- Separate air exchange from air movement and which fan does each.
- Turn room volume, ducting and filter into the delivered airflow a fan must show on its curve.
- Choose the controller from the measured problem and write a spec sheet to compare suppliers.
01 / Two jobs: exchange the air, then keep it moving
Exchange replaces the room’s air. UGA Bulletin 792 says ventilation removes warm, moist air and replaces it with drier air, and that high humidity causes condensation on cool surfaces and tends to increase disease. UMass Extension’s horizontal air flow fact sheet puts the needed movement at 50–100 feet per minute, holds a properly installed house within 2°F, and says moving air removes moisture from the canopy and can reduce foliar disease.
The mechanism is condensation. UMass Extension’s Reducing Humidity in the Greenhouse explains that droplets form when leaf surface temperature falls below the dew point, that they promote germination of fungal spores such as Botrytis and powdery mildew, and that air movement is adequate when the leaves move slightly. A lit tent is a small greenhouse without glazing; University of Minnesota Extension says ventilation is key and adding a fan can help. Room heat also reaches the reservoir, and University of Missouri G6984 notes that warmer water holds less oxygen. Circulation fans differ from exhaust fans: UMass HAF describes 1/10 to 1/15 horsepower units built for little resistance.
Reading: Greenhouses: Heating, Ventilation, and Cooling (Bulletin 792) ↗ · Horizontal Air Flow is Best for Greenhouse Air Circulation (fact sheet, J. W. Bartok, Jr., 2005) ↗ · Reducing Humidity in the Greenhouse (fact sheet, J. W. Bartok, Jr., 2003) ↗ · Small-scale hydroponics ↗ · Hydroponic Nutrient Solutions (G6984) ↗
02 / The rating is free air; you buy delivered air
Airflow is printed in cubic feet per minute (CFM) or m³/h (1 CFM is about 1.7 m³/h), and the figure is measured with nothing attached. UF/IFAS AE020 shows what resistance does: its Fan 1 delivers 10,200 CFM in free air, 9,200 CFM at 1/8 inch of water static pressure and 4,300 CFM at 3/8 inch. A fan runs only where its curve crosses the system’s resistance curve, which AE020 calls the point of operation; your duct, bends and filter are that curve. AE020 says most greenhouse applications stay below 0.10–0.15 inches of water and names AMCA Standard 210 as the basis for comparable ratings; a tent with a filter and a long duct can sit higher.
UGA Bulletin 792 gives the consequence: select fans to operate against 1/8 inch of static pressure, because fans not rated against pressure usually move only 60–70% of rated airflow once installed. A manufacturer page read on 17 September 2026 for a 6-inch inline fan (AC Infinity CLOUDLINE S6) lists 425 CFM, 30 dBA and 503 Pa static pressure with a ten-speed controller, and does not state the pressure at which the airflow was measured; ask for the curve. One example, not an endorsement.
Reading: Fans for Greenhouses (AE12/AE020) ↗ · Greenhouses: Heating, Ventilation, and Cooling (Bulletin 792) ↗ · CLOUDLINE S6, Quiet Inline Fan 6" with Speed Controller (product page) ↗
03 / Size by volume and exchanges, assumption written down
No cited document publishes an air-exchange rule for an indoor tent, so this arithmetic is original and its assumption explicit. UGA Bulletin 792 says one greenhouse air exchange per minute, without evaporative cooling, should keep the house about 8°F above outdoor temperature; half that volume gives about a 15°F rise, two exchanges about 5°F. A greenhouse carries solar load a tent does not: an upper reference, not a target.
A 1.2 × 1.2 × 2.0 m tent holds 2.88 m³. One exchange per minute is 2.88 × 60 = 172.8 m³/h, and 172.8 ÷ 1.699 ≈ 102 CFM, delivered: the fan must show that on its curve at the pressure your duct and filter impose. For movement, UMass HAF’s greenhouse rule is circulation capacity of about two times the floor area in square feet: 1.44 m² ≈ 15.5 sq ft, so about 31 CFM, a greenhouse figure used for scale only. The test is UMass’s: leaves moving slightly.
Reading: Greenhouses: Heating, Ventilation, and Cooling (Bulletin 792) ↗ · Horizontal Air Flow is Best for Greenhouse Air Circulation (fact sheet, J. W. Bartok, Jr., 2005) ↗
04 / Ducting, filters and noise
In practice, and not from any cited document: keep the duct at the fan’s diameter for the whole run, pull flexible duct taut, minimize bends, and put the filter on the intake side. Every reduction, crushed section and tight elbow raises the system curve, and the fan slides down its own curve to meet it. A carbon filter is an odor and dust device: it does nothing for temperature or humidity, adds resistance that grows as it loads, and is replaced on the maker’s schedule.
