For optimal efficiency, what air flow direction should filters be installed according to?

Filters should be installed according to the manufacturer's label for optimal efficiency. This is because the label typically indicates the designated airflow direction, which is essential for ensuring that the filter works effectively. Proper airflow direction allows the filter to trap particles and contaminants as air passes through it, thereby maximizing its performance and lifespan.

When filters are installed in accordance with the manufacturer’s instructions, it ensures that the airflows properly through the filtration media, preventing any bypass of unfiltered air, which could compromise air quality. Additionally, adhering to these guidelines can help in maintaining system efficiency and prolonging the lifespan of HVAC systems.

The other options mentioned, such as the location of the duct, filter size, or the angle of the unit, do not provide specific guidance tailored to the design of the filter itself and are less reliable than the information provided directly by the manufacturer’s label.