Greenhouse Exhaust Fan

Overview

A greenhouse traps solar radiation by design, and on a clear day interior temperature can climb 15–20 K above ambient within an hour of the sun breaking through. The exhaust fan is the active half of the standard ventilation answer: mounted in one gable wall, it pushes air out, and the resulting slight vacuum draws fresh air in through motorised or gravity intake louvres in the opposite wall. One complete air exchange per minute is the usual sizing rule for summer cooling, so a 200 m³ hobby greenhouse needs roughly the 21,000 m³/h this 90 cm fan delivers.

The machine itself is simple and long-established: a six-blade Propeller Assembly in a Fan Housing with a venturi orifice, driven by an induction Drive Motor through a Belt Drive, cycled by a Thermostat, with a Backdraft Shutter sealing the opening whenever the fan rests.

How it works

The propeller is an axial fan optimised for high volume at low static pressure, a greenhouse presents only the 10–30 Pa drop of its intake louvres, nothing like the duct resistance a centrifugal blower is built for. Six stamped aluminium Fan Blades are clamped at fixed pitch in a cast Blade Hub, balanced as an assembly, and locked to the shaft by a Taper Bushing.

Blade-tip clearance dominates propeller efficiency, which is why the housing's Venturi Ring matters: the spun inlet ring closes the gap between blade tips and orifice to about 1% of diameter. Without it, high-pressure air from the blade face simply leaks back around the tips, costing 5–10% of flow.

The Belt Drive exists because large propellers want to turn slowly. A 90 cm blade at the motor's 1,425 rpm would have supersonic-adjacent tip noise and absurd power draw; flow scales linearly with rpm but power with rpm cubed, so the 2.5:1 reduction through the Motor Pulley and heavy Driven Pulley gets the propeller to an efficient, quiet 580 rpm. The propeller shaft rides in two greasable Bearing Blocks on the Cross Frame, and the Belt Tensioner base takes up belt stretch, the Drive Belt is the principal service item, typically replaced every one to two seasons.

The motor is a permanent-split-capacitor single-phase induction machine: a Run Capacitor phase-shifts current in the auxiliary winding of the Stator Assembly to create the rotating field that drags the squirrel-cage Rotor Assembly around. The TEAO (totally enclosed, air-over) Motor Housing is specified because greenhouse air is the worst small-motor environment short of a washdown plant: near 100% humidity, condensation cycles and sulphur sprays. Sealed Ball Bearings and a Thermal Fuse in the winding finish the protection.

Shutter and controls

The Backdraft Shutter is a one-way air valve with no actuator. When the fan starts, about 25 Pa of pressure swings the five light Shutter Vanes open against gravity; when it stops, they fall closed onto their Felt Seals. The Tie Bar forces all vanes to move together, preventing the flutter that would otherwise hammer the Pivot Bushings. A closed shutter matters most in winter: an unsealed 1 m² wall opening would dump heat all night, so vane fit and felt condition directly affect the heating bill.

Control is a bang-bang loop. The capillary-bulb Thermostat hangs at crop height, not at the gable, where stratified hot air would read several degrees high, and switches the motor through a Relay contactor in the sealed Junction Box. Its ~1.5 K differential sets the cycling rate; tighter control wastes motor starts, looser control stresses the crop. The Speed Switch reconnects the two-speed winding for mild days when full flow would overcool, and in larger houses several fans are staged at successive set-points instead.

Sizing and practice

Greenhouse fan ratings follow AMCA-standardised airflow tests, quoted at free air and at 25 Pa static; the 25 Pa figure is the honest one since intake louvres always impose back-pressure. Installers size intakes at 1.25–1.5 times the fan opening area so the limiting orifice is never the inlet. In summer the same fan drives evaporative pad cooling, outside air pulled through wet cellulose pads on the intake wall drops several kelvin below ambient before crossing the crop. Routine maintenance is short: grease the bearing blocks each season, re-tension or replace the belt, wash dust off the blades (a dirty propeller loses measurable flow), and check the shutter falls fully closed.