After 25 years and nearly 300 articles for Contracting Business.com, I realized I’ve never written a comprehensive article about how to balance a residential HVAC system. So let’s take a long-overdue look at the basics of simply balancing a residential system.
Many of articles I write have something to do with air balancing. The reason for this obsession of mine is that balancing is the single-most important step that can be taken to assure your systems produce comfort and operate efficiently. If there is anything I have pleaded with you to do over the years, this one thing will give you the greatest results.
Gather Design Information
While few homes have mechanical plans or specifications to follow, you can reverse engineer the necessary information required to balance most systems.
First, find the equipment model numbers and go online to the manufacturer’s website to get the engineering information on that specific piece of air-moving equipment.
Find the tonnage or heating output to determine required system airflow. Divide the total system airflow so each room has its share. This can be done using Manual J or one of several estimating techniques, including calculating air changes.
You’ll also need to find the fan total external static pressure rating, blower motor horsepower and other key information for the balance report. This data will be used to substantiate and cross reference your field measurements.
Draw a rough floor plan of the house, or each system, and number the supply registers in the order you will be testing and balancing (1, 2, 3, etc.). You may start with the one farthest from the fan. Then number the return grilles after the same manner
R1, R2, R3, etc.).
Before balancing, fill in the design portion of the report. (Email Doc at the end of the article for a free air balance report if you don’t have one.)
Inspect and Start Up the System
Walk or crawl the system and inspect for any defects that could make it un-balanceable. Note any deficiencies and make any minor repairs that are obvious. Since few of us ever really inspect the entire system, be prepared to find and fix disconnected ducts, open dampers, and add duct suspension in a few places.
As you inspect the system, record nameplate data, then verify both fan horsepower and fan speed. Check filters for cleanliness. Start the system by switching the thermostat to heating or cooling. Then adjust the temperature setting to bring the system into full operation.
Drill static pressure and temperature test ports before and after the air moving equipment and a test port five feet after the cooling coil for temperature measurement.
Take Initial Airflow Readings
Using a commercial balancing hood, measure and record the airflow from each supply register in the order you identified on your floor plan and the balancing report. Do not use a powered hood or a duct pressurization fan with a cardboard box, a clothes hamper, or a garbage bag with a coat hanger and stop watch. These are not legitimate air balance tools and do not permit sustainable and realistic air balancing.
Next, measure and record the airflow into each of the return grilles. Should the hood not fit on the grille, perform an airflow traverse of the duct according to ASHRAE Standard 111, the air balancers favorite standard.
Add together and total airflow from each of the supply registers. Then add together and total airflow from each of the return grilles.
Compare Design to Actual Airflows
Compare your initial airflow readings to the design register, grille, and system airflow.
Adjust the fan higher or lower to obtain the required system CFM. Typical air balancing specifications state each should be within 10% of design airflow.
Then compare each supply and return grille to its required airflow and note registers where airflow exceeds 110% of design. Close down the dampers to the high airflow registers until design airflow is reached.
By doing this, you’ll discover the registers that had low airflow have increased. Then re-test the low registers and adjust dampers until all the supply registers are + or – 10% of design airflow.
Next, compare return grille airflows to each grille’s design airflow. Adjust return balancing dampers as needed until each return is + or – 10% of design airflow.
Pass through the system one last time and assure each register and grille is + or – 10% of its design airflow. Mark your final damper settings.
Measurement with an air-balancing hood alone is insufficient to balance a system. Additional testing is essential required on each air balance job. This additional testing includes a variety of measurements that must be in harmony with each other. This is a little understood fact that critics of air balancing have never bothered to learn.
The air balance report provides space to record each of these measurements. These tests include total external static pressure, fan speed, temperature change through the equipment, and electrical measurements.
Each of these readings are compared to the manufacturer specifications and adjusted for certain part-load conditions. Using equipment specifications and fan laws, each value can be weighed against airflow data to verify overall accuracy.
Complete the air balance report with your final conclusions and any recommendations you have to improve the system performance.
Additional Testing and Calculations
System performance measurement and energy analysis calculations have far eclipsed basic residential balancing in recent years. To a basic air balance report you can add equipment and system BTU measurement, system commissioning steps, static pressure and temperature profiles, live duct leakage, refrigerant and combustion adjustment, building pressure analysis and many other tests.
The bottom line is that air balancing has been the prescribed verification test for HVAC system performance for the last 50 years. As you can see, it if finally beginning to take its place in an energy world that is desperately seeking real energy efficiency.
Throughout the life of this industry, air balancing remains the only way to assure your customers that you have delivered what you promised – comfort and efficiency.
Rob “Doc” Falke serves the industry as president of National Comfort Institute -- an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free Residential Air Balancing Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.