Noise is a spec-sheet item. AE020 states that fan noise depends on design, flow rate, total pressure and efficiency, that airstream turbulence is its primary source, that the most efficient fan is also the quietest, and that the only valid comparison is sound power at the required flow and specified static pressure. A dBA figure without flow and pressure beside it is not comparable.
Reading: Fans for Greenhouses (AE12/AE020) ↗
05 / Controllers: what they switch and what they cannot fix
A controller changes when and how fast the fan runs. UGA Bulletin 792 describes fans activated by a thermostat or humidistat and, in a pad-cooled house with three fans or fewer, one fan with a two-speed motor to prevent excessive temperature fluctuation and fan cycling. UF/IFAS AE020 adds that because system pressure is never exactly known, choose a drive that lets fan speed be changed within recommended limits. A timer suits a schedule, a thermostat a heat problem, a humidistat a moisture problem.
No controller cools the room below the air it pulls in or dries it below that air’s moisture. UMass’s humidity fact sheet pairs ventilation with heating, venting two or three times an hour after sundown and at sunrise, with a relay so heater and fans do not run together. A setpoint is a request; verify it with a separate thermometer and hygrometer at canopy height. A speed controller must be the type the fan’s maker specifies for its motor.
Reading: Greenhouses: Heating, Ventilation, and Cooling (Bulletin 792) ↗ · Fans for Greenhouses (AE12/AE020) ↗ · Reducing Humidity in the Greenhouse (fact sheet, J. W. Bartok, Jr., 2003) ↗
06 / Electrical practice, then the spec sheet
General practice, unsourced and jurisdiction-neutral: fan, controller and heater share the ground-fault-protected supply (GFCI/RCD) the commissioning chapter describes, plugs sit above the highest water level with a drip loop in every cord, the fan is unplugged before you reach into the duct, and local electrical rules and the equipment instructions govern.
- Measure the room and compute its volume.
- Write the exchange-per-minute assumption and convert it to delivered airflow.
- List the duct run, bends and filter; get the filter’s pressure drop.
- Ask each supplier for airflow at that pressure from an AMCA-tested curve.
- Choose the controller from the problem: schedule, heat or moisture.
- Add a circulation fan from the floor-area reference, aimed above the canopy.
- Install on ground-fault protection; verify at canopy height with a thermometer and hygrometer.
| Criterion | What to write down | Where the figure comes from |
|---|---|---|
| Room volume | L × W × H in m³ or ft³ | Tape measure |
| Exchange rate | Exchanges per minute assumed; delivered CFM or m³/h | Original arithmetic; UGA B792 upper reference |
| System pressure | Duct length, diameter, bends, filter pressure drop | Filter data sheet; AE020 point of operation |
| Airflow at pressure | CFM at your pressure, AMCA-tested | Supplier’s curve; UGA B792 60–70% caution |
| Movement | Circulation fan CFM and aim | UMass HAF two times floor area |
| Noise | dBA with its flow and pressure | AE020; manufacturer page |
| Control | Timer, thermostat, humidistat or speed; maker-approved | UGA B792; AE020 |
| Power and safety | Watts at running speed; GFCI/RCD, drip loop | Manufacturer page; general practice |
FIELD QUESTIONA 1.2 × 2.4 × 2.0 m tent with a carbon filter, 3 m of 150 mm flexible duct and two bends. What goes on the sheet before you look at fans?
Volume: 1.2 × 2.4 × 2.0 = 5.76 m³. At the one-exchange-per-minute upper reference, 5.76 × 60 = 345.6 m³/h, or 345.6 ÷ 1.699 ≈ 203 CFM delivered. Pressure: the filter’s published drop plus the duct; if a supplier quotes at 1/8 inch of water (about 31 Pa, since 1 inch is about 249 Pa), ask whether your filter alone exceeds that. Movement: 2.88 m² ≈ 31 sq ft, times two ≈ 62 CFM. Controller: thermostat if the log shows heat under lights, humidistat if condensation at lights-off.
Reading: Fans for Greenhouses (AE12/AE020) ↗ · Greenhouses: Heating, Ventilation, and Cooling (Bulletin 792) ↗ · Horizontal Air Flow is Best for Greenhouse Air Circulation (fact sheet, J. W. Bartok, Jr., 2005) ↗ · Reducing Humidity in the Greenhouse (fact sheet, J. W. Bartok, Jr., 2003) ↗ · CLOUDLINE S6, Quiet Inline Fan 6" with Speed Controller (product page) ↗
Parts & buying criteria
Build your own parts & cost worksheet →
Showing aerated deep water culture. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.
01Nutrient reservoir1 · illustrated quantity+
Stores the measured nutrient solution beneath or beside the growing area.
Inspect: Compare the surface with the diffuser and the lower roots; freeboard is deliberate. There is no pump intake in this single-vessel example.
Maintain: Record level before refilling, keep light out and verify temperature, EC and pH.
Opaque, cleanable, intended-use container with volume marks and service access.
