Basics of heat flow measurement
Measuring thermal transmittance (U) and heat flow
The heat transfer characteristics of any structural element depend on the thermal conductivity of the materials used, on the thickness of its various component layers, on its structural geometry (e.g. flat or cylindrically curved walls, etc.), and on the ambient conditions at the structure’s surfaces inside and outside.
The thermal transmittance coefficient (U value) of a structural element describes the quantity of heat that passes through it from one side to the other (no matter how many layers) per second and per square meter surface at a constant difference in ambient temperature inside / outside of 1K. This thermal transmittance coefficient (U) thus also includes the surface heat transfer coefficients, i.e. the thermal energy transferred at the boundary surfaces, interior air - structure - exterior air. The thermal transmittance coefficient (U) is measured in in (W/m²K) and is internationally defined in standard ISO 6946.
A structure’s thermal transmittance coefficient (U) is the reciprocal of its total thermal resistance coefficient (R); R is the sum of the thermal transmission resistances between the structure’s various contiguous layers and also the surface heat transfer resistances between the structure and the ambient media on either side (e.g. air).
Total thermal resistance (R) = thermal transmission resistances through the material + surface heat transfer resistances, inside and out
The thermal transmittance coefficient (U value) is an important rating in civil engineering and the construction industry where it is used to define a building’s transmission heat loss through its various structural elements. Transmission heat loss is the term used to describe the energy-saving qualities of a building’s shell (i.e. the thermal insulation of its roof, outside walls, windows, and floors). In Germany each residential structure is assigned a permissible maximum U value (depending on its external surface area and its internal volume); this is based on the most recently amended version of the Energieeinsparverordnung (EnEV) (German energy-saving legislation)
ALMEMO® Measuring system for Measuring thermal transmittance (U) and heat flow
The thermal transmittance coefficient (U value) is an important rating in civil engineering and the construction industry where it is used to define a building’s transmission heat loss through its various structural elements. It is now possible, with the ALMEMO® measuring system, to measure and record all the physical parameters for the component parts of existing buildings (e.g. walls, etc.) in order to calculate their U value and other relevant thermal energy coefficients.
The measuring principle involved in quantifying heat loss at partition elements, e.g. walls, heating systems, etc., is based on the method which uses a heat flow plate (sensor) fitted on the surface of the structural element and thus incorporated directly in the heat flow. Using the known thermal characteristics of the heat flow plate and the thermo-electrically measured temperature gradient inside the heat flow plate the ALMEMO® measuring system can thus measure the heat flow density q in W/m².
The ALMEMO® measuring system can also be used to measure the surface temperatures on either side the structural element and the respective air temperatures immediately inside and outside; based on these results it is then possible to calculate all the relevant thermal coefficients.
The temperatures and heat flow density data on which these calculations are based are acquired cyclically as average values. Any influence that the structure’s own thermal capacity may have on these calculations (e.g. time shifts between temperature and heat flow, affecting calculation of the U value) will, given a sufficiently long measuring period, become negligible and the calculated average value will certainly be very close to the structure’s actual U value.
To ensure a stable and meaningful U value calculation it is possible to stipulate that measuring operations only be performed subject to certain specified conditions.
- The temperature difference between interior and exterior ambient air must be sufficiently large (typically 20 K, e.g. inside temperature 20°C and outside temperature 0°C).
- Any fluctuations in these temperatures (e.g. day / night) must throughout the measuring period be as small as possible.
- The measured values must be acquired and recorded on-site over a sufficiently long period (e.g. one whole day or even several days) and the parameters must be calculated on the basis of average values