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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,
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.
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.