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RED Calc Free Help DHW Instantaneous Sizing
DHW Instantaneous Sizing Tool User Guide

DHW Instantaneous Sizing
Red Calc Tool User Guide

What this tool can do for you

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.

Photo of Gas-Fired Instantaneous Water Heater
Source: US Department of Energy

Calculated values

  • Temperature rise - the difference between the temperature of the cold water entering the water heater and the heated water leaving the appliance.
  • Peak flow rate - the design hot water flow rate.
  • Input heat value - the required instantaneous energy input rate. Use this value to select an appropriately sized unit.

Tips

  • 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.
  • This is a "solve-all" tool. Select the radio button to the left of the label for which you wish to solve. This label will turn to blue and the input box will disappear.
  • Some instantaneous (demand) water heaters have a fixed output, which increased the water temperature by a fixed number of degrees for a given flow rate. Others have a variable output (firing rate) that increases as the flow rate increases. It is important to know the characteristics of the appliance before sizing.

Inputs and field measurements

  • Recovery efficiency - the rated recovery efficiency of the water heater. Enter as a percent; 78 instead of 0.78. For most water heaters, standby losses are minimal so that the recovery efficiency is essentially the same as the energy factor (EF). A case in which these efficiency values are substantially different would be a tankless coil in a boiler, where standby losses in the summer are a large fraction of the total energy input. In that case, the steady-state efficiency (SSE) is likely to be a good approximation of the recovery efficiency.

Because this is a "solve-all" tool, each of the values below will be treated as inputs unless selected as your result using the radio button to the left of the label.

  • Temperature rise - the difference between the temperature of the cold water entering the water heater and the heated water leaving the appliance.
  • Peak flow rate - the design hot water flow rate. See Table 1, below, for flow rates for mixed (hot + cold) water uses. Use the DHW Volume per Use tool to determine the hot-water-use portion of this flow.
  • Input heat value - the required instantaneous energy input rate. Use this value to select an appropriately sized unit.

 Table 1. Typical Flow Rate Ranges
(Mixed Hot and Cold Water)
 

Water Use       Flow Rate, Mixed     
Gpm (Lpm)
 
 Hand-washing sinks  0.2 to 1.0  
 (0.8 to 3.8)  
 Showers  0.8 to 2.5  
 (3.0 to 9.5)  
 Bathtub fill rates  1.0 to 6.0  
 (3.8 to 22.7)  
 Dishwasher fill rates 1.0 to 3.0
 (3.8 to 11.4)  
 Clothes-washing machine fill rates  1.0 to 6.0 
 (3.8 to 22.7)  
 Residential whole-house recurring peak rates  3.0 to 4.0
 (11.4 to 15.1)  
 Residential whole-house severe-peak rates  6.0 to 8.0 
 (22.7 to 30.3)  
Source: ASHRAE HVAC Applications Handbook. 2011. page 50.25.

Best practices

Select a water heater that has an input heat rate that is equal to or greater than the "Input heat rate" tool result.

References

  • ASHRAE. 2011 HVAC Applications Handbook, p. 50-25. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. This document may be purchased at www.ashrae.org.
    Comment: This document includes a chapter on service water heating and specifically sizing tankless [instantaneous] water heaters - Chapter 50. The information in Table 1 is from this book.

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 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 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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