Venting a Cold Frame Automatically


Normally, you open the window covering of a cold frame yourself on hot days to provide ventilation and prevent overheating. But what if you aren’t home to do it? Photo:

Question: I’m looking into building a cold frame so I can get a head start on certain vegetable crops. The problem is that I have very irregular hours. So, I’m simply not around to lift the cover to let fresh air in on hot days and close it on cold ones. Is there another way to avoid overheating a cold frame*?

Patrick Pelletier

*A cold frame is a bottomless box with a glass top mostly used to grow and harden off seedlings.

Answer: Sure! Just add a heat-activated window opener (also called a greenhouse vent opener). It’s really quite simple. 

Heat-activated window opener. Photo: Lee Valley Tools

It includes an adjustable gas-filled cylinder. When the temperature rises, the gas expands, causing it to lift the sash (the model I use can handle up to 7.5 kg/15 lb) and allow fresh air in. If the temperature is still too great, you can adjust it to open more widely by twisting the knob at the extremity. No electricity, wiring or pipes are involved.

I’ve used this device on my own cold frame for 16 years now. 

This is a good day to show how it works, as it’s cold outside (-2˚C/28˚F), yet sunny, and that heats the cold frame a great deal. I have a digital thermometer in my office linked to a remote sensor in the cold frame so I can keep track of the temperature inside. When the air heats up to around 22˚C/73˚F, the device opens the window just a crack and the temperature drops to about 20.6˚C/69˚F, then it closes. The temperature starts to rise again, so it opens again. This opening and shutting movement has been going on like this all day long. On a warmer day, it opens wider and stays open as long as necessary, even all day. 

Important Detail

This is about as much at a ventilation-controlled window sash will open (and it only open this much on a really hot day), making access to plants difficult. Photo: Lee Valley Tools

What nobody tells you when you buy such a device is that, once it is installed on a window, you can no longer open it fully. This is an irrelevant detail if you use the window opener in a greenhouse setting, as openable greenhouse windows are usually just used for ventilation. A cold frame window, though, is also your only access to the cold frame. You may need to lift it fully to reach the plants inside for watering, thinning, pinching, etc. The result is that, in planning your cold frame, you’d be wise to include two window sashes: one operated by the opener and strictly used for ventilation and a second one you can open completely to reach your plants.

Here you can see my own cold frame coming out of the snow in April. Panels 1 and 3 have a window-opener and open only for ventilation. Panels 2 and 4 have no such constraint and can open fully, offering easy access. Photo:

That’s what I’ve done. I have a long cold frame with four glass panels. Two are fixed to window openers and serve strictly for ventilation purposes (and to let the sun in, obviously) and other two I can open completely to reach the plants. 

Note that this type of ventilation arm is only useful in spring and fall, when the temperature fluctuates a lot. In the summer, when nights stay above 12 ° C/54˚F, I leave the two unfixed windows open permanently to prevent overheating. Besides, I don’t have much growing in the cold frame in the summer. In winter, at least in my climate, the cold frame is entirely covered in snow, so it’s never warm enough for the arm to kick in and, at any rate, temperatures remain below freezing both outside the cold frame and inside almost all the time.

Where to Find a Heat-Activated Window-Opener

I bought my two greenhouse heat-activated window openers from Lee Valley Tools, a company that serves Canada and the USA, but any greenhouse supplier probably carries a similar product. In the United Kingdom, you could try Bayliss Autovents. In Australia, Sproutwell Greenhouses carries it.

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