When selecting a squirrel cage fan for any HVAC or industrial ventilation system, one of the most important decisions you will face is choosing between a single inlet and a double inlet design. As an HVAC professional, I often see performance issues, airflow imbalance, or unnecessary power consumption simply because the inlet type was not matched correctly to the application. Both designs are reliable and widely used, but they serve different purposes depending on airflow demand, space constraints, and system layout. Understanding how each one works in real-world conditions will help you make a smarter, long-term choice rather than relying only on airflow numbers in a catalog.
Understanding How Squirrel Cage Fans Work
A squirrel cage fan, also known as a centrifugal blower, pulls air into the center of a rotating wheel and throws it outward through centrifugal force. The air then exits through the fan housing and into the duct or open space. The inlet configuration determines how air enters the wheel, how evenly it is distributed across the blades, and how efficiently the fan can deliver airflow against resistance. This is why inlet design directly affects noise, power usage, airflow stability, and overall system life.
What Is a Single Inlet Squirrel Cage Fan
A single inlet squirrel cage fan draws air from one side of the impeller only. The other side is typically closed or mounted directly to a motor or housing. From an installation and maintenance perspective, this is the simpler of the two designs and is commonly found in compact HVAC units, small exhaust systems, and equipment where airflow demand is moderate. Because air enters from one side, the fan wheel experiences a slight airflow imbalance, which is already accounted for in proper design and balancing.
Single inlet fans are best suited when space is limited, ducting is available only on one side, or the airflow requirement does not justify a larger blower. In many residential and light commercial HVAC applications, a well-sized single inlet fan delivers stable airflow with lower upfront cost and easier servicing.
What Is a Double Inlet Squirrel Cage Fan
A double inlet squirrel cage fan allows air to enter the impeller from both sides simultaneously. This design feeds the fan wheel more evenly, allowing it to move a higher volume of air at the same rotational speed compared to a single inlet fan. In HVAC practice, double inlet fans are commonly used in air handling units, large ventilation systems, industrial process ventilation, and applications where uniform airflow and higher capacity are essential.
Because air enters from both sides, the impeller operates more symmetrically, which often results in smoother airflow distribution and reduced vibration when properly installed. However, the overall unit is larger, heavier, and usually requires more careful alignment and structural support.
Key Performance Differences Between Single and Double Inlet Fans
The most noticeable difference is airflow capacity. For the same wheel diameter and speed, a double inlet fan can move significantly more air because it draws from both sides. This makes it ideal for high-volume, low-to-medium pressure systems. Single inlet fans, on the other hand, are more efficient for lower airflow demands and tighter installations.
From an HVAC efficiency standpoint, single inlet fans often consume less power for smaller airflow requirements, while double inlet fans become more energy-efficient as airflow demand increases. Noise characteristics also differ slightly, with double inlet fans generally producing smoother airflow noise when operating within their design range, though enclosure and duct design play a major role in overall sound levels.
Installation and Space Considerations
Installation conditions often dictate the final choice more than performance data. A single inlet squirrel cage fan is easier to mount, align, and connect to ducting, especially in retrofits or compact systems. Maintenance access is also simpler since there is only one inlet side to manage.
Double inlet fans require sufficient clearance on both sides for proper airflow. Blocking or restricting one side defeats the purpose of the design and can lead to turbulence, noise, and premature bearing wear. When planning a double inlet installation, always ensure symmetrical airflow paths and solid mounting to avoid vibration issues.
Application-Based Selection Guidance
If you are designing or upgrading a small HVAC system, spot exhaust, appliance ventilation, or localized cooling setup, a single inlet squirrel cage fan is usually the practical and cost-effective solution. It offers dependable performance without unnecessary complexity.
For large air handling units, centralized ventilation, industrial exhaust, or systems requiring high airflow at consistent pressure, a double inlet squirrel cage fan is the better choice. It handles volume more efficiently and maintains airflow stability across larger duct networks.
Practical Steps to Choose the Right Inlet Type
First, calculate the required airflow and static pressure of your system based on duct length, bends, filters, and operating environment. Second, evaluate the available installation space and airflow access around the fan. Third, match the fan type to the system size, keeping future expansion or load changes in mind. Finally, consult performance curves rather than relying solely on size or motor power, as inlet design directly influences real-world efficiency.
Final HVAC Expert Insight
There is no universally better option between single inlet and double inlet squirrel cage fans. The right choice depends entirely on application needs, space constraints, and airflow requirements. In my experience, systems perform best when the inlet design is chosen deliberately rather than as an afterthought. When matched correctly, both fan types deliver reliable airflow, long service life, and stable HVAC performance, which is exactly what any well-designed ventilation system should achieve.