I'm thinking about a very energy efficient way of controlling temperature inside a greenhouse when it's too hot.

The goal is to use the syphon effect in order to draw hot air from the top of the greenhouse to the outside, perhaps using a cooler fan as a trigger for this air motion. The supposed advantage of having that fan is to be able to invert rotation and block/revert the airflow (if temperatures are still within working range, of course)

Would that work at all, similar to how water works? Does the syphon on the outside has to be higher than the inside leg to increase draft? As in when it's full of rising warm air it will create negative pressure on the inside leg and thus suck more hot air which will continue this cycle?

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*edit I was thinking about using a 200mm syphon pipe or wider if necessary; the fan location is illustrative;

The pipe goes down to the ground so existing structure does not need any modifications.

How much flow I need in order to actually cool the greenhouse significantly?

  • 1
    $\begingroup$ Can you not make an opening in or near the roof? Why do you need the plenum to penetrate the wall down near the ground? (P.S., 100mm seems awfully small.) $\endgroup$ Apr 10, 2022 at 16:55
  • $\begingroup$ Down to the ground so the existing structure can be preserved. $\endgroup$
    – enapupe
    Apr 10, 2022 at 22:20

1 Answer 1


This might work, but there's another way that does not need a fan- called the chimney effect, which will work for a pipe of diameter ~at least 4 to 6 inches.

Imagine a tall black pipe standing vertically in the sun, open at both ends. The sun's rays make the air inside get hotter than the air outside and the inside air becomes buoyant, and starts to rise upwards. As it does, it draws in air from the bottom of the pipe, which then gets heated by the walls of the hot pipe, becomes buoyant, rises, etc., etc.

Note that for any such scheme to work, you need a farily tall chimney, and the chimney diameter has to be no smaller than about 3 inches so that air friction does dominate the dynamics.

  • $\begingroup$ Thanks, I should have been more clear about this: I don't want this effect to kick in at any time, but rather when the inside temp is above N. That's why the fan is sort of important so it kicks in when a thermometer tells it's too hot. $\endgroup$
    – enapupe
    Apr 10, 2022 at 22:22
  • $\begingroup$ @enapupe, in your original question, you spoke of the fan being needed to start the air flow, but then you seemed to expect that the air would continue to flow even without help from the fan. So if you want automatic means to start the air flow, does that also mean you want it to autmatically stop when a low temperature threshold is reached? If you Google for "hvac damper," I think you might find some useful ideas. $\endgroup$ Apr 11, 2022 at 0:21
  • $\begingroup$ Indeed I was assuming the fan would be helpful but not the main driver of this motion after it started to happen. I also thought it could perhaps reverse its rotation and break the air flow once a given temperature is reached. Do you think this is reasonable? $\endgroup$
    – enapupe
    Apr 11, 2022 at 12:59
  • $\begingroup$ Maybe with that black chimney idea and a HVAC damper at the top I could induce the air flow to kick in and when given temp is reached its then just a matter of closing the damper again. But I'm back at the core question: would that syphon effect kick in? $\endgroup$
    – enapupe
    Apr 11, 2022 at 13:29

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