An anemometer, a test instrument that measures air velocity is used to determine the average airspeed in the duct. Then the average feet per minute is multiplied by the area of the duct in square feet to determine the airflow moving through the duct.
The best method to accurately measure airflow is with an anemometer. This device has been a standard for many years, and it measures air speed and velocity by using cups attached to a spinning device. The air moves the cups, which are connected to a sensor that reports various data points.
To determine the flow rate in a closed duct, one of the common way is to explore the velocity field. The principle of this technique is to divide the duct cross-section in elementary surface elements, to measure the velocity in each of these elements and to calculate a average velocity weighted over the surfaces.
Obstructed Vent
One of the most common causes for poor airflow is a blocked vent. To properly heat or cool your home, the air needs to be unobstructed. To see if there are any blocked vents, look by the baseboards or on the ceiling. Furniture, bedding, and the vent damper could all be possible obstacles to the vent.
The easiest way to determine Flow Velocity is to measure the Velocity Pressure in the duct with a Pitot Tube Assembly connected to a differential pressure sensor. The Pitot Tube Assembly includes a Static Pressure Probe and a Total Pressure Probe.
The most common and easiest way to measure or with some methods estimate and set airflow is to use one of the following methods: Rotating Vane Anemometer (Measurement, mid airflow range) Pressure drop across the dry evaporator coil (estimate) Total external static pressure method (estimate)
Check the Air Vents
While your AC is turned on, hold your hand up to the vents. If you cannot feel any air coming out of the vents, your blower might be broken or your ductwork might be disconnected. Another potential sign of a broken AC will be air that is warm or lukewarm instead of feeling crisp and cold.
A full HVAC inspection costs between $150 and $500 on average. This comprehensive appointment includes a complete inspection of your heating system, cooling system, and ductwork.
A Simple Test of Duct System Efficiency
Open a door or a window just 1-2 inches, and switch on the furnace/air conditioner blower. If you notice a significant amount of air coming in from outside, it is a good indicator or indoor air pressure loss. This occurs when ducts are leaking outside the conditioned space.
For measurement of low velocity air flow; Vane Anemometer can be used at the discharge of the pipe. A vane anemometer combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument.
Multiplying air velocity by the area of a duct allows you to determine the air volume flowing past a point in the duct per unit of time. Volume flow is generally measured in cubic feet per minute (CFM). Air velocity is measured by sensing the pressure that is produced through the movement of the air.
You can determine if your home has an air flow problem by performing the garbage bag air flow test – so named because the goal is to determine how long it takes for a garbage bag to fill with air (or whether it does it all).
The airflow from your HVAC system should be the same in every room of the home. If it seems like air is flowing out of air vents more slowly or not at all, it indicates a clogged air duct.
There are several reasons your vents are not putting out the amount of air they should. These include: Dirty or clogged air filters. Blockage in the vents.
Persistent Odors: Homes should refresh and clear out odors naturally through proper ventilation. If cooking smells, smoke, or other odors last longer than they should, it's likely your home isn't circulating air properly.
Common issues like leaks, disconnections, or obstructions within the ducts disrupt the flow of cooled air, causing rooms farther from the HVAC unit to suffer the most. In some cases, inadequate insulation of ducts can allow heat to infiltrate, further exacerbating the problem.
Additionally, due to its increased surface area, flex duct may impede airflow somewhat when compared with more streamlined rigid ducts. In some cases, an insulated flexible line may also be necessary if sound dampening or temperature stabilization are desired properties of a ventilation system.
Climb into your attic, crawlspace and/or basement and take a look at the ducts that are visible. Look at each section of duct and connection for obvious gaps, disconnections and tears. Also, look for areas of the ducts which have duct tape affixed. This is a sign that duct leaks have likely been patched before.
Since extractor fan vents aren't flat, the only way to obtain reliable air flow measurements is to use an air flow cone. The simplest set-up is to use a cone with a vane thermo-anemometer, although it's also compatible with a hotwire thermo-anemometer, provided the probe is centred in the middle of the cone.
The static pressure for the air flowing in a duct is typically measured with a device called a Magnehelic. This is a type of differential pressure gauge that is able to measure pressures in the range of millibars or inches of water pressure.