I have noticed that all manufactures of equipment (Trane, Carrier, York) have no power exhaust on side discharge units. I have a 3 unit heat pump system with 15 tons each (2 units will be running at a time, 1 backup). The system max cfm is 12000 cfm (6000cfm/unit), existing exhaust fan for bathrooms, closets, etc is 2000 cfm. If one Heat pump is running i sized one fan for 4000 cfm (6000cfm -2000cfm), and another at 10,000 cfm (12,000cfm-2000). Should I size exhaust fans smaller and let a slight build up in pressure? Is there a rule of thumb for power exhaust units? I noticed that many manufactures use about 1/3 to 1/2 inlet amount to size power exhausts. i.e. 8000 cfm inlet, 4000 cfm power exhaust. Are there any exhaust fans that have 2 speed, that can be controlled by control unit without VAV drive?
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