Greenhouses, Grow Tunnels and Climate Control

Duct fans and controllers sized to the air

Many items here are not structures but climate gear. An inline duct fan is sold by duct diameter, such as 4 inch, and the ducting, filters or hoods you connect must match that diameter or be joined with an adapter; stepping down to a narrower duct cuts airflow. Compare rated airflow with the volume of the space (length x width x height); long runs and bends cut real output, so leave headroom. Speed controllers, with several steps or a dial, let you slow the fan down.

Controllers add the brains. A temperature and humidity sensor lets the fan switch on by itself when the greenhouse heats up or turns damp, which matters when nobody is there to open a vent. Look for adjustable thresholds, a timer for scheduled runs, and a remote or display if the fan sits out of reach. Check that the controller's rated load covers the fan's power draw, and keep plugs and sensors out of direct sprinkler spray.

Glazing, frames and vents on the structure itself

For the greenhouse or tunnel, the covering sets how much heat it holds. Polyethylene film is light and easy to replace but wears out under sun; twin-wall polycarbonate insulates better and lasts longer; glass is clearest and heaviest. Tunnels use bent hoops that are quick to raise but catch wind, so they need ground anchors and a sturdy door end. Whatever the cover, plan roof vents or fans, because a closed greenhouse on a sunny day gets hot enough to scorch plants.

Questions about Greenhouses, Grow Tunnels and Climate Control

How do I size a fan for a greenhouse or grow space?

Work out the space's volume in cubic feet and compare it with the fan's rated airflow in cubic feet per minute. A fan that can move that volume in a minute or two is a sensible start, and ducting or filters will cut the real figure.

Can I connect a 4 inch fan to 6 inch ducting?

Yes, with a 4 to 6 inch adapter. Stepping up to wider ducting costs little or no airflow, though it will not raise the fan's rated output. The reverse, a larger fan feeding narrower ducting, restricts airflow and adds noise. Keep the duct run as straight as you can.

Is polycarbonate better than plastic film for a greenhouse?

Polycarbonate panels hold heat better, resist hail and last many years, but they are heavier and need a stronger frame. Film is lighter and easy to replace, which suits tunnels and seasonal use, though it must be re-covered every few seasons.