Focusing on supply ductwork to address insufficient HVAC airflow is common. However, the issue often lies in insufficient return air (especially lacking a return air vent, common in older home bathrooms).
A simple experiment illustrates this:
- Blow through a straw and cover the other end with your finger – not much air flows through, right?
- Still covering the straw, blow harder (like with a booster fan) – still not much airflow.
- Use a larger straw (increase supply ductwork) with the end still covered – does airflow improve much?
- Now, go back to the original straw and slightly uncover the end (increase exhaust/“return air”) – much more airflow now. Notice that increasing straw size (supply ductwork) or blowing harder (booster fan) will now further increase airflow.
If the bathroom lacks a return air vent to pull air out, that’s the best starting point to improve HVAC effectiveness. Otherwise, trying to reduce ductwork resistance (increase supply/return duct size) is the next best change as it passively reduces duct resistance to increase airflow through the room. Lastly, if neither option is feasible, then using a booster fan may be worth trying to increase airflow through the room, but it may only marginally increase airflow (depending on booster fan quality, other system restrictions), will require additional electricity to operate, and may still be less effective than other solutions.
