How many exhaust fans does a tunnel-ventilated poultry house need?
Short answer: count fans from the air speed you want down the house, not from the house length. Multiply the house cross-section (width × average ceiling height) by the target air speed to get the total airflow, then divide by what one fan really moves at the static pressure your house runs at. Add a margin for dirty shutters, belt slip and wear.
Why "fans per house length" is not enough
A common rule of thumb says a 400 ft (about 122 m) broiler house needs at least seven 48-inch fans and a 500 ft (about 152 m) house needs eight. It works often enough to be popular, but it ignores the two things that actually set air speed: the size of the cross-section the air has to fill, and how much air each fan delivers once the house is under pressure. Two houses of the same length can need very different fan counts if one is wider, taller, or has no drop ceiling.
Step 1 — Pick a target air speed
Tunnel houses are commonly designed to exchange the air in the house at least once per minute. For a 500 ft house, that means an air speed of 500 ft/min (about 2.5 m/s). With heavier birds, many new and retrofitted houses now run at 800 ft/min (about 4 m/s) or more. Your integrator or flock adviser usually sets this number.
Step 2 — Work out the airflow you need
Required airflow = cross-sectional area × target air speed.
Example: a house 50 ft wide with an average ceiling height of 10 ft has a cross-section of 500 ft². At a target of 800 ft/min, it needs 500 × 800 = 400,000 cfm (about 680,000 m³/h).
Step 3 — Divide by the real output of one fan
Use the fan's airflow at the static pressure your house will run at, not its free-air figure. Many poultry houses run between 0.04 and 0.12 inch of water (roughly 10–30 Pa) in tunnel mode; some newer, tighter houses reach 0.15 inch (about 37 Pa) with all fans running.
Continuing the example: if one 50-inch fan delivers 27,000 cfm at your working pressure, 15 fans give 405,000 cfm, or 405,000 ÷ 500 = 810 ft/min down the house. If higher pressure drops each fan to 25,000 cfm, the same 15 fans give only 750 ft/min.
Step 4 — Keep a margin
Fan capacity rarely matches the requirement exactly, and a small surplus is useful:
- Dirty shutters can reduce air delivery by as much as 40%.
- Small losses add up: if several fans each lose 5–10%, the house can lose 25–30% of its air speed.
- Air speed at bird level is usually a little lower than above the birds.
What changes the output of a fan
- Static pressure. As pressure rises, every fan moves less air. Cooling pads add pressure, and wet pads more than dry ones. Light traps and baffles add more.
- Air flow ratio. This is the airflow at 0.20 inch divided by the airflow at 0.05 inch. Values usually run from 0.50 to 0.85; a higher ratio means the fan holds its output better when pressure rises.
- Accessories. Inlet-side shutters can cut airflow by 10–15% and discharge-side shutters by 15–25%. A well-designed housing with a discharge cone can improve airflow by at least 15%.
- Efficiency. Measured in cfm per watt. Independent tests of 50-inch fans at 0.10 inch have shown wide spreads in both airflow and efficiency, so compare tested data at a realistic pressure, not just fan diameter.
How EcoHarvest exhaust fans fit
EcoHarvest supplies three wall exhaust types for tunnel houses: Box (GPF), Hammer-shutter (ZHF) and Cone (GKF, galvanized or FRP). Catalogue air volumes in m³/h are on the exhaust fans page. For your house, airflow and power are confirmed on the written quotation. If the house is dark-out, light traps at the fan wall add resistance and are designed together with the fans.
What to send for a fan quote
- House length, width and average ceiling height (or eave and ridge height)
- Target air speed, if your integrator has set one
- Birds or livestock, and stocking
- Cooling pads and light traps planned or installed
- Supply voltage and frequency
- New house or retrofit; preferred fan type (Box, Hammer-shutter or Cone)
- House type (tunnel, combination or dark-out), country and climate
Email sales@ecoharvestx.com or use the contact page.
FAQ
Can I size fans by house length alone?
Only as a rough first check. The cross-section and the fan's output at working static pressure decide the real air speed.
Should I use the fan's free-air rating?
No. Use the airflow at the static pressure the house runs at in tunnel mode. Output falls as pressure rises.
How much spare capacity should I plan?
Enough to cover dirty shutters, belt slip and wear. There is no single figure for every house; it is set in the design and confirmed on the quotation.
Do cooling pads change the fan count?
They can. Pads add static pressure, so each fan moves less air. The pads and fans should be designed together.