Thermal transmittance

The heat transfer coefficient, and U- value ( in building physics formerly k-value) called, is a measure of the heat flow of a fluid ( a gas or a liquid) by a solid body (such as a wall ) into a second fluid due to a temperature difference between the fluids. In the case of a flat wall it indicates the flow of heat ( thermal energy per time ) per surface of the wall and per Kelvin temperature difference of the two fluids. Its SI unit is therefore W / ( m² · K ) ( watts per square meter per Kelvin). As symbols k is typically used (especially in mechanical and process engineering ) or U ( especially in construction ). The heat transfer coefficient depends on the heat transfer coefficient between the fixed body and the fluid, and the thermal conductivity, and geometry of the solid body. The following statements apply in the construction industry and are special cases of process engineering and thermal apparatus construction.

The heat transfer coefficient is a specific characteristic of a component. It is essentially determined by the thermal conductivity and thickness of the materials used, but also by the heat radiation and convection at the surface.

The reciprocal of the heat transfer coefficient is the thermal resistance RT in (K · m²) / W..

• The higher the heat transfer coefficient, the lower the thermal insulation property of the substance.
• The higher the thermal resistance, the better the thermal insulation property.

Particularly widespread use is the heat transfer coefficient in the building where he used to determine the transmission heat losses through components.

• 2.1 names
• 2.2 Ideal wall
• 2.3 components
• 2.4 Measurement of the U- value of building components and materials
• 2.5 Typical values ​​of construction

Definition and meaning

The heat flux ( the SI unit Watts / meter ²) by a component that is exposed on one side of the outside air temperature and on the other side of the indoor air temperature, at the steady state is proportional to the temperature difference, with the constant of proportionality:

The derived SI unit of the U- value W / (m² · K ) with the unit symbol W for the unit watts, and K for Kelvin.

He thus describes the amount of heat energy in Joules ( = watt-seconds ) that is allowed to pass through the period of one second, an area of ​​one square meter by a dividing wall between two rooms when the mutually adjacent temperatures stationary (ie, not only during the measuring seconds ) by 1 K (equivalent to a difference of 1 ° C) differ. The unit watt-seconds of energy comes in the final unit of the U- value before no longer explicitly, since the time component ( " seconds ") in the energy unit wegkürzt against time component in the denominator ( " per second "). You could be the unit of U accordingly as J / (s · m² · K) conceive.

The thus defined U-value is thus a measure of the ' heat transmission "or the heat insulating properties of components, for example, a particular glazing of a window. A component with a small U- value can not so much as heat by a component with a larger U-value. The during the time period? T by the area A, which entered amount of heat Q is

Seen, the heat flux between the two sides adjacent to a component of the media (for example, indoor air to outside air) here. Not to know the characteristics of the entire component, but the materials used determine ( eg surface temperature inside to the outside ), so instead of the heat transfer coefficient of the component has to be heat conductivity coefficient to use ( see also below).

Limitations of definition

The defining equation ( 1) assumes steady-state conditions and is not suitable to calculate the instantaneous heat flux at each time-varying temperatures. So enter as in a heating process due to the heat storage capacity of the component delay effects, to calculate when attempting the surface heat fluxes using equation (1 ) are not included. However, during the subsequent cooling process, the error occurs in the reverse direction. When heating and cooling take place symmetrically to each other, the two errors cancel. As can be seen, equation (2) applies in the case of strictly periodic running temperature changes as before, when it is used for the calculation of lost during a period? T amount of heat Q and the over the period averaged temperature averages and applied:

The temperature changes occurring in reality are never strictly periodic, the error caused thereby, but is related only with the slightly different heat content of the component at the beginning and end of the period under consideration and is therefore limited. It is against the with increasingly longer viewing time? T ever-increasing total heat loss Q finally completely negligible, provided that the building is exposed to climatic conditions under which it suffers a transmission heat loss in the longer term means:

The U-value is, therefore, despite its initially limited to steady-state conditions defined under real transient conditions is a suitable measure for a longer period of time summed transmission heat losses through the standard cross- section of a component, which are caused by different mean temperatures of the indoor and outdoor air. This is the basis its significance as an important criterion in the energy rating of a building.

Comparison of stationary and non-stationary behavior

In the figure alongside these relationships are illustrated using a concrete example. Consider a 40 cm thick solid brick masonry with a U- value of 1.2 W / ( m² K), which is exposed on the outside of the reproduced in the upper part of the image outside air temperatures, while a temperature of 20 ° C is applied consistently on the inside. When the outside air temperature is fünfminütliche real measurement data from seven days in May 2006.

