HVAC
B y M a r k P ar liam en t an d A lexan d r a W en n berg
Proper airflow is no fictional unicorn E
nsuring proper airflow is one of the most important aspects when installing an HVAC system. Air that is not properly distributed will cause uneven heating and cooling, and will place undue stress on the system.
Unfortunately, unlike other parts of the system, air distribution cannot be seen, so it is extremely important to have a thorough understanding of how air flows in a duct system, and what causes it to be properly distributed. This knowledge will help with successful troubleshooting of air distribution problems.
C H E CK Y OUR ESP It is easy to diagnose a burnt-out blower motor, but understanding why the motor burnt out and fixing it is what separates a technician from a parts changer. External static pressure (ESP) tends to be the unicorn of our industry but it’s really not. The equipment will tell you what ESP it is operating at. Using the following formulas, you can calculate the exact CFM the motor is producing, whether you are on a gas furnace or an electric strip. This, along with the manufacturer’s blower curve, will tell you the ESP the blower is working against. Part of the calculation is to determine the properties of the air to understand the amount of force required to move the air. This will then enable you to calculate the proper CFM. It is also important to remember that the way a blower is designed dictates the amount of flow, or CFM, it can deliver at a given static pressure.
Air type affects performance When designing a duct system, it is important to account for the type of air – wet or dry – that will be distributed through the system. Humid air is heavier and thus harder to move through the system. In a perfect world, measurements would be done to understand air density prior to designing the system, to ensure the ductwork and HVAC appliances are sized properly to distribute air evenly. As we all know, however, we do not live in a perfect world and technicians often end up working on homes where the rooms farthest from the HVAC appliance are at a different temperature than the ones closest to the source. Part of the troubleshooting should therefore be to investigate what is causing the uneven airflow.
FORMULA: If you know the BTU output, you can calculate the CFM as follows: Using a 100,000-output furnace producing a 54˚ temperature rise,
For electric heat, use the following formula, where the Temperature Rise (TD) = 50, to calculate the BTU and CFM:
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Once you know the system’s ESP, look at the velocity of the air being delivered. The recommended velocity for supply trunks is 700 to 900 Velocity Feet per Minute (FPM). Less than 700 FPM will cause uneven air distribution while FPM higher than 900 will create high noise levels. Many systems include the use of volume dampers to adjust airflow due to aesthetics, plumbing or electrical. All of this will have a detrimental outcome on the velocity of the air delivered to the register and improper air delivery will have a negative impact on the customer’s ultimate indoor comfort.