NPL's Rotatable Hot Box Apparatus

Hot box apparatus are used to measure the overall heat transfer through large, non-homogeneous building structures (thermal transmittance or U-value (W/m².K). Although thermal transmittance has the same units as thermal conductance, it differs in that it is applicable to non-homogenous structures (e.g. windows and doors) and that it includes the heat transfer processes associated with the heat transfer into the structure from the warm environment and the heat transfer out of the structure to the cold environment.

There are two classic types of Hot Box Apparatus – Guarded Hot Box and Calibrated Hot Box. NPL have designed and built a special type of hot box apparatus which is referred to as an 'all Guarded Hot Box'.

The general standard for the design and operation of Hot Box Apparatus is BS EN ISO 8990, and both NPL hot boxes comply with the requirements of that standard.

The thermal performance of a structure is influenced by orientation when it contains air cavities and one of the NPL hot box apparatus (pictured right) is capable of being rotated to allow the measurement to be carried out with the test element in any orientation with respect to the horizontal. All orientations are possible, from vertical to horizontal with the cold chamber on top of the warm chamber through to horizontal with the warm chamber on top of the cold chamber.

We also have a special fixture that enables the thermal performance of heavy masonry structures to be measured.

Measurements conditions

These are usually:

• The warm chamber at 22 °C and the cold chamber at 2 °C. The cold chamber of both hot box apparatus can however be reduced to –10 °C
• The air in the warm chamber circulating at < 0.2 m/s and the air in the cold chamber circulating at ~ 2.6 m/s

Time taken to carry out a measurement

• A minimum of 1 day to install the sample in the hot box
• A minimum of 3 days to reach thermal equilibrium and take readings over a 8 hour period

Accreditation

NPL's hot box facility is accredited by UKAS to carry out measurements to:

• BS EN ISO 8990 (Thermal insulation – Determination of steady-state thermal transmission properties – Calibrated and guarded hot box)
• BS EN ISO 12567-1 (Thermal performance of windows and doors - Determination of thermal transmittance by hot box method - Part 1: Complete windows and doors)
• BE EN ISO 12567-2 (Thermal performance of windows and doors - Determination of thermal transmittance by hot box method - Part 2: Roof windows and other projecting windows)

Contact

Customer Services tel: +44 20 8943 6245
E-mail: thermal_enquiries@npl.co.uk

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