Size a greenhouse exhaust fan system at 180–270 m³/h per square metre of floor area for peak summer cooling, which is equivalent to roughly 45–60 air changes per hour in a standard 4 m gutter-height house, then stage the same fans down to 2–4 air changes per hour for winter humidity control. The reason for the wide range is solar gain: a single-layer glass house in a hot climate needs the top of the band, while a double-poly house with 50 % shade screen in a temperate climate can hold temperature at the bottom of it.
Step 1: Calculate the summer cooling airflow
The airflow needed to limit temperature rise is Q = 3,000 × heat gain in kW / allowable ΔT. Peak solar gain through a clear glass roof is typically 0.5–0.7 kW per m² of floor at midday, of which 60–75 % becomes sensible heat once transpiration is accounted for. Allowing a 3–5 K rise from inlet to outlet gives 180–270 m³/h per m². A 1,000 m² house therefore needs 180,000–270,000 m³/h, usually delivered by six to ten 1,250 mm or 1,400 mm fans.
Two corrections matter. Increase airflow by 5–10 % for every 300 m of altitude, because air density falls and each cubic metre carries less heat. Increase it again if the fan-to-inlet distance exceeds about 60 m, since the air warms progressively along the house and the far end runs hotter — the usual fix is a maximum 50–60 m airflow path, not a bigger fan.
Step 2: Choose the ventilation stages
| Stage | Airflow per m² floor | Approximate air changes/h | Purpose | Typical control input |
|---|---|---|---|---|
| 1 — winter minimum | 8–16 m³/h | 2–4 | Humidity and CO&sub2; control | Humidity above 85 % RH |
| 2 — shoulder season | 40–70 m³/h | 10–18 | Mild temperature trim | Air 1–2 K above set point |
| 3 — summer day | 110–160 m³/h | 28–40 | Bulk heat removal | Air 2–3 K above set point |
| 4 — peak with pad cooling | 180–270 m³/h | 45–60 | Evaporative cooling duty | Air 3–4 K above set point |
| Recirculation only | 15–30 m³/h (internal) | 4–8 | Destratification, leaf drying | Continuous or timed |
Staging matters more than total capacity. A house that jumps straight from off to full flow swings 4–6 K around set point and stresses the crop; three or four stages, or better still variable-speed control on two of the fans, typically holds the hall within 1–2 K.
Step 3: Static pressure and pad-and-fan design
Greenhouse fans are very high-flow, very low-pressure machines, so selection is dominated by airflow per watt rather than by pressure capability. Bare exhaust fans with motorised louvres see only 15–30 Pa. Adding wet evaporative pads raises the total to 30–50 Pa, and adding insect screening at the inlet can add a further 15–40 Pa depending on mesh — 50-mesh anti-thrips screen is the single most common cause of under-performance in tropical greenhouses.
- Pad face velocity of 1.0–1.5 m/s for 100 mm thick cellulose pads and 1.3–1.8 m/s for 150 mm pads; higher velocity carries water droplets into the house.
- Pad area of roughly 1 m² per 6,000–9,000 m³/h of design airflow.
- Evaporative cooling efficiency of 75–85 % for a well-wetted 100 mm pad, giving a 6–12 K dry-bulb depression in a dry climate but only 2–4 K in a humid coastal one.
- Water circulation of 8–12 L/min per linear metre of pad to keep it fully wetted and avoid dry channels.
- Maximum 50–60 m from pad wall to fan wall, so the temperature gradient along the house stays under about 4 K.
Fan efficiency: the number that decides operating cost
Greenhouse fans run thousands of hours per season, so airflow per watt dominates lifetime cost. A well-designed 1,250 mm direct-drive fan with a cast aluminium impeller and an efficient cone typically achieves 15–22 m³/h per watt at 25 Pa; a cheap flat-blade belt-driven unit may deliver only 8–12 m³/h per watt. Over a 2,000-hour season, ten fans moving 25,000 m³/h each cost roughly 25,000 kWh at 20 m³/h/W but 50,000 kWh at 10 m³/h/W — the whole capital difference is usually recovered in one or two seasons.
