Airflow is a critical component to air conditioning and designing duct systems is both an art and a science. Understanding some design aspects can help to diagnose airflow issues to quickly resolve service issues. We cover these topics in the 3rd year of our apprenticeship program; however, I‘d like to discuss some of the basics here.
In a previous article (March 2022) we talked about measuring airflow methods. Here I’d like to bring up a point that some may find controversial. To the left we have an image of the traditional type of Air Duct Calculator. This slide rule was used, and may still be used, to size flex ducts. However, this calculator is not designed for flex ducts. The “duct diameter” round size listed is for metal. So, what’s the difference? Round metal is smooth and has less resistance (friction) on the air moving through. Flex duct has a mylar liner with a wire helix coil which offers higher resistance (friction) than the smooth metal duct. The ACCA Duct Calculator shown on the next page has different duct materials (referencing their associated resistance) and is much more accurate for the type of duct material you are designing. Due to many designers using the wrong calculator for flex duct we could run into undersized ductwork. We often see a 5-ton split system (nominal 2,000 CFM) with a single 18” flex duct for the return, which is approximately 1300 – 1400 CFM (dependent on the friction rate). Now to compound this issue further, we moved to new higher efficiency systems with evaporator coils that have more “fins per inch.” We will see performance issues. That is, unless we address the duct system.
The steps to properly design a duct system include a load calculation to determine how many BTUH the space requires. Select the system which meets the load calculation. Convert the BTUH and the system’s airflow capability into cfm. Find the correct friction rate based on the application. Design a duct system that delivers the required amount of BTUH/CFM – supply and return. Many think this is only done during new construction projects; although, this can be done on retrofits when necessary. So how does all this information play into airflow diagnostics? We must understand the design aspects to recognize and diagnose improper airflow. Poor airflow can lead to scenarios such as multiple failed evaporator motors, low cooling, multiple compressor failures to name a few. High static pressure can lead to motor failures, since the electronically commutated motors (ECM) are programmed to meet a specific CFM, they tend to fail in high static situations. If we do not address the static pressure issue, the replacement motor(s) will fail. This is different than PSC type motors which tend to “slip” under higher pressure. Higher static pressure additionally will lower the amount of air getting to the desired space. If the airflow is low, we cannot get the refrigerant charge correct. This can lead
to low capacity and low cooling. As our last example, I mentioned compressor failures. A poor duct system results in low airflow which causes lower coil temperatures (the refrigerant not getting enough heat to cause evaporation), sending liquid refrigerant back to the compressor. Compressors are designed for vapor not liquid so liquid refrigerant causes compressor damage and failure.
When working in an existing home, the toughest question might be how much airflow does “this” room need? If we have access to design load calculation or plans, we can identify the room and locate the reference with the CFM identified. What happens if we do not have access to this information, how do we solve for “X.” I can say from experience there are a few options to get us in the right direction. The textbook answer is, if the load calculation or plans are not available, we would do our own load calcs. This can be time-consuming on a service call and not really a good option. The first step is to identify the concern. Is it a load issue, i.e., sliding glass door with a west exposure – it’s warm in the afternoon. Perhaps there is an option to “treat” the window with blinds, curtains, or solar film. Or is it an airflow issue that can be addressed by checking for ducts that have become disconnected or crushed. More than once I’ve diagnosed a warm room and found the duct completely unattached. However, if it’s an issue that can be solved by adjusting the airflow and we want to know what CFM should be there. We can run a simple formula to determine CFM by using the air changes per hour (AC/HR). There are published AC/HR rates. Let’s say our example problem is a bedroom, we can look up in several resources what the expected AC/HR for a bedroom is. It is approximately 6 AC/HR. We can measure the room’s volume by the AC/HR. Divide that result by 60 (minutes in an hour). In example we have a 15’ x 18’ bedroom that has 12’ ceiling our room volume is 3,240 cubic feet. Multiply that by 6 AC/HR and we have 19,400 which would divide by 60 giving us 324 CFM. Determine what the existing CFM is and move to this number. This method is not as accurate as the load calculation, but it gets us in the right neighborhood. I wish you all a happy and safe holiday season!
Required CFM = Room Volume x AC/HR
Kelly McCann is an accomplished Heating, Ventilation and Air Conditioning (HVAC) professional with over 30 years in the industry. He has experience in several capacities; installation, service, service management, O.E.M technical support and territory management. Kelly has also provided professional instruction since 2000, teaching at the Florida Air Conditioning Apprenticeship Association. His current position is Retail Sales Manager at Custom Mechanical Inc. in West Palm Beach, FL.