RED Calc Free

v1.0, Legacy Tool

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Summary: Determines 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. See Tool Guide. Click on any input or result label for help. By using this tool (application) you agree to the Terms of Use.

This legacy tool is NOT optimized for mobile devices. Stay tuned for the mobile-optimized version 2.0.