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Running a positive pressure system in my house since many years, I agree that sucking in fresh air from outside and filter it with a HEPA filter makes more sense.

You probably need to make sure you use a fan that creates enough pressure to get the air through the trickle vent as well as a filter. A duct fan might be more suitable than a PC fan for this.

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The pressure drop across a HEPA filter is a function of flow rate per unit area (i.e. velocity), and it can be very very low if the velocity is low. You only need some 15 cfm per person, which isn't very much. The real issue that that a HEPA filter that will achieve extremely low pressure drop is rather bulky.

Thank you both! I'll start with a small research into available HEPA filter sizes, pressure drops they entail, and noise vs power ratio of available fans. Then I'll see if any combination fits my trickle vent setup and theoretically gives enough air as an output.

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