VFD energy benefits shall acrue only if 2-way valve are provided. Otherwise, if VFD is not considered any of 2-way or 3-way valve may be used. One problem noticed with 2-way valve is balancing of system particularly in large network few circuits may get very little or scarce water.
You can learn...
No there is no rule of thumb for air exchanges. Ideally in HVAC, the ventilation rate shall be determined by the number of persons as recommended by ASHRAE 62.
The air exchange method is not an energy efficient method and is recommended for hazardous areas for dilution of indoor contaminants...