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DHW Instantaneous Sizing

What this tool does

Illustration of an on-demand water heater.
On-demand water heater. (Source: U.S. Department of Energy)

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.

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).
  • 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.
  • 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.

Background

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)
Table data from ASHAE Handbook of Fundamentals.

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.

Disclaimer

This content is a work created with funding provided by the United States Department of Energy under Contract no DE-AC05-076RL01830 for the operation of Pacific Northwest National Laboratory. The information and guidance provided by Pacific Northwest National Laboratory (PNNL) in the content are intended solely for educational purposes only and do not constitute formal training or certification. It is provided with the explicit understanding that neither the United States Government nor the United States Department of Energy, nor the Contractor, nor any or their employees, nor any jurisdiction or organization that has cooperated in the development of these materials, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness or any information, apparatus, product, software, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Viewers assumes full responsibility for all actions that they may take from information provided in this content including ensuring the safety, code compliance, and proper functionality of any products they choose to install. Installation and use of such products should be performed in accordance with local regulations and manufacturer instructions.