How to calculate static pressure drop across a coil
How coil pressure drop scales with face velocity and how to read manufacturer coil data at the actual design airflow rather than at the tested reference condition.
What this means
A coil's pressure drop is published at a specific tested face velocity and wet or dry condition, and it does not transfer directly to a different airflow or a different coil face area. Pressure drop rises with roughly the square of face velocity, so reading a coil's rated drop at the manufacturer's test condition and applying it unchanged to a system running a different airflow or a coil resized to a different face area understates or overstates the real figure the static pressure budget needs.
Equipment and model context
- Cooling and heating coils selected as part of an air-side static pressure budget
- Worked figures illustrate the method and are not a rating for any product
This explains how coil pressure drop scales with face velocity and what the manufacturer's rated figure actually represents. It does not calculate pressure drop for a specific coil selection. That requires the manufacturer's actual performance data at the design airflow, face area, and wet or dry condition for the selected coil.
What this covers
- Why a coil's rated pressure drop applies at one specific tested condition, not universally.
- How pressure drop scales with face velocity and why that relationship is not linear.
- Why a wet cooling coil carries higher pressure drop than the same coil dry.
- What happens to a static pressure budget when coil drop is read at the wrong condition.
What changes the result
- Applying a coil's published pressure drop figure directly to a system running a different airflow than the manufacturer's test condition.
- Using a dry coil pressure drop figure for a cooling coil that will actually operate wet during active dehumidification.
- Assuming pressure drop scales linearly with airflow rather than approximately with its square, understating the effect of even a modest airflow increase.
- Selecting a coil with a smaller face area than the original design assumed, which raises face velocity and pressure drop without a corresponding note in the static pressure budget.
Why the rated figure is a single point, not a universal constant
Coil manufacturers publish pressure drop at a specific tested face velocity, the airflow rate divided by the coil's face area, along with a stated wet or dry condition. That single figure describes the coil's resistance at exactly that tested combination, not at every airflow the coil might see once installed in a system running a different total airflow or paired with a coil of a different face area than the tested unit.
Face velocity is the variable that actually drives pressure drop, not airflow in isolation, which is why the same coil model in a larger face area, spreading the same airflow across more area and therefore lowering face velocity, carries a lower pressure drop than the identical coil construction in a smaller face area at the same airflow.
Why the relationship is roughly square, not linear
Fluid friction losses scale with velocity squared rather than velocity directly, a relationship that appears throughout air-side and water-side HVAC calculations, from duct friction to pump system curves, and coil pressure drop follows the same pattern. Doubling face velocity roughly quadruples pressure drop rather than doubling it, which is why even a modest increase in airflow above a coil's tested condition can produce a disproportionate rise in the pressure drop the static pressure budget has to accommodate.
This is the specific reason coil pressure drop cannot be scaled by a simple ratio from the manufacturer's tested figure. The manufacturer's own performance curve, plotting pressure drop against a range of face velocities rather than a single tested point, is the correct source for a design running at a different airflow than the reference condition.
Why a wet coil carries more pressure drop than a dry one
A cooling coil actively removing moisture accumulates condensate on its fin surfaces, and that film of water narrows the effective airflow passage between fins and adds surface friction beyond what dry air alone would experience. Manufacturer data publishes both a dry pressure drop figure and a separate, higher, wet pressure drop figure in most coil selection tools, and the wet figure is the one that applies during active cooling and dehumidification, which is most of a cooling coil's actual operating time in a humid climate.
Using the dry figure for a cooling coil's static pressure budget understates the resistance the system will actually run against during normal cooling operation, a gap that shows up as airflow below design once the coil is wet in service even though the same measurement dry would have matched the budget.
Where this feeds into the rest of the static pressure budget
Coil pressure drop is one component subtracted from the equipment's total external static pressure rating to arrive at available static pressure for the ductwork, alongside filter and grille losses, in the friction rate calculation covered separately. Reading coil drop at the wrong condition, whether the wrong face velocity or the wrong wet or dry state, produces an available static pressure figure that is too optimistic, and every duct sized against that inflated figure ends up undersized for what the system actually needs.
Denser fins and more rows raise pressure drop while also lowering bypass factor, the same trade discussed in the coil selection article, so a coil chosen to improve moisture removal often costs static pressure in return. This is why coil selection and duct design are not sequential, independent steps performed once each: a coil selected late in the process, after duct sizing already assumed a particular pressure drop figure, needs the static pressure budget rechecked against the coil's actual data at the actual design airflow and wet condition, not against an assumption carried forward from an earlier stage of the design.
For one coil rated at 0.35 inches of water column pressure drop at its 450 feet per minute test face velocity, this shows how pressure drop actually changes as face velocity varies from that reference condition.
- Actual pressure drop
- If drop scaled linearly with velocity
- At the manufacturer's tested 450 feet per minute, pressure drop reads 0.35 inches of water column, the single figure most likely to appear on a spec sheet.
- Raising face velocity by a third to 600 feet per minute nearly doubles pressure drop to 0.62, far more than a linear scaling from the tested figure would predict.
- A linear assumption, the dashed line, understates the real figure at higher velocity and overstates it at lower velocity, in both cases feeding an inaccurate number into the static pressure budget.
- This is the same square-law relationship that governs duct friction loss, applied here to the coil rather than to straight duct or fittings.
| Factor | Effect on pressure drop | How much it typically matters |
|---|---|---|
| Face velocity above tested condition | Rises roughly with velocity squared | Often the largest single source of an inaccurate static pressure budget |
| Wet versus dry coil condition | Wet condition carries meaningfully higher drop | Significant for any actively dehumidifying cooling coil |
| Coil face area for a given airflow | Larger face area lowers face velocity and therefore drop | A real design lever, not just a measurement condition |
| Fin density and row count | Higher density and more rows raise drop at a given face velocity | Trades against latent performance, covered in the bypass factor calculation |
Questions people ask about this
Where do I find a coil's pressure drop at airflows other than the tested condition?
Manufacturer expanded performance data includes a table or chart of pressure drop across a range of airflows or face velocities, not only the single reference condition sometimes highlighted on a summary spec sheet. Requesting or locating this full performance data, rather than relying on a single headline figure, is what makes an accurate calculation at the actual design airflow possible.
Does coil pressure drop matter for a heating-only coil the same way?
The face velocity and square-law scaling principles apply identically to a heating coil, but a dry heating coil does not carry the wet-versus-dry distinction a cooling coil does, since there is no condensate film forming on a coil that is only heating air. The calculation is otherwise the same method applied to a simpler, single-condition case.
Can increasing coil face area always reduce pressure drop without a downside?
A larger face area lowers face velocity and pressure drop for the same airflow, but it also increases the coil's physical size, which may not fit the available space in the air handler or ductwork, and it adds cost. The trade is real and worth making where space and budget allow, not a change without any cost attached.
How does filter pressure drop interact with this calculation?
Filter pressure drop is a separate line item in the same static pressure budget the coil drop feeds into, calculated from the filter's own rated resistance at the design airflow, and a higher-MERV filter adds more resistance than a lower one at the same airflow. Both figures are subtracted from the equipment's total external static pressure rating together, alongside grille and duct losses, to arrive at what remains available for the ductwork itself.
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