What is the significance of units for H and I in S-390?

• A. H is typically in Btu/ft^2 and I in Btu/ft·s; using consistent units is essential for correct calculations
• B. H and I can be any units; consistency is not important
• C. H is in liters per square meter and I in liters per second
• D. H is in Btu/ft^2 and I in Btu/ft; identical to each other

Answer: (A)

Understanding how H and I are used in S-390 comes down to keeping energy units consistent so the math makes physical sense. H is a measure of heat per unit area, so it carries units of Btu per square foot (Btu/ft^2). I is a heat input rate along a line or front, so it carries units of Btu per foot per second (Btu/ft·s). When you compute total heat, these units line up cleanly: multiplying H by an area (ft^2) gives total heat in Btu, and multiplying I by a length (ft) and a time (s) also gives total heat in Btu. This consistency is essential for accurate energy balances in fire behavior calculations.

If you tried to mix arbitrary units, or use different unit systems (like liters) that aren’t tied to energy, you’d lose dimensional consistency and end up with meaningless results. Similarly, pairing incompatible forms (one based on area, the other on length) would prevent correct aggregation of heat inputs. Keeping these units as Btu-based per area and per length-per-time ensures the math captures the intended physical meaning and yields correct, comparable results.