Air Flow Efficiency

The normal range of acceptability for airflow efficiency is 90% to 105% as measured against the industry standard of 400 cfm per ton of conditioning. If the efficiency is below this range, then there is inadequate airflow. This leads to moisture buildup in the air handler, excessive energy consumption, excessive equipment wear, and moisture-related issues in time (e.g., microbial growth). Additionally, this lower airflow can cause temperature differences between the air handler interior and the external ambient temperature that exceed design specifications. This will result in thermal failure and the possibility of condensation on the outside of the air handler, which also leads to moisture-related issues in time.

Efficiency above the acceptable range is indicative of excessive airflow. This prevents sufficient airflow “resident time” on the evaporator coil inside the air handler and consequently does not allow the air to become dry enough for adequate conditioning. This will cause the conditioned area to feel cool but damp. This is often referred to as a “clammy” feeling. This too, in turn, leads to moisture-related issues over time since water vapor is not properly removed from the airflow.

Static Pressure

Static pressure measurements are taken immediately before (return side) and after (supply side) the air handler. The two values are added together to provide the Total External Static Pressure (TESP). The pressure is measured in inches of water column (in wc). Think of an open ended U-shaped tube partially filled with water. Under equilibrium conditions the water level is equal on both sides. Applying pressure on one side will cause the water level on that side to drop and to rise on the other side. This difference is measured in inches. Hence, inches of water column.

The maximum magnitude of static pressure that an air handler can work with is determined by the manufacturers. This pressure is a “resistance” to airflow and is caused by sharp bends or constrictions in the air passageways as well as by undersized passageways. These passageways are commonly called ducts. If a duct is too small, has a series of sharp bends, or is constricted as it passes through a truss, then the static pressure will rise. An appropriate analogy here is high blood pressure caused by clogged arteries.

The higher the static pressure the more work is required of the air handler fan. High static pressure will cause excessive energy consumption, shorten the life of the fan motor, reduce latent heat (moisture) removal, and in some instances actually “pull” water out of the condensation drain pan and add it to the air stream. This will lead to serious moisture-related issues in time. Low static pressure is good. However, if low static is accompanied by low airflow, then this is a good indication that there are leaks in the duct work.

The air handler fan is a pump. All pumps are far better at pushing than they are at pulling. Consequently, high static pressure on the pulling side (return) of an air handler has dramatic effects on its performance. Return static pressure should always be kept at an absolute minimum. This is the rationale behind Florida Building Code* requirements on the return duct work. Any reduction in return static pressure has far greater effects than similar supply static reductions.

*§918.3 Heat Pumps. The minimum unobstructed total area of the outside and return air ducts or openings to a heat pump shall not be less than 6 square inches per 1,000 Btu/h (13 208 mm2/Kw) output rating or as indicated by the conditions of the listing of the heat pump. Electric heat pumps shall be tested in accordance with UL 1995.


Differences between the supply and the return air flow are indicative of leakage. For example, if the supply airflow is at 100% and the return is only 85%, then there is a 15% return leak. Currently, the regional limit for leakage is 10%. Any leakage in excess of 10% is considered worse than the limits of acceptability.

A return leak is caused by openings in the return duct work, connections, or openings at the air handler. If the ductwork or air handler are located outside of the conditioned space, then these leaks introduce dusty, moist air, and (if the air handler is in the garage) exhaust fumes into the air stream.

Supply leaks likewise are caused by openings in the supply ductwork. These push conditioned air into unwanted locations like the attic or garage. If the duct work is located outside of conditioned space, then the home is placed under a condition of negative pressure. Negative pressure (see below) pulls unconditioned, contaminated air into the home. Unlike the situation with return duct leaks, this air is not passing through the air conditioner before entering the home and thus sets up adverse conditions with respect to comfort and moisture levels.

Building Pressure

Returns remove air from a room or zone. Supplies provide air to a room or zone. The supply and return to a given area should be in balance. If they are not, then that area is under a pressure differential. If the supply exceeds the return, the area is under positive pressure and air is forced out of the room or zone. If the return exceeds the supply, then the area is under negative pressure and outside air is forced into the room or zone.

Positive Pressure: If the supply exceeds the return, the room or zone is under positive pressure because more air is entering the area than is being removed. As a result, air is forced out of the room or zone. For example, a master bedroom suite with two supplies, a supply in the master bath, master water closet, and master closet and no return is placed under a sizeable positive pressure. When the air handler comes on, the bedroom door is slammed shut as the air is being forced out of the room by the positive pressure.

Another common cause for positive pressure is leaks in the return duct work. In this scenario, attic air is drawn into the return ducts in lieu of air being drawn from the conditioned space. This means that hot, moist attic air is entering the system and placing additional burden on the system. In this case it is absolutely critical that these leaks be repaired. Studies have shown that return leaks of 15% can reduce the effective capacity and efficiency of a cooling system by approximately 50%.

Negative Pressure: If the return exceeds the supply, the room or zone is under a negative pressure because more air is leaving the area than is being delivered. As a result, air is forced into of the room or zone. Hot, moist air is drawn in at every opportunity when doors are opened, and through ceilings and weaknesses in seals around doors and windows. This unconditioned air places additional burden on the system and can lead to moisture-related issues in time.

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