Fan Wall Calculator — Fan Count, N+1 Redundancy & Failure Scenarios
A fan wall replaces one large fan with several smaller EC fans in parallel. This tool works out how many fans you need for a duty point, what the array does when one or two fans fail, how much the survivors must speed up, and the spares to keep — the questions every data-center and AHU retrofit asks.
No signup · Works on mobile · Formulas shown below · Updated 2026-08-23
Fan Array Sizing & Redundancy
Parallel fans share airflow at the same pressure. Redundancy = extra fans beyond the number needed for the duty point.
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
How fan arrays behave
Fans in parallel add airflow at the same pressure. If one stops, the system curve does not change, so the remaining fans must each move more air — which on a steep system curve (filters, coils) is easy, and on a flat one (ductwork, large free areas) needs a lot of extra speed. The tool uses the per-fan maximum airflow at your pressure to check whether the survivors can do it.
Fans needed N = ⌈ Qdesign ÷ Qfan(p) ⌉ Array total = N + redundancy After k failures each survivor must deliver Qdesign ÷ (total − k) ≤ Qfan,max(p) Power, all running ≈ total × Wfan × (share)³, share = Qdesign ÷ (total × Qfan) // cube law: spreading the load over the spare fan too saves energy instead of wasting it
Design each fan at 80–90 % of its maximum airflow at the system pressure. That 10–20 % headroom is what makes N+1 real: it lets 7 fans do the work of 8. Without headroom, "N+1" is just an extra fan that cannot compensate.
Fan wall design checklist
Backdraft dampers on every fan (or the array recirculates through the dead fan and loses 10–20 % airflow).
Spacing: centre distance ≥ 1.2 × inlet-ring diameter; leave ≥ 0.5 D clear upstream for even inflow.
Control: one 0–10 V or Modbus setpoint to all fans; Modbus gives alarm, speed and power feedback per fan. Stage fans off at low load only if the system pressure is flat — otherwise keep all running slow.
Electrical: individual fuses per fan; inrush of EC fans is low, but size the supply for simultaneous restart after power loss.
Noise: many small fans at moderate speed are typically 3–6 dB quieter than one large fan at the same duty, and the BPF shifts higher (easier to attenuate).
Service: each fan should be removable through the access door without disturbing the others — a 500 mm EC plug fan is a one-person job; a 1,000 mm belt-drive fan is a crane job.
Spares: 5 % of the installed base, minimum one per fan size per site; identical firmware/model so controls need no reconfiguration.
Worked example: AHU retrofit
Given: 40,000 m³/h at 700 Pa. Candidate 500 mm EC plug fan: 9,000 m³/h at 700 Pa at 85 % speed, 12,000 m³/h at full speed.
One failure: 5 survivors × 12,000 = 60,000 ≥ 40,000 → each at 8,000 m³/h, i.e. 89 % of design duty — 100 % airflow maintained
All 6 running: each at 6,667 m³/h (74 % of 9,000) → power ≈ 6 × 3.0 × 0.74³ ≈ 7.3 kW vs 15 kW for 5 fans at duty speed — the spare fan saves energy.
At 70 % annual average load: ≈ 2.5 kW average → 22,000 kWh/yr.
Frequently Asked Questions
How many fans should a fan wall have?
Divide the design airflow by one fan's airflow at the system pressure (at 80–90 % speed), round up, then add one (N+1) for redundancy. Typical arrays use 4–12 fans; more than ~16 rarely pays because installation and control cost grows faster than the efficiency benefit.
What does N+1 redundancy mean for fans?
N is the number of fans needed to meet the design airflow; N+1 adds one more so that any single fan can fail without loss of capacity — provided the surviving fans have enough speed headroom to pick up its share.
Does a fan wall use more energy than one big fan?
Usually less. Several smaller EC fans at 70–85 % speed, each near its best efficiency point, typically beat a large belt-driven fan by 15–30 %, and running the redundant fan as part of the array (all fans slower) saves additional energy via the cube law.
What happens when one fan in a fan array fails?
Without a backdraft damper, air recirculates backwards through the stopped fan and the array loses more airflow than the failed fan's share. With dampers and a controller that raises the speed of the rest, airflow stays at 100 % as long as the survivors have headroom.
How many spare fans should I stock?
About 5 % of the installed base, rounded up, with a minimum of one per fan model per site; two for unattended or remote sites. With Modbus fan monitoring you can replace on alarm rather than on schedule.
Link to or Cite This Tool
This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:
<a href="https://www.longwellfans.com/resources/fan-wall-redundancy-calculator/">Fan Wall & N+1 Redundancy Calculator</a> by LONGWELL Fans
