Understanding External Static Pressure

What external static pressure means

External static pressure, or ESP, is the static pressure a unit's fan has available at its connections to overcome the resistance of everything outside the unit: ductwork and fittings, dampers, attenuators, grilles and diffusers, and louvres on outdoor air intakes. It is expressed in pascals (Pa) and always relates to a stated airflow.

What counts as internal varies between products and data sheets. Filters, coils, heat recovery devices and casing losses may be included within the unit's rating or left for the designer to add, so the basis of any published ESP figure should be confirmed before it is compared with a duct calculation.

Ducted fan coil units and ducted split or VRF indoor units often offer a choice of fan settings for different external static pressures. Leaving a unit on its factory setting when the ductwork needs another is a common cause of poor airflow after installation.

How the duct system creates resistance

Every metre of straight duct adds friction loss, and every bend, transition, branch and component adds a further loss. Adding these along the index run, the path with the highest resistance, gives the pressure the fan must supply at the design airflow.

Resistance rises roughly with the square of airflow. Increasing airflow by 10% raises system resistance by about 21%, which is why small changes to the design airflow have a larger effect on fan duty than expected. Poor connections close to the fan, such as a bend directly at the outlet, add losses that calculations based on ideal fittings do not capture.

What happens when the estimate is wrong

If the real system resistance is higher than the figure used for selection, a fixed-speed fan delivers less air. Heating and cooling capacity fall with it, rooms at the end of long runs are affected first, and direct expansion coils can run too cold at reduced airflow. EC and variable-speed fans can compensate within their limits, at the expense of higher power and noise.

If resistance is overestimated, the fan is selected for a duty it never needs. Airflow is then reduced by closing balancing dampers or lowering fan speed. Speed reduction recovers most of the energy, whereas throttling with dampers wastes it and can create noise at the dampers and terminals.

Filters, margins and variable airflow

Filter pressure drop rises as filters load, so the design should state whether the fan is selected at clean, mid-life or final filter resistance. Where airflow must stay constant, a common approach is to select at final resistance and let the fan speed rise to hold airflow as the filters load. Differential pressure monitoring across each filter stage then shows when filters should be changed, rather than relying on a fixed interval.

Margins added separately by the designer, the contractor and the equipment selection can compound into a fan that is significantly oversized. A single, clearly stated allowance is easier to manage. In variable air volume systems, the pressure required changes with demand, and resetting the duct static pressure set point as terminals open and close reduces fan energy.

Checking pressure on site

During commissioning, static pressure measured at the unit's inlet and outlet connections, together with measured airflow, shows where the fan is operating on its curve. A large difference from the design figure points to problems such as closed fire or volume control dampers, crushed flexible ductwork, missing fittings or blocked filters.

When replacing existing equipment, measuring the pressure and airflow of the system as it runs today is more reliable than relying on original drawings. Ductwork is often modified during fit-outs, and a like-for-like replacement selected from the original design duty may not deliver the airflow the building now needs. Recording measured values at handover also gives the service team a baseline for diagnosing airflow complaints later in the building's life.

Stating ESP in a specification

A clear specification avoids most of these problems. Each fan duty should be stated with the following information.

• Design airflow in m³/s or l/s
• External static pressure in Pa at that airflow
• Components included within the unit's rating
• Filter condition assumed at selection
• Any margin already applied
• Minimum airflow for variable volume systems