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No reason a ventilation fan can't be installed - that's standard on modern air-conditioned and insulated houses (a "whole-house fan") precisely in order to rotate the air.

Yes, but ventilating the house itself plus the surrounding greenhouse is much more expensive than just ventilating the house itself. Reason being that the volume of air is much larger.

I'm not an expert, but all greenhouses I've personally seen have natural ventilation with vents high up (such as at the peak of the roof) and near the ground.

The greenhouse effect makes the air inside a lot warmer than outside. This reduces its density, and it becomes buoyant and spills out the roof vent.

There may also be fans, but you can get some ventilation for free (energy-wise).

You can see the open vents on the ridge in the first photo.

That might help, but it's not quite the same as a nice summer breeze flowing through your house.

It literally is a summer breeze flowing through your house.

In the summer, the greenhouse opens up.

Then they will lose all the advantages of the green house if they get cold air from outside...

Balanced ventilation (not sure what the English term is) is standard these days and recovers about 90% of the energy (heats air going in using the air going out). I have one at home.

Dont know if they have one set up, but they could. But possibly the sun already warms it enough even in winter that it is not needed even with natural ventilation.

> Balanced ventilation (not sure what the English term is)

Heat exchanger: https://en.wikipedia.org/wiki/Heat_exchanger#HVAC_air_coils

That may be. The word we use is more a property of the ventilation system ... that you blow as much air into the house as out of it, active airflow in both directions. (As opposed to older ventilation where only exhaust has a fan.) This facilitates plugging in a heat exchanger.

To be clear I did not mean HVAC, which requires (some) energy in itself.

At home I do not have HVAC. However, the outbound air passes through a rotating metal structure that sucks up the heat from outbound air and releases it into the colder inbound air. Just using the temperature gradient that is there naturally. Passive energy transfer.

Same principle can be used with hot water (e.g. from showers).

Then one can put HVAC on top of this for active warming.

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