The Top 3 Reasons You Shouldn’t Oversize Inverter-Based Equipment

1. Airflow Requirements

It is the job of HVAC professionals to add or remove heat (Btu) from a space. Air is the vehicle that transports those Btu, and it must be correct from the start. Otherwise, the equipment won’t operate at its rated capacity.

In 2018, the Department of Energy (DOE) reviewed a study titled “Residential HVAC Installation Practices: A Review of Research Findings.” The study found that fewer than one-in-three residential systems move enough airflow. In other words, you could say that most systems have inadequate fan airflow.

Static pressure and fan airflow go hand in hand. You need to consider both to understand the full picture. National Comfort Institute (NCI) analyzed static pressure tests from contractors across North America and found that the average HVAC system operates at a Total External Static Pressure (TESP) of 0.82 inches of water column (in. w.c.). Most residential fans are rated at 0.50 in. w.c., indicating that most equipment is running at excessive TESP.

Increasing equipment size increases the required fan airflow. If most systems are operating at high static pressure and low fan airflow, what do you think happens when you install a larger unit? If you said the problem gets worse, you would be correct. Static pressure moves at the square of airflow. As you increase fan airflow, the static pressure can go through the roof.

Problems caused by high static pressure and low fan airflow include:

• Reduced efficiency — high blower watt draw;
• Blower motor faults/failure;
• Heat exchanger failure;
• Compressor failure; and
• Frozen evaporator coils.

A common method to counteract this is to lower the fan speed setting. The industry standard is 400 cfm (cubic feet per minute) per ton, and the solution is to commission the air-moving equipment at 350 cfm per ton.

Keep in mind, by doing so, the equipment will have a lower sensible heat factor (SHF). A lower SHF means less sensible capacity and lower efficiency. If you commission a system to move less Btu by lowering the fan speed, what is the point of installing the larger system to begin with?