Every air-to-air heat exchanger sits between two competing masters: it must recover as much energy as possible while letting the fan push air through with the least resistance. The measure of that resistance is pressure drop, and it is one of the most misunderstood numbers in HVAC specification. A low pressure drop looks cheap on paper, but it can mean an undersized core that recovers little energy. A very low face velocity buys you effectiveness only at the cost of a huge, expensive casing and a fan that still has to move the same air. Understanding the trade-off is the difference between a recovery system that pays for itself and one that quietly costs more than it saves. This article explains what pressure drop really is, how it trades against effectiveness, and how to size a core that gets the balance right.<\/p>\n
Pressure drop, often written as \u0394P, is the static pressure a fan must add to force air through the exchanger core, the filters, and the ductwork. It is expressed in Pascals (Pa) or inches of water gauge. In an air-to-air core the dominant contributor is the narrow corrugated channel between plates: the smaller the hydraulic diameter and the higher the velocity, the larger the frictional loss. For a given core, pressure drop rises roughly with the square of face velocity, so a 20 percent increase in airflow can add nearly 45 percent to the resistance. That non-linearity is why oversizing a fan to “be safe” is expensive and why the recovery core must be matched carefully to the design flow.<\/p>\n
The fan pays for every Pascal. Fan power scales with the product of flow and pressure, so a core specified with an extra 50 Pa of drop can add kilowatts of continuous motor load across a building’s operating hours. When you evaluate a unit, always read the pressure drop next to the effectiveness\u2014never one without the other. Our heat exchanger selection guide<\/a> explains how we balance these in practice.<\/p>\n Effectiveness\u2014the fraction of available energy actually recovered\u2014improves as air spends more time in the core, which means a longer path, a deeper core, or a lower face velocity. But lowering face velocity to chase a few more points of effectiveness makes the core physically larger and, per unit of recovered energy, often raises first cost more than it saves. There is a sweet spot, and it is different for every duty. The table below shows the typical relationship for a counter-flow plate core at a fixed airflow.<\/p>\nThe effectiveness\u2013pressure-drop trade-off<\/h2>\n