Hot Box

Experimental procedure and uncertainty analysis of a
guarded hotbox method to determine the thermal
transmission coefficient of skylights and sloped
glazing

Elmahdy, A.H.; Haddad, K.

NRCC-43378

A version of this document is published in / Une version de ce document se trouve dans :
ASHRAE Transactions, v. 106, pt. 2, 2000, pp. 601-613

www.nrc.ca/irc/ircpubs

ExperimentalProcedure and Uncertainty Analysis of a Guarded HotBox Method to
Determine the Thermal Transmission Coefficient of Skylights and Sloped Glazing
A. H. Elmahdy and K. Haddad
ABSTRACT
A laboratory test procedure and a guarded hot box facility to determine the
thermal transmission coefficient of skylights are described. Skylights can be tested at an
angle of inclination that can be varied from 20 to90o off the horizontal. The
experimental procedure, to determine the skylight R-value, is based on a correlation for
the convective heat transfer on the warm side and the weather side of the test specimen.
This correlation is obtained from calibration measurements using a calibrated transfer
standard (CTS) in the inclined position. The results show that the convective heat
transfer along aninclined CTS is substantially different from that along a vertical CTS.
Three-dimensional simulations of the conduction heat transfer within the CTS assembly
are found to be in good agreement with the experimental results. A complete error
analysis for the calibration experiments and for skylight testing is presented. It is found
that the experimental relative uncertainty associated with thethermal transmission
coefficient of the test specimen without the frame is of the order of ±6%.
INTRODUCTION
For years, the overall thermal transmission coefficient (U-factor) of fenestration
systems were determined in guarded hot box facilities, where the specimens are mounted
in a vertical frame separating the two environmental chambers. Recently, the
determination of the overall thermaltransmission coefficient of skylights and sloped
glazing at an inclined position was discussed in the research and technical communities
as a requirement to assess the energy rating and labelling of this type of products.
Correlation of film heat transfer coefficient on a vertical surface in a guarded hot
box environment was documented earlier in the literature [Elmahdy, 1992]. However,
scarceexperimental data about the variation of the film heat transfer coefficient over the
surface of fenestration products in inclined position is extremely scarce. The focus of
this paper is on the development of a research class guarded hot box facility to enable the
accurate determination of the overall heat transfer coefficient (U-factor) of skylight and
sloped glazing systems. This is inresponse to the needs of industry, researchers and
standards writing authorities. Also, the results of this research work are critical for
product certification and labelling, which is demanded by industry and building code
officials.
ENVIRONMENTAL TEST FACILITY
The guarded hot box facility was designed originally to test windows in the
vertical position. Detailed description of this facility andthe associated uncertainty
analysis can be found in Elmahdy (1992). In order to accommodate the testing of
skylights mounted at an angle off the horizontal, the calorimeter and the surround panel

assembly were rotated about its central axis, as shown in Figure 1. Presently, the facility
can be used to test special fenestration systems with inclinations ranging from 20 to 90o
off thehorizontal.
A total of three fans were installed on the weather side to mix the air and to obtain
the desired heat transfer coefficient on this side of the test sample, by means of changing
the speed of the fan motors. On the room side, the heat was delivered through a constant
temperature baffle and a convection heater. The baffle and the convection heaters are
evaporating/condensing heat...