Collection: Window opener

The Problem an Opener Solves

On a clear day the temperature inside a greenhouse can rise ten to fifteen degrees above the air outside within minutes of the sun coming round. That happens whether or not anyone is there, and it is the single most common cause of losses under glass.

Opening vents by hand works if you are present. Most people are not, and the moment that matters is usually mid morning on a weekday. Timers and forecasts are no substitute, because what governs the house is the sun on the glass, not the hour or the prediction.

How It Works

An automatic greenhouse window opener uses a sealed cylinder filled with wax. As the air warms, the wax expands and drives a piston that pushes the vent open. As the air cools, it contracts and the vent closes under the weight of the frame.

There is no power, no wiring, no battery and no controller. The mechanism responds directly to the temperature around it, which means it works on an allotment plot with no supply as readily as in a garden.

Setting the Opening Temperature

Most models are adjustable, usually by turning the cylinder against a threaded stem to change how far it has to expand before the vent starts to move. Typical settings begin opening somewhere between fifteen and twenty degrees.

• Set it lower for seedlings and leafy crops, which suffer sooner.
• Set it higher for tomatoes, peppers and chillies, which tolerate more.
• Check the setting each spring, since the wax response shifts slightly with age.

A maximum and minimum thermometer tells you whether the setting is right. If the recorded peak is still too high with the opener fitted, either the setting is too high or one vent is not enough.

How Many You Need

One unit serves one vent panel. A small frame may manage with a single roof vent; a house above ten square metres will usually need two or more, plus louvres at low level so cool air can be drawn in.

Opening a vent at the top without an inlet at the bottom moves far less air than people expect. The ventilation range covers louvres and additional vents.

Fitting and Upkeep

Most openers fit standard roof vents on aluminium frames and can be retrofitted to an existing house. Check the frame profile and the weight of the vent, since a heavier glazed vent needs a stronger cylinder.

Maintenance amounts to keeping the arm clear and replacing the wax cylinder when it stops responding, typically after several years. Replacement cylinders are stocked in the spare parts range.