Back

Guarded hot box apparatus for measurement of thermal transmittance of building elements such as doors and windows.

There are two types of hot-box – the guarded hot-box (an absolute method, illustrated right) and the calibrated hot-box (a secondary method similar in form but which requires calibration using panels or materials of known thermal conductance). They are used at ambient temperatures to measure the thermal performance of building envelope components such as walls, roofs and windows.

Hot-boxes measure the overall heat transfer (normally in the steady state but a calibrated hot-box can also make dynamic measurements) through large, inhomogeneous structures. A hot-box measures either thermal transmittance (or U-value) which determines the overall heat transfer through a structure; or it measures thermal conductance and thermal resistance, which are concerned with heat flow within the structure.

The measurement principle is straightforward: a large specimen panel, usually in excess of 1.5m x 2.5m and typical of the construction form, is well instrumented on each surface with temperature sensors arranged to provide a representative temperature distribution across the surface of the panel. It is placed between hot and cold chambers operating at controlled fixed temperatures, humidity and air flow conditions. Temperature sensors are placed at positions approximately opposite those in the specimen to obtain the corresponding air temperatures. The d.c. power required to maintain the temperature difference across the external environments is measured together with the air temperatures, the temperatures of the surfaces in radiant exchange with the specimen and in some circumstances, the specimen surface temperatures (for thermal conductance or resistance).

In the guarded hot-box, a central metering box that covers a representative area of the panel is surrounded by an outer guard box (see figure). The temperature and air-flow conditions in the guard box are arranged to reduce the heat transfer through the metering box walls to negligible levels. The total measured power into the metering box from all sources is taken as the power though the specimen (with some small corrections).

In the calibrated hot-box there is no inner metering box. The outer walls of the hot chamber are made with very thick insulation to minimise conduction losses and the power flow through those walls is measured for a range of hot chamber and laboratory temperatures, using the calibration panels. The power through the specimen is then derived by appropriately correcting the measured power into the hot chamber. Ideally, both types of apparatus need to be designed so that measurements can be made at a variety of orientations, in order to include the important effects of convection which can be a significant component of eat transfer, particularly in roof specimens.

Hot-box measurement standards

The design and operation of both types of hot-box are specified in the standard EN ISO 8990:1996. And there are several international and European standards specifying hot-box measurement procedures for specific structures, as follows:

EN 12412-2 Thermal performance of windows, doors and shutters — determination of thermal transmittance by hot box method — Part 2: frames

EN 12412-4 Thermal performance of windows, doors and shutters — determination of thermal transmittance by hot box method — Part 4: roller shutter boxes

EN ISO 12567-1 Thermal performance of doors and windows – determination of thermal transmittance by hot box method

Key requirements

1.		Ensure that the metering section is truly representative of the inhomogeneous construction being measured
2.		Ensure that sufficient temperature sensors are used in appropriate positions on the specimen surfaces and in the air opposite, in order to provide representative area- weighted temperatures of each
3.		Good attachment of temperature sensors on the specimen surfaces is critical. For the guarded box ensure that all power sources (fans etc) within the guard section are considered for inclusion in the measurement of the total power

References

[1]		K R Solvason, 1965, ASHRAE Transactions, Vol. 65
[2]		J R Mumaw, 1974, Heat Transmission Measurements in Thermal Insulations, ASTM STP 544, Ed R P Tye, ASTM International, West Conshohocken, PA, pp 193-211
[3]		J R Mumaw, 1980, Thermal Insulation Performance, ASTM STP 718, Eds D L McElroy and R P Tye, ASTM International, West Conshohocken, PA, pp 195-207
[4]		E L Perrine, P W Linehan, J W Howanski and L S Shu, 1981, Thermal Performance of the Exterior Envelopes of Buildings, ASHRAE SP28, American Society of Refrigeration and Air-Conditioning Engineers, Inc, New York, NY, pp 237-249 (see also P R Achenbach, ibid, pp 308-319 and J L Rucker and J R Mumaw, ibid, pp 237-249)
[5]		A G Lavine, J L Rucker, K E Wilkes, 1983, Thermal Insulations, Materials, and Systems for Energy Conservation in the 80s, ASTM STP 789, Eds F A Govan, D M Greason, J D McAllister, ASTM International, West Conshohocken, PA, pp 234-247
[6]		Van Geem, G J Martha, J Testing and Evaluation, May 1987, pp 178-184
[7]		E Bales, 1988, ASTM, Hot box round-robin, ORNL/Sub/84/97333/2, Oak Ridge National Laboratory, Oak Ridge, TN

Back