Focusing on supply ductwork to address insufficient HVAC airflow is common. However, the issue often lies in inadequate return air paths (especially lacking return vents, typical in older bathrooms).
A simple experiment illustrates this:
- Blow through a straw with your finger covering the other end – not much air flows, right?
- Keep your finger covering the straw and blow harder (like adding a booster fan) – still not much airflow, right?
- Blow through a larger straw (increasing supply duct size) with the end still covered – does airflow improve much?
- Now, use the original straw and slightly uncover the end (increasing exhaust/“return air path”) – you’ll notice significantly increased airflow. Note that increasing straw size (supply duct) or blowing harder (booster fan) will further increase airflow.
If your bathroom lacks a return vent to exhaust air, that’s the best starting point for improved HVAC efficiency. Otherwise, trying to reduce ductwork resistance (increase supply/return duct size) is the next best change, passively reducing restriction to increase airflow. Lastly, if those aren’t feasible, a Register Vent Booster Fan might be worth trying to increase airflow, but may only marginally improve it (depending on fan quality and system limitations), requires electricity, and may still be less effective than other solutions.
