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controlled ventilation. (not OP, just speaking in generic terms)

DIY ventilation with co2 monitor, some motors to control the incoming air. The amount of air needed to recycle the air is so small it doesn't make a dent on the temperature.

The typical solution is to use a heat recovery ventilator (HRV) or a energy recover ventilator (ERV). They exhaust old air outside and bring fresh air in, but they also attempt to transfer the heat of the outgoing air to the incoming air.