- Prefer direct drive over belt drive: no belt slip, 3–6 % higher efficiency, and far less maintenance in a dusty, humid house.
- Specify a discharge cone; it recovers velocity pressure and typically adds 8–15 % to airflow at the same power.
- Use IE3 or better motors, IP55, with tropicalised windings and stainless shafts for the humid environment.
- Choose galvanised or polyester-coated steel housings; bare steel in a fertigation atmosphere corrodes within two or three seasons.
Keruite manufactures horticultural axial fans at an 80,000 m² plant in Dezhou, China, with an in-house AMCA-210-style test laboratory, and every impeller is balanced to G2.5 and performance-tested before shipment.
When to choose which ventilation approach
- Choose natural ridge and side vents alone when the house has at least 15–20 % of floor area as vent opening, the climate is temperate, and crop value does not justify mechanical plant.
- Choose mechanical exhaust fans with inlet louvres when the house is wide, multi-span, or screened for insects, because natural ventilation collapses once screens and internal shade cloth are fitted.
- Choose pad-and-fan evaporative cooling when summer design wet-bulb depression exceeds about 8 K — typically continental and arid climates — where it can hold the crop 6–12 K below ambient.
- Choose high-pressure fogging with mechanical ventilation instead when the climate is humid and pad cooling would yield only 2–4 K, or when uniform humidity is more important than dry-bulb depression.
- Choose horizontal airflow recirculation fans in addition to exhaust fans when the crop suffers fungal pressure; 15–30 m³/h per m² of internal circulation dries leaf surfaces and evens out temperature stratification of 2–4 K between floor and gutter.
- Choose variable-speed control whenever the house grows a temperature-sensitive crop; the cube law means running at 70 % speed uses only about 34 % of full-speed power.
Winter operation and humidity
In winter the fan's job changes from cooling to moisture removal. Transpiration continues at 0.5–2.0 L per m² per day, and if that vapour is not exhausted the house saturates, condensation drips on the canopy and disease follows. A minimum of 2–4 air changes per hour, interlocked with the heating system so incoming cold air is tempered, holds relative humidity below the 85 % threshold where most fungal pathogens become active. Where heating energy is expensive, an air-to-air heat exchanger recovering 50–70 % of the sensible heat makes the minimum ventilation rate affordable.
FAQ
How much airflow does a greenhouse need for summer cooling?
Allow 180 to 270 cubic metres per hour per square metre of floor area, roughly 45 to 60 air changes per hour in a house with four metre gutter height. Use the top of the range for clear glass in hot climates and the bottom for shaded double-poly houses.
What static pressure do greenhouse exhaust fans work against?
Very little. Bare fans with motorised louvres see only 15 to 30 pascals, evaporative pads raise this to 30 to 50 pascals, and fine insect screening can add another 15 to 40 pascals. Selection is therefore governed by airflow per watt, not pressure capability.
How effective is pad and fan evaporative cooling?
A well-wetted 100 mm cellulose pad achieves 75 to 85 percent saturation efficiency. In a dry inland climate that yields a 6 to 12 kelvin dry-bulb depression, but in a humid coastal climate the same pad may deliver only 2 to 4 kelvin of useful cooling.
What fan efficiency should I look for in a greenhouse fan?
Target 15 to 22 cubic metres per hour per watt at 25 pascals for a direct-drive fan with a cast impeller and discharge cone. Low-cost flat-blade belt-driven fans often manage only 8 to 12, which can double seasonal electricity consumption for identical airflow.
Why does a greenhouse need ventilation in winter?
To remove moisture. Crops transpire 0.5 to 2.0 litres per square metre per day all year, and without exhaust the house saturates and condensation drips onto the canopy. Two to four air changes per hour, interlocked with heating, keeps humidity below the 85 percent disease threshold.