(Wall-Mount Exhaust Fan)

Wind-Tunnel Design Tips!
Fresh Air Inlets

Use windows and doors — close ceiling inlets, hayholes, gutters, & sidewall fans in summer to prevent "short-circuiting" the end-to-end air flow.


For 2-1/2 MPH Air Flow = Height x Barn Width x 220
For 3 MPH Air Flow = Height x Barn Width x 264
Note: Total Fan CFM should provide at least 450 CFM for each large-size holstein cow in hot weather. An air speed of 3 MPH helps evaporate water off cow.

  • The beauty of tunnel ventilation is that ALL of the fresh air which is pulled into the back end of the barn is drawn through the barn as a block of air (ceiling to floor and sidewall to sidewall), flowing over, under and around cows. The result is more complete air movement without "dead air" spots. All the air sucked into the tunnel (barn) is pulled through it (just like sucking through a straw!). Inside temperature is kept at or slightly below outside temperature. Many farmers claim it's the next best thing to air conditioning!

  • 2-1/2 — 3 MPH summer air flow cools cows so they will eat and milk more. It cools down the barn faster in evening and carries away heat, humid air, flies, dust, disease organisms and ammonia odor. Air flow dries feed alleys and reduces feed spoilage, keeping cows cleaner and extending equipment life.

  • Keep windows and doors closed — open only at the end of barn opposite fans. Barn must be "tight" to pull air through entire length.

  • Haymow installation illustrated — fans draw up more of warmer ceiling air, fans quieter, further from humans and animals, first floor windows can be kept for light, and fans remain easier to close off in winter. However, first floor installation eliminates need for hayholes and plywood air duct (preventing loss of mow space), but requires OSHA fan guards and possibly cattle-grate guards to protect fans (available from factory).

  • Fan(s) can also be mounted along sidewalls, close to end, if end wall space is limited. L-shaped barns should have all fans in one end and inlets at the extreme opposite end, not at the juncture of the two wings. T-shaped barns will need more total fan capacity to avoid different air-velocities in the different wings.

  • Position fans to utilize prevailing wind if possible. However, consider fan exhaust odors, if a house is nearby. The breeze from air being discharged into barn yard will reduce fly control costs.

  • If using thermostats , locate halfway down the barn. (Some farmers line them up with related fans for "more accurate temperature control"). Set about 3° to 5° apart, starting at about 45°. Thermostat and on/off switch can be wired to a small pilot-light box, which lights up when related fan in haymow is operating.

  • Air pulled up through "hayhole type" opening (use inlet formula below to size opening). Duct Height = top of fan.

  • Use 1/2" to 5/8" plywood with 2x4 framing. Caulk joints. If hay bales are to be stacked on top of exhaust duct, use thicker plywood and heavier post and beam construction.

  • Insulated plywood door (1-1/2" foam board) — hinged and opened with rope and pulley — closed in winter. Duct can have separate door to run just one fan (smallest) in winter.

  • Summer Inlet square feet needed (doors and windows) = Total Fan CFM / 400 or 500 or 600 (number to use varies between "experts").

  • Hayhole-type exhaust opening in mow floor requires same square footage as above formula (adjust upward to allow for width of joists).

  • Undersized inlets will starve fans and reduce air flow by as much as 20% or more.

  • Increasing inlet openings will reduce static pressure and increase air flow. An inexpensive wind meter aids in adjusting window and door openings to obtain correct air flow speed. A chemical smoke gun can also be used to locate dead spots (or shake and squeeze a bottle of talcum powder to check micro-currents). Smoke from a cigarette or other heat source may not accurately show airflow patterns, because warm smoke tends to rise vs. follow air stream.

  • When temperature is below 60°, adjust inlets so outside air does not flow directly over cows. Use many smaller inlets vs. 1 or 2 large inlets. Close inlets in cooler areas of barn to slow air flow and open more inlets in warmer areas as required to speed air flow.

  • If you run one fan in winter, reduce inlet openings to slow air flow (slotted inlets, mounted toward or in ceiling, are often used). In coldest weather, you only need 32 CFM per 1,300 Ib. cow (65 CFM for mild winter conditions and 130 CFM for spring and fall). Use timer and run fan only at intervals. Monitor cows to avoid freezeburn.