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Air Flow

There are a variety of ways to measure air flow rate in a field test setting. Unlike other fluid flow rate measurements of water or natural gas, air flow is not necessarily constrained to a pipe. Air flow rate in ducts is one place that air flow can be measured, but there are other places in the home that air flow rate may be desired, such as at the outlet of a vent. Some methods are more applicable in a short-term test situation, rather than a long-term installation. Collecting air flow rate measurements would be an important task when investigating the performance of heating, ventilation, and air conditioning (HVAC) equipment or when looking for duct leakage, for instance. Some common instruments for measuring air flow rate are described below:

Pitot Tube

A pitot tube is an instrument used to measure fluid flow rate and is typically used to measure air flow rate in a field test application. A pitot tube is a curved tube with an inlet that points toward the flow direction with no outlet, which is used to measure the total pressure of the flow.

Hot Wire Anemometer

A hot-wire anemometer, or thermal anemometer, is a device used to measure fluid speed. The wire is generally made of either platinum or tungsten and is heated by running current through it. The change in current required to maintain a constant wire temperature is directly related to the wire heat loss to the fluid. Based on convective heat transfer, the heat loss can be used to calculate the fluid speed.

Flow Hood

A flow hood is a device that measures the air flow rate into or out of a register while the air handler is operating. The flow hood channels the air through a short fabric duct containing thermal anemometers to measure the flow of air entering or exiting the register. During normal air handler operation, the flow hood is held over each supply and return register and the flow rate for that register is recorded. These measurements are used to check the total air flow for the house and room-to-room air flow balance.

Orifice Plate

An orifice plate, also known as a flow plate, is a calibrated airflow measurement device, typically used to measure return airflow at an air handler unit. It is placed in the filter slot, and sealed to the sides of the air handler case. Round holes are located in a flat plate, and the differential pressure across the plate is measured while the air handler blower is operating. This pressure is correlated to the volumetric flow through the orifices.

Powered Flow Hood

A powered flow hood is a type of flow hood, primarily used for airflow measurement at a supply or return register. When properly used, it is able to provide a measure of airflow without altering the register pressure and flow as much as non-powered flow hoods can. Powered flow hoods are comprised of a standard flow hood plus a fan, pressure gauge, and controller. The fan speed is set to ensure the register pressure is the same as the ambient room pressure. In that way, there is no back-pressure from the flow measurement component (orifice plate or nozzle) on the duct system.

The Trash Bag Method

The trash bag method is a simplified and reliable method for measuring supply register airflow during short-term testing at a field test site. It requires a trash bag, small metal or wood frame, tape, stopwatch, and a properly sized piece of cardboard (or similar).

Relevant publications:

LBL has done work on both laboratory and field evaluations of airflow meter devices.