The temperature rise through any convection heating unit can be measured but must be done in a specific manner to get meaningful results. If the rise is known, and the BTU output is known, then the CFM circulated airflow can be calculated and is more precise than most field measuring devices. Be certain that the unit has been supplied with prescribed gas pressures, oil pressures or voltage (depending on fuel to be used). Once the furnace has been started and has been given time to reach level off temperatures, the input should be confirmed and the output calculated based on the unit thermal efficiency. (This information is usually provided on a rating plate attached to the appliance). On gas-fired equipment, the manifold pressure should be checked to make sure it is at the prescribed setting. A check of the gas flow at the meter can also be made, however, you must be certain that all other gas burning equipment has been shut off.
Measure the time (T) in seconds for one cubic foot of gas flow - Then, using the following formula, determine the BTUHINPUT.
BTUHINPUT = 3600 ÷ T x Heat Value (BTU) for 1 ft3 gas
Measure the inlet air temperature on a continuing basis, being sure to observe any fluctuations in this temperature.
Discharge temperatures should be measured at a distance from the heat exchanger that minimizes radiational effect on the measuring device. A distance of four feet should be adequate. Measure the discharge temperature by making at least ten measurements across the discharge duct. Be sure to pick up measurements top to bottom and side to side. Single measurements are not reliable due to temperature gradients in the discharge air.
Once the measurements are completed, the multiple discharge readings must be averaged. Then the temperature rise is determined by deducting the average inlet air temperature observed during the test.
CFM is then determined by the following formula.
CFM = BTUHINPUT x Efficiency ÷ 1.085 ÷ Temperature Rise
NOTE: Efficiency can be determined by referring to unit rating plate = (output ÷ input)