HVAC airflow problems can make or break a duct system design. After all, the cornerstone of the heating and cooling industry is comfort. Without proper HVAC airflow, comfort goes out the window — pun intended. The end result is a building owner left with a duct system absent its purpose.
All the more reason for HVAC contractors to focus on common HVAC airflow problems and their solutions. Improper HVAC airflow can be costly for contractors, manufacturers and wholesalers in the form of replacements and service calls.
In a 2015 RSES Journal article, republished in SNIPS magazine in 2018, Chris Van Rite foretells, “If the HVAC industry does not correct its airflow problem, one predictable outcome is that we will see regulations address it through building codes and local ordinances.” He adds, “Another long-term outcome could be an increase in the use of ductless mini-splits.”
For that reason, it’s best for everyone in the HVAC industry if contractors get HVAC airflow right the first time. Here’s how.
The Basics of Duct System Airflow
HVAC unit’s evaporator coil. The evaporator coil is what absorbs the heat from the conditioned space.
The general rule of thumb for the HVACR industry is 400 cubic feet per minute (cfm) per ton of air across the evaporator coil; for a heat pump, 500 cfm, and 350 cfm for cooling high latent heat applications, the result from an increase or decrease in the amount of moisture held by the air.
Therefore, a 3-ton residential cooling HVAC system should have 1,200 cfm of air across its evaporator coil. A 5-ton system should have have 2,000 cfm, a 6-ton system should have 2,400 cfm, and so on and son.
Measuring how much airflow measurements deviate from that math is a quick way to diagnose airflow issues in a residential duct system.
Common HVAC Airflow Problems
Think of the evaporator coil as the Goldilocks of HVAC airflow. The air that flows across the coil has to be just right. Otherwise the duct system won’t function properly. Occupants will lose comfort, and the building will lose efficiency.
Too much air across the evaporator coil will cause low sub-cooling in the HVAC system. The resulting high compressor discharge temps and will increase refrigerant temps, putting more pressure on the condenser to cool the refrigerant, resulting in lower sub-cooled refrigerant.
The obvious result is that lower sub-cooled refrigerant means increased humidity in the occupant space.
On the other hand, too little air across the evaporator coil will cause low suction pressure for HVAC airflow. The most common sight of this symptom is an iced evaporator coil.
Undersized ductwork, duct leaks, dirty air filters are often the culprit in this situation. Although it may seem like a simple issue, returning liquid can cause compressor failure.
Ultimately, HVAC techs measuring the air flow across the evaporator coil is the first step to diagnosing airflow issues. Obvious red flags in the occupant space include hot and cold spots, pressure imbalances, and the absence of heating and cooling controls.
Temperature Rises & HVAC Airflow
“If a forced-air furnace also is installed on the system, a technician can use the furnace to determine the airflow across the evaporator coil, “ explains Joe Marchese in a 2005 article published in the RSES journal.
The mechanics behind this idea are fairly simple: measure the rise in furnace temperature during heating mode while the fan is set to the cooling mode speed. However, a big part of this process relies on a HVAC tech knowing the exact Btu output of a furnace and efficiency. Or, they can estimate.
“Using estimated values will yield a value that is not precise, but should be accurate enough for the testing procedure,” Marchese writes.
The Role of Static Pressure Drop in HVAC Airflow
Anytime air changes direction, it loses static pressure. When air passes the evaporator coil pressure drops. The more air that passes across the evaporator, the greater the static pressure loss.
To help HVAC tech’s measure static pressure, manufacturers often publish the static pressure values of their HVAC units. Keep in mind manufacturers often include two values because HVAC airflow changes depending on if the evaporator coil is wet or dry.
Carrier Enterprise even went as far as developing a training curriculum for HVAC professionals around airflow issues, which is available through Carrier University’s HVAC Mechanical Pro Troubleshooting course online.
The point being, proper HVAC airflow is crucial to a healthy HVAC system. Understanding that as an HVAC contractor is the first step in ensuring that the system you install lasts its full lifetime.
This story originally appeared in the March 2020 issue of SNIPS magazine.