Quick answer: Greenhouse air tubes (perforated polyethylene "fan-jet" or distribution tubes) are fed by a fan at one end and release air through rows of holes along their length. They do three different jobs: bring in and mix cold outdoor air in winter without chilling the crop, distribute heated or dehumidified air evenly, and circulate air under benches or between rows. For even airflow, the hole pattern must match the fan and tube length, the tube must stay fully inflated, and the fan must deliver its rated airflow against the tube's pressure.
Quick answer
- Three jobs: winter ventilation, distributing heated or dry air, circulation.
- Even discharge needs an inflated tube and holes sized for the fan.
- Placing dehumidifiers on tubes spreads dry air through long houses.
The three jobs
| Job | How the tube is used | Typical layout |
|---|---|---|
| Winter ventilation | Fan draws cold outdoor air through a shuttered inlet into the tube; jets mix it with warm house air before it reaches plants | Overhead, along the house |
| Air distribution | Heater or dehumidifier discharges into the tube; air spreads along the house | Overhead, or under gutters, or under benches |
| Circulation | Fan recirculates house air through the tube | Overhead or under the canopy |
Don't confuse them: a tube sized for winter ventilation may not deliver enough dry air for a night dehumidification duty, and vice versa.
Why air arrives unevenly
Air in a tube loses speed as it escapes through the holes, while static pressure along the tube changes. If the holes are too large or too many, most air leaves near the fan and the far end gets little; the tube may also collapse at the far end. If they are too few, the fan works against high pressure and delivers less air.
Practical rules:
- Hole area: keep the total hole area within the range the tube or fan maker specifies relative to the tube's cross-section; too much open area starves the far end.
- Tube length: the longer the tube, the harder it is to keep discharge even; stay within the maker's maximum length, or feed from both ends or use two tubes.
- Fan match: the fan must deliver its airflow at the pressure the tube creates; check the fan curve.
- Inflation: a tube that sags or flaps is under-pressurised.
Wageningen research covers air distribution and climate uniformity in greenhouses (WUR).
Hole pattern and direction
- Holes are usually in two rows, angled downwards on both sides, so jets spread across the house.
- Jets entrain several times their own volume of house air, which is why tubes mix air well.
- Avoid holes blowing straight onto plants; cold or very dry jets damage leaves.
- Under-canopy tubes should blow along rows or slightly upwards into the crop.
Using tubes with dehumidifiers
In long or multi-span houses, a dehumidifier in one corner leaves the far end humid. Connecting dehumidifiers to air tubes:
- spreads dry air along each bay;
- puts dry air where condensation and disease start (in or just above the canopy);
- lets several smaller units share a distribution system.
Check that the dehumidifier's fan can push air through the tube (external static pressure), or add a booster fan. Ducted models list external static pressure; see our ceiling-mounted and ducted dehumidifiers and ducted vs freestanding greenhouse dehumidifiers.
Common failures
| Failure | Cause | Fix |
|---|---|---|
| Far end deflated, humid | Too much hole area or tube too long | Fewer or smaller holes; shorter tubes; feed both ends |
| Cold spots under the inlet | Winter jets aimed at plants | Re-angle holes; raise tube; reduce outdoor air fraction |
| Fan noisy, low airflow | Fan working against high pressure | Check fan curve; add holes; larger fan |
| Torn or dirty tubes | Age, UV, dust | Replace tubes periodically; clean |
Winter vs summer
- Winter: small outdoor-air volumes mixed into warm air; tubes prevent cold drafts and help purge humid air.
- Summer: tubes cannot provide the large airflows needed for cooling; use roof and side vents or exhaust fans.
- Nights in spring and autumn: tubes distribute heated and dehumidified air to keep the canopy dry; see greenhouse humidity control methods.
FAQ
Frequently asked questions
How do greenhouse air tubes work?
A fan inflates a perforated tube; air leaves through holes along its length as jets that mix with house air.
How long can a greenhouse air tube be?
Follow the tube and fan maker's maximum length. Long houses usually need tubes fed from both ends or several tubes.
Can I connect a dehumidifier to a greenhouse air tube?
Yes, if the unit's fan can handle the tube's pressure or a booster fan is used. It spreads dry air evenly along the house.
Why is one end of my air tube flat?
Too much hole area for the fan, or the tube is too long.
Sources
- Wageningen University & Research, greenhouse air distribution research (edepot 202121).
- UMass Extension, Reducing humidity in the greenhouse.
