National Oilheat Research Alliance 17 Air Velocity Measurement Instruments Velometer—Measures the velocity of airflow. Most velometers give direct readings of air velocity in fpm. Some can even provide direct reading in cfm. Rotating vane anemometer—Consists of a lightweight, air-propelled wheel, geared to dials that record the feet of air passing through the instrument. Anemometers read feet of air per minute (FPM) or cubic feet per minute (CFM). The rotating vane anemometer is placed in the airstream and readings are taken for a measured amount of time. Thermal Anemometer—Operates on the principle that the resistance of a heated wire will change with its temperature. The probe is placed in the airstream and the velocity is indicated. Flow hood—Accurately determines the airflow volume in CFM from an outlet by collecting the air supply, directing it through a one foot square opening where a calibrated manometer provides the CFM readout. Flow hoods are considered the most accurate measurement of airflow through registers and grilles. Troubleshooting airflow problems A Department of Energy report, http://www.nrel.gov/ docs/fy05osti/30506.pdf, has indicated that ducts may have efficiencies of 60-70%. This significantly decreases the overall efficiency of the system. A system with an 85% efficient furnace and a duct efficiency of 60% will deliver a little over 55% overall efficiency to the building (.85 x .65 = 55.25%) . When measuring airflow, it should be checked and verified. For example, if we check airflow by performing a Total External Static Pressure test, we could verify by checking the temperature rise or amperage draw. This chart (Figure 15) shows the correlation between airflow, static pressure and temperature rise for a specific furnace. Figure 15 For PSC systems, the 1/2 hp motor is equipped with 4 speeds. The unit is set for the mid-fire temp rise @ 66°F. See table for poper blowe motor setup. Alterations Required for A/C @ Design External Static Pressure Cooling Unit Heating Speed by Input Recommended CLG Speed Low Fire Mid Fire High Fire 24,000 Low ML MH Low 30,000 Low ML MH Med Low 36,000 Low ML MH Med High 42,000 Low ML MH Med High 48,000 Low ML MH High Furnace Airflow - CFM vs. External Static Pressure (in W.C.) Speed Tap/ Static Pressure 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Low 930 915 912 910 822 774 730 ML 1155 1152 1130 1126 1085 1042 920 MH 1442 1432 1418 1382 1334 1293 1230 High 1802 1762 1705 1635 1569 1493 1428 Furnace Motor Current Draw (Amps) vs. External Static Pressure (in W.C.) Low 3.3 3.1 3.0 2.9 2.6 2.5 2.4 ML 4.2 4.0 3.9 3.7 3.6 3.3 3.0 MH 5.4 5.2 5.0 4.7 4.4 4.2 4.0 High 6.6 6.4 6.0 5.7 5.5 5.2 5.0 High Fire Temperature Rise vs. External Static Pressure (in W.C.) Speed Tap/ Static Pressure 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Low 90 91 91 92 101 108 114 ML 72 72 74 74 77 80 91 MH 58 58 59 60 62 64 68 High 46 47 49 51 53 56 58 Mid Fire Temperature Rise vs. External Static Pressure (in W.C.) Speed Tap/ Static Pressure 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Low 72 73 73 73 81 86 91 ML 58 58 59 59 61 64 72 MH 46 47 47 48 50 52 54 High 37 38 39 41 42 45 47 Low Fire Temperature Rise vs. External Static Pressure (in W.C.) Speed Tap/ Static Pressure 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Low 60 61 61 61 68 72 76 ML 48 48 49 49 51 53 60 MH 39 39 39 40 42 43 45 High 31 32 33 34 35 37 39
NORA Advanced Air Flow
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