Near the end of September, we see the heat of summer fading away, and mornings feeling crisp. It’s time to shift from cooling season to heating season. Let’s take a look at how to calculate required system heating airflow to assure fans are set correctly, and your diagnostics remain accurate.
There are various methods you can use to determine heating airflow. The most popular is the temperature rise method. The method we discuss in this article is easy to calculate and adjust based on the type of gas furnace you diagnose.
Gas-Fired Furnaces
To calculate required airflow, begin with the rated Btu input of the furnace. Divide the rated Btu by 10,000. Then multiply this number times the following cfm factors:
Natural Draft Furnaces (70%) 100 cfm per 10,000 BTU of rated Btu input
Induced Draft Furnaces (80%) 130 cfm per 10,000 BTU of rated Btu input
Condensing Furnaces (90%) 150 cfm per 10,000 BTU of rated Btu input
Example: You check the furnace nameplate and find the rated Btu output is 60,000 Btu. 60,000 Btu divided by 10,000 equals six.
You inspect and see this is an induced draft furnace that requires 130 cfm per 10,000 Btu input.
Multiply six times the required airflow per 10,000 Btu of 130 cfm to find this furnace requires 780 cfm.
In cooling mode, say this furnace has a three-ton coil and condensing unit. Three tons multiplied by 400 cfm per ton equals 1200 cfm. Heating season airflow would be only 65% of cooling season airflow. (780 cfm/1200 cfm = 65%.) That’s a big difference that will affect system efficiency, heat rise, and the occupant comfort.
Higher Altitudes
If you service gas-fired heating systems at higher altitudes, this rule changes because air density decreases as altitude increases.
Email Doc for additional information if this applies to your situation. If he receives enough requests, we’ll do a follow-up article about how to calculate required airflow at higher altitudes.
For example, this same 60,000 Btu system requiring 780 cfm at sea level will require more than 1000 cfm at an elevation of 7,000 feet.
Heat Pumps
Heat pumps require the same airflow in heating mode, as they do in cooling mode. The magic number is 400 CFM per rated nominal ton.
Customers often complain of cool supply-air temperatures in heating mode. Many service technicians will reduce fan speed to heat up supply air temperature. This solves one problem but creates a bigger issue a few weeks later when the temperatures fall, and the building fails to heat as expected. When required airflow over the coil isn’t available to remove heat generated by the heat pump, total system Btus take a nosedive.
Verify Equipment Engineering Data
Equipment manufacturers continually find more creative ways to squeeze a little more heat out of their systems in the name of energy efficiency. Some of these new methods may require different airflow rates than recommended in this article. Take time to review manufacturer engineering data from time to time and make sure it agrees with these simple rules.
Manufacturer installation instructions and specifications are a valuable source of information. The best technicians take time to study and soak up knowledge from these and other valuable resources on a regular basis. This habit ensures you stay on top of your game and catapults you forward in your career.
Engineering data contains endless interesting facts and figures that may change some of your long-held assumptions. Conversations with better technicians confirm they regularly study industry information. This sets them apart from the average technician and keeps them at the top of the industry.
One technician recently told me that he discovered a new function available through dip switch changes that increase the performance of the furnaces he works on by nearly 15% when coupled with combustion efficiency adjustments. Knowing how to maximize system performance isn’t easy to come by, but the best contractors and technicians spend time with their faces buried in their manufacturer’s engineering publications.
Keep on Testing
The primary reason to calculate required airflow is so you can interpret airflow and system performance readings. Whether you verify system airflow by interpreting total external static pressure using manufacturer fan performance tables or read airflow using an airflow traverse or air balancing hood, understanding required airflow is essential to verifying live installed system performance. This assures you give your customers your very best.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free procedure to calculate system airflow at higher elevations, 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.