Originally Posted by 235
1) ..........Should my exhaust fan dump directly into the main HVAC return or rather into an adjacent room? If a 200 CFM fan is blowing into the main HVAC return which already has lower pressure because main fan is ON then does it effectively become more than 200 CFM overall (logarithmic math maybe)?........
First, will your HVAC always be on when your exhaust fan is running? If not, this may not be the best configuration as the fan will be pressurizing the line to the nearest registers in order to exhaust your room. Depending on the size and length of the those lines you may not get the full 200 cfm your fan is rated for. You also need to consider which rooms it will be exhausting into under those circumstances.
If your HVAC will be on, then the total flow will depend on the system. You will probably get some increase above the 200 cfm, but it will likely not be additive. As an example, if your HVAC would normally pull 300 cfm from that line, you will probably not get 500 cfm once you add the inline fan. At some point, the fan will become a restriction in the line rather than boosting your flow. IIRC, that fan can only create a certain pressure differential, and once that differential is exceeded, the fan can't do any more to improve the flow. If the HVAC establishes that pressure differential while running (remember that fan is a restriction in the line), then turning on the fan will not produce any additional flow.
Someone may be able to give you a definitive answer based on your duct sizes, etc. But suffice it to say, I'm subscribed and felt the need to post something