Air Flow Glossary

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 we have inadequate airflow which leads to moisture buildup in the air handler, excessive energy consumption, excessive equipment wear and moisture related issues in time.

Efficiency above this range is indicative of excessive airflow. This prevents sufficient airflow "resident time" on the evaporator coil inside the air handler and consequently not allowing 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. It can also lead to moisture related issues over time since the water vapor is not properly removed from the airflow.

The static pressure measurements are taken immediately before (return side) and after (supply side) of 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. The water levels in both sides are equal. 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 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. Think of this in terms of trying to breath when your neck is being squeezed or 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 condensate 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 there are probably leaks in the duct work. Additionally, this lower airflow can cause larger temperature differences than designed between the air handler interior and the external ambient temperatures. This will lead to thermal failure and the possibility of condensation on the outside of the air handler which also leads to moisture related issues in time.

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 affects on its performance. Return static 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 affects than similar supply static reductions.

When there is a difference between the supply and the return air flow that delta is leakage. If the supply airflow for example is at 100% and the return is only 85% then we have a 15% return leak. Currently, the regional limit for leakage is 10%. Therefore any leakage in excess of 10% would be 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 is outside of conditioned space, then these leaks are introducing dusty, moist air and (if the air handler is in the garage) exhaust fumes etc. into the air stream.

Supply leaks likewise are caused by openings in the supply ductwork. They push conditioned air into unwanted locations like the attic or garage. If the duct work is outside of conditioned space, then the home is placed under a negative pressure which drives unconditioned, contaminated air into the home. Hot moist air is drawn in at every opportunity when doors are opened and through ceilings and weaknesses in seals around doors and windows. 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 within the home itself.

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

Positive Pressure: If the supply exceeds the return, the room or zone is under a positive pressure because there is more air entering the room than there is being removed. Therefore, 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. In fact studies have shown that return leaks of 15% "…can reduce the effective capacity and efficiency of a cooling system by about 50%".

Negative Pressure: If the return exceeds the supply, the room or zone is under a negative pressure because there is more air leaving the room than there is being delivered. Therefore, air is forced into the room or zone. This means that unconditioned air can be forced into the conditioned space which can lead to moisture related issues in time.