The orange curve in the lower part shows the heat flow through the outer surface of the wall, as he was also determined by means of an unsteady calculation program in five- minute increments from the data (positive currents flow into the wall, negative flows out ). The strong fluctuations of the heat flux clearly show the transient nature of the situation. The mean value of outside air temperature during the observed seven days is 11.9 ° C. Therefore, the U- value indicates an average heat loss

Advance. This value is entered as a blue line. The red curve in the lower part shows the cumulative average value of the heat flow, so the mean value of successively five minutes, ten minutes over fifteen minutes, etc. up to the right, finally, the mean value over the entire seven days is obtained. As can be seen clearly convey with increasing averaging period, the transient fluctuations of the heat flow away quickly and approach within the seven days already almost perfectly predicted by the U-value mean to.

The cumulative average is initially systematically over the U-value - result, because according to previous cooler days (not shown here ) warming up the wall first above-average heat flow required into the wall. Even this difference is no longer relevant after several days averaging.

For simplicity, heat inputs were not recognized by solar radiation here. You could take into account, for example by appropriately increasing the outside air temperatures ( to so-called radiation or combined air temperatures outside temperatures ). This has no effect on the mathematical relationships and the general behavior.

Calculating the U-value of building components and materials

The calculation of the heat transfer coefficient for the public proof in civil engineering is based on the calculation steps in accordance with EN ISO 6946, where more complicated construction-related cases are treated. The design values ​​required are specified in EN 12524 and DIN 4108-4.

Designations

Designations for heat transfer coefficient of windows, Unit W / (m² K):

Ideal wall

In the event of a flat, infinitely extended wall, which is composed of consecutive layers of the thickness and the thermal conductivity, the proportionality constant is calculated by:

With

Components

The thermal transmittance of a component depends on the thermal conductivities of the materials used and their layer thicknesses as well as on the component geometry ( plane wall, cylindrical curved pipe wall, etc.) and the transition conditions to the component surfaces.

In general, the thermal resistance from the sum of the thermal resistances of each successive component layers and the heat transfer resistance is to the surrounding fluids (air, water, etc.) on the two surfaces together and is the reciprocal of the heat transfer coefficient:

Measurement of the U- value of building components and materials

The determination of accurate heat transmission coefficient for the certification of building material characteristics are made of material research and testing institutes on behalf of the manufacturer of complex test equipment to ensure comparable conditions.

With a special temperature sensor for U- value determination, a compatible meter and another temperature sensor of the heat transfer coefficient (U - value) can be non-destructively determine a component of the application ( eg building site). The U-value is an important indicator for assessing the thermal performance of the building envelope. For the measurement of the U- value are determined:

• Ambient temperature Ta
• Internal temperature Ti
• Surface temperature Tw of the component (s).

To measure the outdoor temperature radio sensor is being used. All data are recorded on a measurement program in the meter, stored and then analyzed using the software and documented.

Measuring the respective temperatures and determining the differences is easy. For reasonably reliable measurement results, the following requirements must be met:

• Temperature difference between inside and outside, ideally > 15 K
• Constant conditions
• No sunlight
• No heat radiation in the measuring range.

On the other hand is more difficult to determine the heating power supplied, the yes causes the temperature differences between inside and outside. Waste heat from adjacent rooms, lighting, measuring devices, eg Computer, give off heat. The staff can help with about 80 W per capita for heating and affect the result.

Therefore, there are mainly the night or early morning hours before sunrise. This also applies to the thermography, which has also been used in the past for the approximate determination of the U- value.

The comparability of the results, and the relevance of the U- value for the assessment of heat loss in a building is despite clear measurements disputed by some, for example, even when they themselves have participated in the measurements ( Bossert ).

Typical values ​​of construction

Importance for heat insulation

After which came on 10 October 2009 in Germany in force regulation amending the Energy Saving Ordinance (EnEV ) have the annual primary energy demand QP and the specific transmission heat loss H'T (for non-residential buildings: on the nature of the components mean heat transfer coefficient of heat transfer surrounding surfaces ) to be constructed of a comply with building certain limits. U- values ​​are used to calculate the transmission heat loss and this in turn in the calculation of primary energy demand. Furthermore, the Energy Saving Ordinance prescribes limit values ​​of the heat transfer coefficients of certain components when they are replaced in existing buildings or built new.

Standardize

• EN ISO 6946, DIN as: 1996-11 Parts - Thermal resistance and thermal transmittance - Calculation method
• EN ISO 7345, DIN as: 1996-01 Thermal insulation - Physical quantities and definitions (replaces DIN 4108-1 )
• EN ISO 9346, DIN as: 1996-08: Thermal insulation - Mass transfer - Physical quantities and definitions
• EN 673, DIN as: 2003-06 Glass in building - Determination of thermal transmittance ( U value) - Calculation method
• EN 12524 Building materials and products - Thermal properties - Tabulated design values
• DIN 4108 Thermal insulation in buildings, provides additional requirements for U- values ​​of components, but not with the goal of energy saving, but the prevention of structural damage ( minimum energy )