AT-15-022 -- Evaluation of Airflow Measurement Methods for Residential HVAC Returns ✓ Most Recent

This study was undertaken to address the concern thattypical equipment used in the field by contractors for measuringtotal airflows at return grilles in residential forced-airsystems may not be accurate enough for the purposes ofmeasuring return flows for use in Title 24 applications. Themeasurement errors are a combination of errors in determiningthe airflow through the measurement device and the insertionloss effects of actually performing the measurement. Thetechnical reasons for insertion loses are that most devices usedfor airflow measurement introduce additional airflow resistance,leading to reduced total system airflows or, more significantlyfor multiple-branch systems, reduced airflow onlythrough the branch currently being measured. The experimentalwork comprised a series of tests where airflow wasmeasured using a range of measurement techniques anddevices. An experimental apparatus was built that could beconfigured as a one-, two-, or three-branch return system. Theapparatus used a typical gas furnace with two different blowermotors—one permanent split capacitor and one brushlesspermanent magnet. The measured airflows were compared toreference airflow measurements using inline airflow metersbuilt into the test apparatus. The experimental results showedthat some devices had reasonable results (typical errors of 5%or less) but others hadmuch larger errors (up to 25%). Becausemanufacturers' accuracy estimates for their equipment do notinclude many of the sources of error found in actual fieldmeasurements (replicated in the laboratory testing in thisstudy) there is a need for a test method that could be used todetermine the actual uncertainty in this specific application.Toaddress this need, a draft test method has been prepared fordetermining the uncertainty of airflow measurements at residentialHVAC terminals. This test method, when finalized, canbe used by entities to specify required accuracy of measurementdevices used to show compliance with standards.