Parallel Operation of Axial Fans: When Two Fans Do Not Double the Flow

By the NextAir Systems engineering team. Last reviewed October 2026.

Adding a second fan feels like a simple way to double the airflow. It is not. Two identical fans side by side deliver twice the flow at the same pressure, and a duct system does not hold the pressure still: more airflow means more resistance, so the pressure rises and each fan delivers less than before. How much you gain depends on how steep the system is.

We calculate the gain for five system resistances with a simple fan model, and list the practical issues that turn a bank of parallel fans from an easy answer into a problem.

The short version

• In parallel, the fans share the system, each carrying half the flow at the common pressure. The combined curve is the single-fan curve with the flow axis doubled.
• In our model, a low-resistance system gives a gain of about 58% from the second fan, and a high-resistance system only about 8%.
• The steeper the system curve, the smaller the benefit, so parallel fans work best on open, low-resistance systems such as wall banks and roof extraction.
• Fans of the same type from different manufacturers react to a system differently, which AMCA 201 flags as a reason to be careful when mixing them.^[1]

How parallel fans combine

For identical fans connected to a common system, each runs at the same pressure and carries half the flow. So the combined performance curve has the same pressure axis as one fan and twice the flow. The operating point is where this combined curve crosses the system curve.

We use the same illustrative fan as in the system effect article: pressure falls from 800 Pa at zero flow to zero at 20 m³/s, following P = 800 × (1 − (Q ÷ 20)²). The system curve is P = k × Q², where k is the resistance in Pa per (m³/s)². We solve for the flow with one fan and with two.

Worked example. On a system with k = 0.5, one fan delivers 17.9 m³/s and two deliver 28.3: a gain of 58%. On a system with k = 8, one fan delivers 8.9 and two deliver 9.7, a gain of only 8%. The second fan is the same fan, with the same motor and the same cost. The system decides what it is worth.

Fan curves of real axial fans have a stall region and are not parabolas, so the exact figures differ, but the trend is general, and it follows from the physics: with a steep system curve, the pressure rises quickly with flow, and each fan is pushed up its own curve toward lower flow.

When parallel works well

• Open systems, with low resistance: wall and roof extraction banks, direct discharge to atmosphere, short ducts.
• Systems with a flat curve and a duty well below the fans’ maximum flow.
• Applications where staging saves energy. Running one fan at part load and bringing in the second when needed can be more efficient than one big fan throttled, as covered in one large fan or several small ones.

When it goes wrong

• Stall and unstable operation. If the combined operating point lands in the fan’s stall region, flow can hunt between fans. Check each fan’s curve against the combined point, as in how to read an axial fan curve.
• Fans of different size or curve. Unequal fans do not share load in proportion to size. The stronger one can push the weaker back into its stall zone.
• Backflow through a stopped fan. A fan that is off in a bank lets air recirculate through itself unless it has a backdraught damper or shutters.
• Interference between neighbours. Fans placed too close disturb each other’s inlets. That is system effect again, as in the system effect article.
• Starting current. Starting both fans together can be a large load on the supply. Stagger the starts.
• Control. Two fans on VFDs should be controlled together, at the same speed, or their curves drift apart.

A worked reading of the table

Imagine a roof extraction system with a short duct and no filters. Its resistance is low, so a second fan adds a large share of a fan’s flow, between the first rows of the table. Now imagine a duct system with a coil, filters and a long run. The resistance is high, the second fan adds little, and the better investment is a larger fan, a higher-pressure fan or a lower-resistance duct. The same fan, doubled, is a good buy in one case and a poor one in the other. That is why the system curve, not the fan count, belongs at the centre of the design conversation.

Checking a parallel design

1. Draw the single-fan curve, then the combined curve with the flow doubled at each pressure.
2. Draw the system curve using the real resistance, including system effect.
3. Read the operating point. Compare it with the single-fan point to see what the second fan really adds.
4. Check that both the single-fan and the combined points are clear of stall.
5. Decide how the fans are controlled, and how a failed fan is isolated.

Where this fits with NextAir

NextAir will check a parallel arrangement against the fan curves and the system curve for your site, and supply backdraught dampers and controls. Send the layout and duty to our axial fan manufacturer team. Our axial fan guide covers selection, and the factory ventilation page covers where parallel banks are most common.

Sources

1. AMCA / ANSI Webstore. AMCA 201-23 Fans and Systems.
2. Plant Engineering (US DOE Motor System BestPractices material). Building energy efficiency into fan systems.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

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