Temperature Rise Measurements Temperature rise can also be used to verify the measurements of pressure that have been performed. To do a temperature rise assessment, find the furnace output ratings from the equipment rating plate if available. If not, you can determine the Btu output by checking the nozzle size and fuel pump pressure to determine the Btu input. Then do a combustion efficiency test to find output Btu and use the following formula: Btu Input x Combustion Efficiency 1.08 x Temperature Rise For Example • A furnace with 105,000 Btu input • 85% combustion efficiency • 75 degree temperature rise 105,000 x .85 89,250 1.08 x .75 81 Many manufacturers have done these calculations and they are usually in the installation instructions. Using Static Pressure Drop to Measure System Airflow The volume of air moved by a blower is dependent upon the pressure that the blower is working against. The airflow can be accurately determined by measuring the pressure drop across the blower or any component in the airflow and using the manufacturer’s specifications. To determine the amount of air a blower is moving: 18 National Oilheat Research Alliance 1. Measure the total static pressure difference across the blower, filter, or evaporator coil (Figure 15). 2. Compare the static pressure difference to the manufacturer’s specifications to determine the system airflow. Static pressure charts are model specific. Most equipment is rated to give the best performance and maximum airflow at around .50 to .60 static pressure. This information can be found on most equipment manufacturers’ labels (Figure 16, above). Measuring Airflow at Registers and Grilles The easiest and most accurate method for measuring the airflow at registers and grilles is with a flow hood. The flow hood is placed over the register or grille, forcing all the air passing through the register or grille on to the flow hood. The CFM is read directly on the flow hood. Figure 15 Figure 16 = CFM =1,102 CFM
NORA Advanced Air Flow
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