Wobbe Index (Fuel-Gas Interchangeability)
The single number that governs fuel-gas interchangeability: WI = HHV / sqrt(specific gravity), the heat delivered through a fixed orifice at a fixed pressure. Two gases of EQUAL Wobbe deliver the same heat input through the same orifice, so an appliance set for one runs correctly on the other without an orifice change. Natural gas (1,000 BTU/ft^3, 0.60 SG) is ~1,291; propane (2,516, 1.52) is ~2,040 -- far apart, which is exactly why an appliance must be CONVERTED between them. A utility holds its Wobbe in a narrow band so appliances stay in tune as composition shifts (propane-air peaking, LNG, hydrogen blending move it). Wobbe captures orifice/heat-input but NOT flame speed / yellow-tipping / flashback -- a full check uses the AGA indices too. The supplier's certified analysis, the appliance listing, and the conversion kit and AHJ govern.
Formula and source
wobbe_index_btu_ft3 = hhv_btu_ft3 / sqrt(specific_gravity). Two gases of equal Wobbe deliver the same heat input through the same orifice at the same manifold pressure.
Wobbe index / fuel-gas interchangeability (AGA interchangeability practice; ISO 13686 natural-gas quality), by name; the gas supplier's certified analysis, the appliance listing, and the conversion kit and AHJ govern.
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