Upgrade to RED Calc Pro
Save your calculations to RED Cloud ♦ Organize your work in folders ♦ Save time with templates ♦ Print and save PDF reports ♦ Customizable report headers and footers ♦ Preferential support ♦ Multi-tab/window syncing ♦ Fully functional offline mode ♦ No ads

DHW First-Hour Rating Tool Guide

What this tool does – [Tool]

Heat pump water heater. Source: EPA EnergyStar

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).


  • 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.