The Fan Affinity Laws: Why Doubling Airflow Can Require 8x the Power
In a hurry? Skip straight to the numbers.
Open the Combustion Air Blower Power Calculator →This calculator's efficiency-adjusted power formula gives an accurate power requirement for one specific flow rate and pressure rise - but a genuinely important related question it doesn't directly answer is how that power requirement changes if flow rate needs to increase, and the answer follows a specific, well-established engineering relationship called the fan affinity laws.
The Fan Affinity Laws: Flow, Pressure, and Power Don't Scale the Same Way
Pressure ∝ Speed²
Power ∝ Speed³
For a given fan or blower operating on a fixed system, the fan affinity laws describe how these three quantities scale together as fan speed changes: volumetric flow rate scales linearly with rotational speed, but pressure rise scales with the square of speed, and power requirement scales with the cube of speed. This means doubling a fan's speed to double its flow rate doesn't just double the power requirement - it increases power requirement by a factor of eight, since pressure rise alone already quadruples at double speed, and power combines both the doubled flow and the quadrupled pressure together.
Why This Matters Directly for This Calculator's Power Formula
This calculator's power formula (flow rate times pressure rise, adjusted for efficiency) correctly calculates power for one specific operating point - but if a system's required flow rate increases (perhaps from a burner upgrade or capacity expansion), simply recalculating power using this same formula at the new flow rate implicitly assumes pressure rise stays unchanged. In reality, pushing more air through the same fixed ductwork and burner passages typically increases system resistance and required pressure rise too, meaning the true power increase needed for a flow rate increase is often considerably steeper than a naive linear scaling of this calculator's formula alone would suggest - exactly the cubic relationship the fan affinity laws describe.
The System Curve: Why Blower Selection Isn't Just About Matching a Single Power Figure
| Consideration | Why it matters |
|---|---|
| System resistance curve | Describes how required pressure rises as flow rate increases through the actual ductwork and burner |
| Fan performance curve | Describes what flow rate and pressure a specific fan model can deliver at various speeds |
| Operating point | Where these two curves intersect - the actual flow and pressure the installed system will settle at |
Proper blower selection matches a specific fan's performance curve against the actual system resistance curve of the ductwork, burner, and any other flow restrictions in the path - rather than simply picking a fan sized to hit one calculated power figure at one assumed operating point, since the real installed system will settle at whatever flow and pressure combination the two curves actually intersect at, which may differ from the single design point this calculator's formula was based on.
Applying This to a Calculated Blower Power Figure
The power figure this calculator produces is accurate for the specific flow rate and pressure rise entered - but if evaluating how power requirements would change under a different flow rate, or selecting an actual blower model for procurement, the fan affinity laws' cubic power-versus-speed relationship, and a proper system curve match against the blower's own performance curve, are both essential additional considerations beyond simply recalculating this formula at a new flow value.
Ready to Put This Into Practice?
Now that you understand how it works, plug in your own numbers and get an instant, accurate result.
Use the Combustion Air Blower Power Calculator Now →