We had quite a number of calls about my previous article regarding the correct amounts of fresh air to bring into HVAC systems. One recurring question was: “Is there a way to measure fresh air if I don’t have a full set of air balancing instruments?”

The amount of fresh air can be measured and calculated using a simple pocket thermometer, and by applying a simple formula.

Sometimes field test conditions don’t allow for accurate or convenient measurement of outside air flow because there may be poor access at the fresh air inlet, or it may be too windy to get a good reading. The percentage of outside air can be calculated by using three simple temperature measurements: outside air temperature, return air temperature and the mixed air temperature.

This economizer test has been used in air balancing for nearly 100 years.

OAT - Outside Air Temperature. This is the temperature of the air entering the system or equipment from the outdoors. Measure the air temperature in a shaded area (such as under the eyebrow of an economizer) so your thermometer doesn’t absorb the radiant heat of the sun. Also be sure the reading is not affected by the roof temperature or any other source of heat.

RAT - Return Air Temperature. This is the temperature of the return air entering the equipment. This temperature may be different from the temperature entering the return grilles due to duct loss or gain. Find a test location in the return duct system, near the fan, near the blower motor, or a cooling coil. Remember that return air temperature can be affected by the outside air.

MAT - Mixed Air Temperature. This is the air temperature past the outside air inlet where the temperatures of the return and outside air have mixed together. This may be in the return plenum, or you may need to test for it in the blower compartment. There are times you may need to hunt by your thermometer readings to interpret the point between return air and fresh air where the two types of air are completely mixed.

Now, here’s the formula:




Here’s an example of how the formula works:








Or, 50 percent of the total airflow of the system is being pulled into the system from outside. If the fan is moving a total of 2,000 cfm, the amount of air being introduced from outside would be 1,000 cfm.

The test is not perfect, but when time is short, and you don’t have access to good quality air balancing instruments, this test can provide valuable data.

Also when indoor and outdoor conditions are nearly the same, the formula doesn’t provide good results. If the indoor temperature is 80 degrees, the return air is 80 degrees and the mixed air is 80 degrees, the test will always show zero outside air in the system.

The formula works best when there are 10 degrees or more difference between the indoor and outdoor air temperatures. If there is less than a 10 degree difference, measure the air temperatures down to the tenth of a degree to increase accuracy.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a procedure to measure fresh air using an anemometer, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.