Refrigerant blends are commonly used in various HVAC applications, including air conditioning, refrigeration, and heat pump systems. Specific blends, such as R-410A and R-407C, are designed for use in air conditioning systems, while others, like R-404A and R-507, are used in refrigeration systems. The choice of refrigerant blend depends on the specific requirements of the HVAC system, including temperature range, pressure, and safety considerations.