Standard Test Method for Measuring the Steady-State Thermal Transmittance of Fenestration Systems Using Hot Box Methods
This test method details the calibration and testing procedures and necessary additional temperature instrumentation required in applying Test Method C 1363 to measure the thermal transmittance of fenestration systems mounted vertically in the thermal chamber.
4.2 The thermal transmittance of a test specimen is affected by its size and three-dimensional geometry. Care must be exercised when extrapolating to product sizes smaller or larger than the test specimen. Therefore, it is recommended that fenestration systems be tested at the recommended sizes specified in Practice E 1423 or NFRC 100.
Since both temperature and surface heat transfer coefficient conditions affect results, use of recommended conditions will assist in reducing confusion caused by comparing results of tests performed under dissimilar conditions. Standardized test conditions for determining the thermal transmittance of fenestration systems are specified in Practice E 1423 and Section 6.2. The performance of a test specimen measured at standardized test conditions is potentially different than the performance of the same fenestration product when installed in the wall of a building located outdoors. Standardized test conditions often represent extreme summer or winter design conditions, which are potentially different than the average conditions typically experienced by a fenestration product installed in an exterior wall. For the purpose of comparison, it is essential to calibrate with surface heat transfer coefficients on the Calibration Transfer Standard (CTS) which are as close as possible to the conventionally accepted values for building design; however, this procedure can be used at other conditions for research purposes or product development.
Likewise, it would be desirable to have a surround panel that closely duplicates the actual wall where the fenestration system would be installed. Since there are such a wide variety of fenestration system openings in North American residential, commercial and industrial buildings, it is not feasible to select a typical surround panel construction for installing the fenestration system test specimen. Furthermore, for high resistance fenestration systems installed in fenestration opening designs and constructions that have thermal bridges, the large relative amount of heat transfer through the thermal bridge will cause the relatively small amount of heat transfer through the fenestration system to have a larger than desirable error. For this reason, the Calibration Transfer Standard and test specimen are installed in a homogeneous surround panel constructed from materials having a relatively high thermal resistance. Installing the test specimen in a relatively high thermal resistance surround panel places the focus of the test on the fenestration system thermal performance alone. Therefore, it is important to recognize that the thermal transmittance results obtained from this test method are for ideal laboratory conditions, and should only be used for fenestration product comparisons unless the thermal bridge effects that have the potential to occur due to the specific design and construction of the fenestration system opening are included in the analysis.
4.5 This test method does not include procedures to determine the heat flow due to either air movement through the specimen or solar radiation effects. As a consequence, the thermal transmittance results obtained do not reflect performances that may be expected from field installations. It is possible to use the results from this test method as input to annual energy performance analyses which include solar, and air leakage effects to get a better estimate of how the test specimen would perform when installed in an actual building. To determine the Solar Heat Gain Coefficient of fenestration products, refer to NFRC 2010. To determine air leakage for windows and doors, refer to Test Methods E 283 and E 783.
It is important to recognized that the thermal transmittance, US, value determined in Section is the only true experimental measurement result of this test method. The “standardized” thermal transmittance value, UST, obtained by either the Calibration Transfer Standard (CTS) or Area Weighting (AW) methods described in Section include adjustments to the thermal transmittance value bases on results from calibration runs described in Section 6. The standardized thermal transmittance is useful for two reasons; it facilitates comparison of test results between different laboratories with different thermal chamber geometries and configurations, and it improves the comparison between test results and computer simulation results. Due to the differences in size, geometry, and climate chamber air flow permitted by this test method, Test Method C 1363, and Practice E 1423, there can be significant variations in the local surface heat transfer coefficients on the same test specimen installed in different laboratories even though these laboratories measured identical surface heat transfer coefficients on their Calibration Transfer Standards. Inter-Laboratory Comparisons conducted by the NFRC have shown that the effect of this variation is reduced if the standardized thermal transmittance is used for comparison instead of the thermal transmittance. The standardized thermal transmittance is also a useful tool for the evaluation and comparison of experimental results of fenestration systems with computer calculations of the thermal transmittance. that are made because the current Historically, computer calculation methods (NFRC 100) for determining the thermal transmittance were not capable of applying the actual surface heat transfer coefficients that exist on the test specimen while testing at standardized conditions. These current computer calculation methods assumed that uniform standardized surface heat transfer coefficients exist on the indoor and outdoor fenestration product surfaces. Although the next generation of computer simulation programs includes improved radiation heat transfer algorithms, which generate non-uniform surface heat transfer coefficients, the standardized thermal transmittance remains to be a useful tool when comparing test results to computer modeling results.
4.6.1 It is important to recognized that due to radiation effects, the room side or weather side temperature (th and tc, respectively), has the potential to differ from the respective room side or weather side baffle temperatures (tb1 and tb2, respectively). If there is a difference of more than ±1 °C(±2 °F), either on the room side or weather side, the radiation effects shall be accounted for as described in Sections 6 and 9 to maintain accuracy in the calculated surface heat transfer coefficients. Calculating the radiation exchange for highly conductive test specimens or projecting fenestration products as described in A1.1 is not a trivial task.
