Fluke 922 Fluke Low Pressure Differential Meters - HVAC Pressure Applications - Page 3

Combustion air blower pressure differential: Category IV direct vent gas appliances typically use a combustion air blower to ensure airflow through the vent system and appliance before ignition. In most cases, a pressure differential switch monitors pressure differential across an orifice. Greater pressure differential indicates more airflow, lower differential indicates less airflow. In this case, two tees are needed to tap into both the high and low pressure sensing tubes connected to the pressure differential switch. This is an important value to benchmark during appliance installation. Pressure differential depends on the length of connected vent and the application. If this value is benchmarked when installed, degradation to differential created by heat exchanger, condensate or vent problems is more likely to be detected before a failure occurs. If not benchmarked, there's no way to evaluate degradation. Follow guidelines from the manufacturer. Fan effects on combustion pressures: When checking stack draft, over-fire draft or combustion air inducer/blower pressure differential, an abrupt change in pressure when the blower starts may indicate a compromise between the flue passages and the air passages. Fan pressure can easily be 15 times or more greater than flue passage pressure (-0.03" wc draft vs 0.5" ESP), so abruptly changing draft pressures commonly indicate heat exchanger leaks. Gradual changes in draft pressures most likely mean combustion air supply problems or vent problems, especially when accompanied by rising CO and CO2, and falling O2 levels in the combustion products. This can be caused by insufficient make-up air or return leaks, especially in spaces that are considered confined (less than 50 cu ft per 1,000 Btu input) or tight construction. Use the Fluke 922 to compare equipment room pressure to outside pressure before and during equipment operation. Simply adding a small supply register in the equipment room can often compensate for this engineering design deficiency. VAV (Variable Air Volume) control: Pressure sensors are often used to control VFD (variable frequency drive motor) RPM. For instance, a supply duct pressure sensor can modulate the RPM of a VFD, based on pressure changes caused by a combination of open and closed zone dampers. Exhaust VFD blowers may modulate RPM based on pressure differential between the occupied space and outdoors. In each of these cases, the VFD varies RPM to maintain a relatively consistent pressure or pressure differential. The Fluke 922 can be used to monitor or check the calibration of these sensors. CAVB (Constant Air Volume with Bypass) damper control: Equipment with a constant RPM blower may need to simulate VAV in the supply duct, when zoning is used. The control is similar to a true VAV system. A pressure sensor monitors supply and modulates a bypass damper. The damper relieves pressure from the supply to the return, maintaining a consistent supply static pressure. The Fluke 922 can monitor or check the calibration of these sensors. Pressure differential defrost control: Some enthalpy wheels use a differential pressure sensor to determine if a defrost cycle is needed during cold weather. A pressure differential increase (indicating a frost build up) will cycle the outdoor air intake blower off , so it can use the indoor air to defrost the wheel. The Fluke 922 can be used to monitor pressure differential, check the calibration of the sensor, or test defrost operation. Some air-to-air heat pumps use a pressure switch to monitor pressure drop across the outdoor coil to determine if frost build-up warrants a defrost cycle. In that circumstance, the Fluke 922 can be used to monitor defrost pressure differential, since restricted airflow may indicate a frosted coil. Airside devices: Pressure drop across airside devices is published by manufacturers and is necessary for design considerations, including selecting equipment for satisfactory blower performance and for duct design considerations. Once installed, these same performance tables are useful in estimating airflow and determining end of life for filters. Evaporators, for instance, will have pressure drops with associated cfm's for a clean dry coil and for a clean wet coil. High performance filters will have pressure drops with associated cfm's for a clean filter, and pressure drops with associated cfm's for that same filter when it has reached the end of its life. The Fluke 922 can be used with a choice of pressure sensing devices to check pressure drop across an airside device. If the approach to and from the device is straight, a Pitot tube can be placed on each side of the device and the difference in total pressure readings will result in pressure drop. The tube connected to the upstream side Pitot tube is connected to the 922 "+" port, the tube connected to the downstream side Pitot tube is connected to the "-" port, the pressure reading displayed will be the device pressure drop. If the approach is not straight to the device, then use the static sensing side of the two PitotStatic tubes, or use two static pickups connected to the meter in the same manner. Confirming pressure drop across the air filter bank.  Fluke Corporation HVAC pressure applications with the Fluke 922