Duct Sizing & Physics
Sizing ductwork is not something that can be determined with a rule of thumb or educated guesswork. Duct sizing boils down to science – specifically, physics – and despite the inclination of some HVAC contractor to fit a tight budget by undersizing ductwork, physics is indifferent to money. An cooling system with undersized ductwork simply won’t perform properly.
A contractor at a recent seminar shared an anecdote about receiving a call from an angry homeowner. A different HVAC contractor had recently replaced two old 1.5-ton A/C units with two new 3-ton units. In theory, the airflow on each unit would go from approximately 600 cfm to 1,200 cfm, but no changes were made to the existing ductwork. Not surprisingly, the expensive new system was severely underperforming, and when complaints to the installing contractor fell on deaf ears, she reached out to a different HVAC company for a second opinion.
The new contractor immediately understood that the problem was due to an undersized duct system. Static pressure readings on supply and return ductwork were both extremely high; airflow was a fraction of what it should have been, and the fan was basically starved of air. In fact, the ductwork was found to be too small for the original 1.5-ton systems, never mind the new 3-ton systems. The homeowner had been led to believe that a new unit would solve all of her cooling problems – sadly, a non-uncommon occurrence when it comes to unscrupulous and/or uneducated contractors. The typical home or business owner can’t be expected to know that bigger does not automatically mean better when it comes to HVAC.
Proper airflow and air distribution provide correct heat exchange rates across evaporator and heating coils, distributing the correct airflow to all areas of the home for the correct air changes per hour. They allow for proper humidification and dehumidification, along with filtration. All HVAC technicians should understand how to use a ductilator or computer software to size metal and flex duct for proper airflow. This knowledge can also aid in identifying problems during service calls. If static pressure is too high, indicating low airflow, installing new equipment without addressing undersized ductwork is never the answer.
In the case of the irate customer, the second contractor called in changed out the majority of the ductwork throughout the home, and some excessive-length flex duct was replaced with metal duct. The duct system was brought into equilibrium with the new units, and in the end the customer was finally satisfied with the performance of the HVAC system.
