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DHW First Hour Rating

What this tool does

Illustration of a water heater installed in a basement.
Heat pump water heater. (Image credit: ENERGY STAR)

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,

Calculated values

Peak hour demand (1st-hour rating).

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).
  • 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.

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.

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.