This test method details the calibration and testing procedures and necessary additional temperature instrumentation required in applying Test Methods C 236
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
It should be recognized that the only true experimental measurement is the thermal transmittance, US, value determined in Section 7. The “standardized” thermal transmittance value, UST, obtained by either the Calibration Transfer Standard (CTS) or area weighting (AW) methods described in Section 8 include adjustments to the US value that are made because the current computer calculation methods (NFRC 100-97) for determining the thermal transmittance are not capable of applying the actual surface heat transfer coefficients that exist on the test specimen while testing at standardized conditions. The current computer calculation methods assume that uniform standardized surface heat transfer coefficients exist on the indoor and outdoor fenestration product surfaces, which is not the case. Until such a time that the computer calculation methods are upgraded to have the actual surface heat transfer coefficients applied to the actual fenestration product geometry, the modification of the true tested thermal transmittance value, US, to a standardized value UST, is necessary for rating and comparison (measured with calculated) purposes.
It should be noted that the standardized surface heat transfer coefficients, hh and hs, as calibrated prior to testing a fenestration product using an appropriately sized Calibration Transfer Standard (CTS) may 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 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 should be 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 coef........
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