The formula to calculate the new Impeller Speed (N2) is:

\[ N2 = N1 \left( \frac{Q2}{Q1} \right)^{1/2} \left( \frac{H2}{H1} \right)^{3/4} \]

Where:

• \(N1\) is the initial impeller speed (rpm).
• \(N2\) is the new impeller speed (rpm).
• \(Q1\) is the initial air flow rate (cfm).
• \(Q2\) is the new air flow rate (cfm).
• \(H1\) is the initial fan head (in. H2O).
• \(H2\) is the new fan head (in. H2O).