What is Correct Airflow?
HVAC School - For Techs, By Techs
Bryan Orr
4.9 • 1K Ratings
🗓️ 28 September 2020
⏱️ 60 minutes
🧾️ Download transcript
Summary
The great Ed Janowiak (Jon-Oh-Wok) joins us to talk about what correct airflow really looks like. He also explains how to design for it appropriately.
The ACCA design series (Manuals J, S, and D) all go hand in hand to design HVAC systems properly for a given space. Correct airflow will depend on how a technician or designer uses the ACCA design series. When we say "correct airflow," we mean that the CFM per ton matches the sensible and latent load for a space while maximizing comfort for building occupants. In many cases, 400 CFM per ton is the rule-of-thumb baseline for many systems, but it's not a one-size-fits-all solution. The point of the ACCA manuals is to use math to determine solutions tailored to a specific space and avoid rules of thumb.
Many technicians prefer higher airflow in the field because it leads to fewer technical problems. However, the occupied space can suffer from reduced latent removal when you have higher airflow. Variable-speed technology helps a bit to allow longer runtimes to help with dehumidification, but consumers may not be in the market to purchase those solutions.
We can use airflow grids to determine the CFM on a running system. When those grids determine that the CFM per ton is below 300, that means the equipment is likely failing to match the required sensible BTUs. Airflow also affects pressurization, which you can measure with a manometer. Overall, you will want to track airflow trends and work to optimize the airflow.
Ed and Bryan also discuss:
- Using software for calculations
- Friction rate
- Sensible heat ratio (SHR)
- Equipment selection and code compliance
- Relative humidity targets
- Intermittent ventilation
- Ancillary dehumidification
- Duct sweating
- Residential vs. commercial equipment design gap
- Blower door testing
- Testing delivered capacity and balancing
- Zonal pressure testing
- Extended performance data
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Transcript
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