Check size, materials and operating conditions with your chosen supplier before ordering.02Lid & root support1 · illustrated quantity+
Holds the plant above the chamber or reservoir without pinching its crown.
Inspect: Separate the cover and inspect the basket or collar opening.
Maintain: Check fit as plants grow and keep unused openings covered.
Rigid compatible support with removable, correctly fitted baskets or collars.
Check size, materials and operating conditions with your chosen supplier before ordering.03Air pump & diffuser1 · illustrated quantity+
Delivers air below the water surface in this aerated DWC example.
Inspect: Trace the thin line from the dry pump to the diffuser. Bubbles are illustrative.
Maintain: Check actual delivery and dissolved oxygen; preserve backup air and prevent back-siphoning.
Rated output at the installed depth and diffuser resistance; accessible replaceable parts. Power it through ground-fault protection (GFCI/RCD) and keep the pump and its plug above the water line.
Check size, materials and operating conditions with your chosen supplier before ordering.Evidence beside the lesson.
Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.
Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.
- University of Georgia Cooperative ExtensionGreenhouses: Heating, Ventilation, and Cooling (Bulletin 792) ↗
Ventilation replacing warm, moist air with drier air and high humidity causing condensation and increased disease; one air exchange per minute without evaporative cooling keeping the house about 8°F above outdoor temperature, half that volume about 15°F and two exchanges about 5°F; selecting fans to operate against 1/8 inch static pressure because unrated fans usually move only 60–70% of rated airflow when installed; fans activated by thermostat or humidistat; one fan with a two-speed motor in pad-cooled houses with three fans or fewer to prevent excessive temperature fluctuation and fan cycling.
- UMass Extension Greenhouse Crops and Floriculture ProgramHorizontal Air Flow is Best for Greenhouse Air Circulation (fact sheet, J. W. Bartok, Jr., 2005) ↗
Air movement of 50–100 feet per minute; no more than 2°F difference in a properly installed system; moving air removing moisture from the canopy and reducing foliar disease; total circulation fan capacity of about two times the floor area with the 30 × 100 ft example; 1/10 to 1/15 horsepower circulation fans built for little resistance as distinct from exhaust fans.
- UMass Extension Greenhouse Crops and Floriculture ProgramReducing Humidity in the Greenhouse (fact sheet, J. W. Bartok, Jr., 2003) ↗
Condensation on plants when leaf surface temperature is below dew point; droplets promoting germination of fungal spores such as Botrytis and powdery mildew; adequate air movement when leaves move slightly; ventilation combined with heating, cycled two or three times per hour after sundown and at sunrise; a relay to keep the heater and fans from running together.
- University of Minnesota ExtensionSmall-scale hydroponics ↗
Root support, transplanting, light, aeration and cleaning.
- University of Missouri ExtensionHydroponic Nutrient Solutions (G6984) ↗
Solution preparation, elemental nutrition and measurement.
- University of Florida IFAS ExtensionFans for Greenhouses (AE12/AE020) ↗
Fan rating table in which Fan 1 delivers 10,200 CFM in free air, 9,200 CFM at 1/8 inch and 4,300 CFM at 3/8 inch static pressure; the point of operation as the intersection of fan and system curves; most greenhouse static pressures no higher than 0.10–0.15 inches of water; AMCA Standard 210 testing; choosing a drive that allows speed change because system pressure is never exactly known; fan noise as a function of design, flow, pressure and efficiency, turbulence as its primary source, the most efficient fan being the quietest, low outlet velocity not assuring quiet operation, and comparison only at the required flow and specified static pressure.
- AC Infinity · manufacturerCLOUDLINE S6, Quiet Inline Fan 6" with Speed Controller (product page) ↗
Manufacturer specification accessed 17 September 2026: 6-inch duct size, total airflow 425 CFM, total noise 30 dBA, static pressure 503 Pa, maximum power draw 70 W, average power usage 38 W and a wired ten-speed controller, with no measurement pressure stated for the airflow figure; a product example, not an endorsement.
The next useful connections.
Light, temperature and humidity
Measure the growing environment at the canopy and root zone, then adjust one limiting condition at a time.
Interactive tool · 6 min + guided practice →Build, operate & troubleshootCommissioning and daily care
Turn assembled equipment into a repeatable operating system, with a first-crop checklist and a useful log.
6 min + guided practice →Build, operate & troubleshootOperating costs and sensible scaling
Build a complete cost ledger, measure energy and labor, and expand from usable production evidence.
Interactive tool · 6 min + guided practice →ComparisonsReady-made kits vs DIY hydroponics: sites, light, pods and repairs
A boxed countertop garden and a drilled bucket meet the same needs; they differ in who chooses the light, the consumables, the meter and the spares.
8 min + guided practice →Buying guidesChoosing grow lights for leafy greens: buy the DLI, not the watts
A grow light is bought against the daily light integral your greens need at canopy height; photon flux and efficacy get you there, watts do not.
Interactive tool · 8 min + guided practice →