supply ducts, replacement air is drawn into the return ducts and register. In a system with no leaks, no additional airflow should be required to maintain the pressure once the duct has been pressurized. There are several ways to conduct a leak test. The standard method is the use of an orifice tube. The orifice is certified and designed for a specified flow curve. A pressure source is installed to pressurize the duct system. Typically, an axial fan (duct blaster) with an inlet damper to control airflow and limit pressure to the duct system is used. Two manometers are used, one installed to measure pressure drop across the orifice and the other to measure static pressure in the ductwork at the farthest point from the orifice. The pressure supplied to the system should not exceed the maximum design ratings that are specified for the system. The technician should read the pressure drop across the orifice and then compute the flow rate using the flow curve data. The actual leakage rate can then be compared with the allowable leakage rate of the duct system. Most systems should have a loss rate no greater than 2%. Check local codes to determine the acceptable leakage rate in that jurisdiction. Once the inspection and leak test are completed, a report should document the test for future reference. If the duct system is out of design tolerance, the system should be corrected to meet design conditions before attempting to balance the system. System Startup and Air Balancing When starting up a new system, design setup information including unit specifications, control systems details, job layout and airflow quantities for each register and grille should be included in a commissioning report for future reference. The report may also be useful in assessing warranty claims. 20 National Oilheat Research Alliance After gathering all the system data, the overall system airflow should be measured and compared to the required airflow in the design specification. The actual airflow must not vary more that 5% from the required airflow. Until the actual overall airflow is correct, there is no reason to evaluate airflow at the registers. The next step is to use a flow hood, velometer, or anemometer to measure the airflow at each supply register and return grille. The sum of all CFM measurements should be within 10% of the system CFM at the blower. Leaks in the return duct system will cause the air entering the return air grilles to be less than the total amount of air the blower is moving. If the difference is more than 10%, the leaks in the return duct must be sealed. Similarly, the total of all the CFM measurements at the individual supply registers should be within 10% of the system CFM at the blower. Leaks in the supply duct system will cause the air leaving the supply registers to be less than the total amount of air the blower is moving. Once the total delivered airflow is within tolerances, the supply register measurements should be compared to the design specifications for each area. Dampers can now be adjusted to achieve the correct airflow for each register. Start with all dampers open, then partially close the dampers to the registers that are delivering too much air. Begin with the registers that exceed design specifications by the biggest margin. Measurements should be taken after each adjustment to determine the effect of the adjustment on the other registers in the system. Testing Heat Exchangers For Leakage Heat exchangers separate combustion gases from the heated air that the blower distributes
NORA Advanced Air Flow
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