of the water heater.

Inputs and field measurements

  • Hot water use for each appliance each time it is used.
  • Times in peak hour - the maximum number of times the appliance/use is used during the maximum-use hour.

Best practices

Select a storage water heater that has a first hour rating that is equal to or greater than the "Peak hour demand (first hour rating)" tool result.

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 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 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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