The average residential HVAC system is a 3- to 5-ton system typically referred to in our industry as a matched split system. “Matched” means that the indoor unit and the outdoor unit are the same size.

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For purposes of explanation, we’ll base our analysis on a 4-ton residential matched split system and of course we must assume the system is properly sized for the home.  Typical residential AC systems run around 400 to 500 cubic feet per minute (CFM) per ton, depending on the manufacturer, model, fan type and system parameters (duct size, duct length, register/grille type and size, damper positions, etc.), while commercial systems run closer to 400 cfm/ton. These residential units also operate at around .50 to .70 inches of water column (IWC) static pressure. 

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The functional piece of the system that creates available static pressure is the fan / fan motor. Everything else eats into that available static pressure; losses from fittings and transitions (where the unit connects to the ductwork and where duct fittings are located throughout the duct system), heating system (typically a gas-fired heat exchanger), air conditioning (cooling) coil, ductwork (both supply and return), terminal devices (diffusers, registers and grilles), and filters. 

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Filtration has two primary parameters, which are tied to other secondary but related parameters. Primary parameters are particle size being filtered and the related pressure drop. Secondary parameters are air flow and cross-filter airflow velocity. Particle size is defined by the Minimum Efficiency Reporting Value or MERV Rating. Lower MERV ratings allow more larger particles to move through the filter media while larger MERV ratings allow fewer smaller particles, so higher is generally better. But higher MERV ratings also cause a higher static pressure drop, so we must balance these two so that we get the filtration we want while not imposing too high a static pressure on the fan and motor. The secondary parameters of airflow and cross-filter airflow velocity are related to the pressure drop in that the lower the velocity through the filter, the less static pressure drop there is.

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Bottom line up front, adding the Crown Vent cover and filter to your existing residential AC grilles should not greatly impede the airflow or adversely affect the operation of your home HVAC system. The system is designed to operate properly, assuming it has been properly maintained, across a range of external static pressure values – which is the parameter of operation affected by adding filtration devices to the system. The external (usable) static pressure capacity, as typically measured in inches of water column (IWC or “ WC) of a standard residential AC system is generally around .65” WC, and most furnaces allow for a maximum safety threshold of 0.8” of WC.

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Testing of the Crown Vent cover with filter media indicates a 7 to 11% drop in airflow. This is a static pressure drop of around .05” to .07” WC in a typical system. In conclusion the pressure drop is well within system design tolerance, and the additional filtration is worth a minor reduction of air flow.