ANSI/ASHRAE Standard 120-2017 – Flow Resistance of HVAC Ducts and Fittings

An ANSI/ASHRAE 120-2017 HVAC duct near ceiling.

Pressure is key for HVAC system operations, and the many factors that can adjust the airflow, thus influencing alterations in pressure, should be measured and monitored to keep track of the system’s performance. For HVAC ducts and fittings, this is addressed in ANSI/ASHRAE Standard 120-2017 – Method of Testing to Determine Flow Resistance of HVAC Ducts and Fittings.

ANSI/ASHRAE Standard 120-2017 is the latest revision of the standard specifying uniform laboratory testing methods of HVAC ducts and fittings. These methods are carried out to determine the resistance of ducts and fittings to airflow, and the new edition of the standard, like its predecessors, is primarily intended to be used for assessing the change in pressure correlating with the airflow in these two features.

Specifically, the standard can be used for rigid and flexible ducts of rectangular, round, flat oval, or other shapes that have been constructed to contain airflow, as well as any fittings that can be used with the aforementioned duct types. Elastic materials that change size greater than 5 percent of any linear dimension with flow conditions are not addressed by this standard.

ANSI/ASHRAE Standard 120-2017 employs the use of test ducts and test chambers with nozzles or orifices that measure airflow for carrying out its laboratory procedures. There are two types of test setups presented by the document:

  1. Draw-through with the flow measuring apparatus and fan downstream of the test duct or fitting.
  2. Blow-through with the fan and flow measuring apparatus upstream of the test duct or fitting.

The materials, methodology for carrying out the testing, and guidance on conducting observations and making calculations are all detailed in the standard.

With this information, the users of ANSI/ASHRAE Standard 120-2017 can use the test results to determine duct flow losses in pressure loss per unit length. Fitting losses are to be reported as local loss coefficients.

The standard does not, however, cover the interpretation of test data.

ANSI/ASHRAE Standard 120-2017 contains one significant change: the length upstream of the test duct or fitting was increased from 10 to 15 hydraulic diameters.

ANSI/ASHRAE Standard 120-2017 – Method of Testing to Determine Flow Resistance of HVAC Ducts and Fittings is available on the ANSI Webstore.

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