Date: May 23, 2012
When creating an energy efficient building, don’t overlook the importance of the Airflow Measuring Station (AMS). These extremely important HVAC components are your frontline defense against wasted energy, starting with the amount of Outside Air entering your building. Too much can lead to inefficient BTUH exchange across Hot and Cold Water Coils, too little could lead to stale odors and sick building syndrome. And those are just a few examples.
Ensuring the calibration and function of the Airflow Monitoring Station should be at the top of the priorities list for the Test and Balance Contractor. The equipment should be verified to hold calibration at both Economizer and Minimum Modes, showing that control damper positions do not create “eddies” or unstable flow characteristics at the Air Monitoring Station.
As per AABC Standards – Airflow measuring stations are used for determining total airflow, outside air, return air, relief air, and exhaust quantities of systems.
Each monitor shall be field – verified by a Pitot tube traverse. If the airflow reading by the AMS does not match the traverse ( within +/- 5 % ), the AMS display has to be corrected.
A typical HVAC Specification might call for providing AMS in the main supply, return and outside air ducts of all VAV systems, and to ensure that the AMS is installed with the manufactures recommended straight sections of ductwork on both sides of the measuring station.
While most of the AMS readings can be corrected using a flow factor and the control contractor adjusting his readout, there are a few cases that may need additional troubleshooting.
Some of the additional problems may be as follows:
• Airflow in ductwork changes dramatically, an example would be the airflow in a 30 x 28 outside air duct goes from 6000 cfm 1029 fpm to a minimum of 1500 cfm 257 fpm. This will cause the velocity pressure to go from 0.07″ wg to 0.004″ wg which is below what the AMS can read, resulting in a system error.
• Airflow rate is less than 200 fpm as recommended by most manufactures.
• The AMS is located too close to an intake or louver.
These are just a few items that might require additional time in completing the Testing and Balancing specifications of a project. At times these small steps are “Value Engineered” out of project requirements because of cost. This is a misnomer as these small steps lead to “exponential” energy savings over the lifetime of buildings when applied correctly.
Here are the links to additional information on this topic:
“Troubleshooting Air Flow Measuring Stations (Variable Volume Systems with Fan Tracking),” By W.B. Whitt, TBE, Systems Analysis, Inc.
Or if you like here is a link to the full pdf: https://www.aabc.com/resources/tabjournals/Winter06active.pdf
(The article is on page 14 of that issue.)
Albert Englehart – Mechanical Testing, Inc. AABC/TBE
Brian Venn – Mechanical Testing, AABC/TBE
When creating an energy efficient building, don’t overlook the importance of the Airflow Measuring Station (AMS). These extremely important HVAC components are your frontline defense against wasted energy, starting with the amount of Outside Air entering your building. Too much can lead to inefficient BTUH exchange across Hot and Cold Water Coils, too little could [...]