How do automatic greenhouse window openers work?

Greenhouses are fantastic for giving your plants some extra warmth in spring and autumn, but in the summer they can be a cookhouse. Without ventilation, temperatures can reach over 50 degrees centigrade, which is more than enough to damage your plants. Along with greenhouse shading, some kind of ventilation is essential. Opening the door is the first thing to do in the morning during the summer, but that doesn’t allow heat in the roof to escape and if you only have one door, there’s no through-breeze; you need a window open too … but what if you’re away? Or you just forget?

Thankfully, there’s a simple and reliable answer. Automatic greenhouse window openers are a tried and tested way to make sure that your greenhouse windows open when it’s hot, and stay firmly shut when it’s not.

Automatic window opener as fitted to a Harvst mini greenhouse.

So how do automatic greenhouse window openers work?

There are various models, but they all operate on the same principles. A cylinder is filled with mineral wax, which expands and contracts depending on the heat around it. As the wax expands, it pushes the piston open. This works with a metal scissor mechanism to multiply the range of movement, and open the window.

Unlike water, which expands when it cools into ice, waxes can expand when they melt – typically expanding in volume by 5-20%. This principle has been used for nearly a hundred years in wax thermostatic elements, starting in early cooling systems and combustion engines. An automatic greenhouse window opener cylinder is essentially a wax motor.

When the wax cools, it contracts again. The weight of the window combined with springs in the mechanism pull the piston back in, and the window closes. No electricity is needed.

The temperature at which the greenhouse window opener starts to open can be adjusted by screwing the piston in and out. Typically the range is 18 to 25 degrees.

There are three main designs of automatic greenhouse window opener for domestic greenhouses;

Automatic greenhouse window opener

The lightweight single spring greenhouse window opener is usually fitted to windows in the roof of a greenhouse. It needs space below and in front of the window for the piston and mechanism to swing.

Univent automatic greenhouse window opener

The double spring greenhouse window opener has a slightly different scissor design, and is suitable for mounting in a cold frame or on the very edge of a greenhouse, or for side windows, where the single spring design would foul the vertical wall.

Megavent greenhouse window opener

Thirdly there is a larger, more powerful single spring model which can be used either in a roof window or a side windowm and can lift a heavier weight.

Harvst mini greenhouses are fitted with automatic window openers – especially important in a small greenhouse where the temperature can rise quickly. They are also made from horticultural twinwall polycarbonate, which provides a good level of shading all year round, preventing plants from being scorched in the hot sun.

Automatic greenhouse window openers are available from a number of suppliers;

Replacement window opener wax cylinders are available for all models. They last a long time, but are not indestructible and sometimes do need replacing.

Automatic window openers also need regular maintenance to keep them running smoothly year after year. Have a look a this blog post to read our advice.