The most efficient ventilation system would only operate on demand, when ventilation was "needed". Running the ventilation system all the time at a low flow rate, a rate sufficient to match the ASHRAE recommended 0.35 ACH is a crude control approach to an optimum system. Conditions in a house are not constant. On average, a constant flow rate ventilation system will work reasonably well and certainly is simple, but there are more energy efficient approaches, and users tend to shut down systems that run constantly.
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