4.6.2 The calculation of the standardized thermal transmittance assumes that only the surface heat transfer coefficients change from the calibrated standardized values for the conditions of the test. This assumption may not be valid if the surface temperature differentials for the standardized calibration conditions are different from the surface temperature differential that exists on the test specimen during the test. Currently, specifications for the Calibration Transfer Standard give it a thermal transmittance of 1.7 W/(m2·K) [0.3 Btu/(hr·ft2·°F)]. Accordingly, the calculation of the standardized thermal transmittance produces the least error when performed on test specimens with a similar thermal transmittance.
It is important to note that the standardized surface heat transfer coefficients, hh and hc, as calibrated prior to testing a fenestration product using an appropriately sized Calibration Transfer Standard (CTS) have the potential to differ from the surface heat transfer coefficients that exist during a hot box test on a specific test specimen. Fenestration systems usually have frame and sash surfaces that introduce two- and three-dimensional convective heat transfer effects which result in variable surface heat transfer coefficients, which differ from the uniform standardized values. As a result of this, the test specimen surface heat transfer coefficients will differ from those obtained with the non-framed, essentially flat Calibration Transfer Standard tested under the same conditions. In this standardizing procedure, it is assumed that the differences are small enough so that the calibration surface heat transfer coefficients can be used to calculate equivalent test specimen average surfaces temperatures, t1 and t2, in order to estimate the actual test specimen surface heat transfer coefficients. It is important to recognized that this assumption will not be accurate for all fenestration products, especially for high thermal transmittance products where the surface heat transfer coefficients are a major portion of the overall thermal resistance and also for fenestration products with significant surface projections (for example, skylights, roof windows, garden windows) where the surface heat transfer coefficients are quite different from the standardized values.
In these situations, it is important to attempt to measure the test specimen surface temperature distributions and then calculate directly the test specimen average area weighted surfaces temperatures, t1 and t2. This area weighting (AW) method also has problems in that the placement of temperature sensors to get an accurate area weighting is not known, especially on high conductivity horizontal surfaces that act as heat transfer extended surfaces (that is, fins). In addition, the placement of many temperature sensors on the test specimen surfaces will affect the velocity fields in the vicinity of these surfaces which will affect the surface temperatures and surface heat transfer coefficients.
1.1 This test method covers requirements and guidelines and specifies calibration procedures required for the measurement of the steady-state thermal transmittance of fenestration systems installed vertically in the test chamber. This test method specifies the necessary measurements to be made using measurement systems conforming to Test Method C 1363 for determination of fenestration system thermal transmittance.
Note 1—This test method allows the testing of projecting fenestration products (that is, garden windows, skylights, and roof windows) installed vertically in a surround panel. Current research on skylights, roof windows, and projecting products hopefully will provide additional information that can be added to the next version of this test method so that skylight and roof windows can be tested horizontally or at some angle typical of a sloping roof.
1.2 This test method refers to the thermal transmittance, U of a fenestration system installed vertically in the absence of solar radiation and air leakage effects.
Note 2—The methods described in this document may also be adapted for use in determining the thermal transmittance of sections of building wall, and roof and floor assemblies containing thermal anomalies, which are smaller than the hot box metering area.
1.3 This test method describes how to determine the thermal transmittance, US of a fenestration product (also called test specimen) at well-defined environmental conditions. The thermal transmittance is also a reported test result from Test Method C 1363. If only the thermal transmittance is reported using this test method, the test report must also include a detailed description of the environmental conditions in the thermal chamber during the test as outlined in 10.1.14.
1.4 For rating purposes, this test method also describes how to calculate a standardized thermal transmittance, UST, which can be used to compare test results from laboratories with vastly different thermal chamber configurations, and facilitates the comparison to results from computer programs that use standard heat transfer coefficients to determine the thermal transmittance of fenestration products. Although this test method specifies two methods of calculating the standardized thermal transmittance, only the standardized thermal transmittance result from one method is reported for each test. One standardized thermal transmittance calculation procedure is the Calibration Transfer Standard (CTS) Method and another is the Area Weighting (AW) Method (see Section 9 for further descriptions of these two methods). The Area Weighting method requires that the surface temperatures on both sides of the test specimen be directly measured as specified in Practice E 1423 in order to determine the surface heat transfer coefficients on the fenestration product during the test. The CTS Method does not use the measured surface temperatures on the test specimen and instead utilizes the calculation of equivalent surface temperatures from calibration data to determine the test specimen surface heat transfer coefficients. The AW shall be used whenever the thermal transmittance, US, is greater than 3.4 W/(m2·K) [0.6 Btu/(hr·ft 2·°F)], or when the ratio of test specimen projected surface area to wetted (that is, total heat transfer or developed) surface area on either side of the test specimen is less than 0.80. Otherwise the CTS Method shall be used to standardize the thermal transmittance results.
1.5 A discussion of the terminology and underlying assumptions for measuring the thermal transmittance are included.
1.6 The values stated in SI units are to be regarded as the standard. The values given in parentheses are provided for information purposes only.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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