Brake horsepower (BHP) is the mechanical power required to operate a fan at specified airflow and pressure conditions. Expressed in horsepower or kilowatts, BHP represents actual power delivered to rotate the fan shaft and move air through the system. Typical residential HVAC fans require 0.25-2.0 BHP depending on size and operating conditions.
Calculation and Determination
BHP is calculated using the formula: BHP = (CFM × Total Pressure) / (6356 × Fan Efficiency). A 3000 CFM fan producing 1.5 in. wc total pressure with 70 percent efficiency requires approximately 1.02 BHP. Fan manufacturers provide BHP curves showing power requirements across operating ranges and speeds.
Motor Selection
Motor horsepower must exceed calculated BHP by 10-15 percent to account for service factor and acceleration loads. Oversizing motor horsepower by more than 25 percent reduces operating efficiency and increases starting current. Variable-speed EC motors automatically adjust power to match load requirements, improving efficiency.
Energy Significance
BHP directly correlates to electrical consumption and operating costs. Reducing fan speed by 20 percent decreases BHP by approximately 50 percent due to cubic relationship with velocity. Proper system design minimizes required BHP, reducing energy consumption by 2000-4000 kilowatt-hours annually for residential systems.