Greenhouse fans: circulation vs extraction

Choose the job before the fan

If leaves in a crowded corner stay wet after the rest of the greenhouse has dried, an internal circulation fan may help. If the whole greenhouse becomes hot and humid, air must leave and be replaced. These are different jobs, and a product labelled “greenhouse fan” may do only one of them. Start by checking the existing roof and low-level vents in the ventilation guide.

A small clip fan can smooth out still pockets around plants and reduce a sharp hot or cold spot. It should produce a gentle movement of leaves, not a harsh wind on a single tray. Reposition it as the canopy fills the space. A clip must grip a sound structural member rather than brittle glazing, and its cable must not cross a wet path, sharp edge or moving vent.

Why circulation cannot replace extraction

A circulation fan does not add fresh air or remove heat from a closed greenhouse. In fact, its motor adds a little heat. If roof vents and an inlet are blocked, the same warm, moisture-laden air is simply moved around. The humidity guide explains why this distinction matters when condensation persists.

An extraction fan is installed to expel air outdoors. It needs a suitably sized replacement-air opening, protection from rain, insect screening that does not choke flow, secure mounting and control suitable for the crop. A larger fan is not automatically better: an undersized inlet can limit its real performance, while excessive draught can stress plants. In a small domestic greenhouse, improving passive roof-and-low ventilation may be simpler than cutting the structure for an extractor.

Electrical safety is part of the design

Greenhouses are damp and can contain hoses, condensation and splashes. A maker's IP marking describes a particular tested level of ingress protection; it does not make a fan universally safe in every wet position. Keep plugs and controls away from water, follow the specific manufacturer's instructions, and do not improvise an extension lead under a wet bench. A permanent supply or extractor installation should be designed by a competent electrician with appropriate outdoor protection. Electrical Safety First's garden guidance explains why RCD protection and suitable outdoor equipment matter.

The AC Infinity fan below is offered as a circulation example only. Its maker lists ten speeds and IP54, but the proposed mounting and supply still need checking for a greenhouse. Do not treat a grow-tent product listing as an approval of a particular greenhouse installation.

Test the result

Observe the crop and record maximum temperature, morning condensation and whether leaves dry by midday. With a circulation fan, check the previously still corners; with an extractor, confirm that outside air enters at the intended inlet rather than pulling against a closed structure. Clean guards and blades according to the maker's instructions with power isolated. If the problem is primarily bright-day overheating, use the overheating guide to address ventilation and shade before adding powered equipment.