# Transmission heat loss

The transmission heat loss is one of the power losses that occur in a heated building through the release of energy to the environment.

Transmission heat loss is based on a temperature difference and thus on the 1st law of thermodynamics . The transmission heat loss (and the ventilation heat loss) is an energy flow that is specified in the unit of power, watts . The transmission heat loss over a certain period of time gives an amount of heat . This has the unit Ws .

Further energy losses occur in buildings such. B. by ventilation and waste water ( specific heat capacity of water, air and latent heat in water vapor), the heat storage of the building mass as well as any heat input from outside by solar radiation and atmospheric counter-radiation must also be taken into account in the calculation. If the internal temperature is to be maintained, the sum of the heat losses must be replaced by thermal heat. The heating power required for this is called the heating load .

## Meanings of the term "transmission heat loss"

The word "transmission heat loss " describes a heat flow ( power loss , in watts ) or a quantity of heat ( work , mostly in watt hours ).

The following compilation lists all common uses of the term “transmission heat loss”, shows the relationships and names their useful applications in building physics and in EnEV.

### Transmission heat loss

In the EnEV, this is called the transmission heat transfer coefficient or heat transfer coefficient for transmission .

${\ displaystyle H _ {\ mathrm {T}} \ qquad \ mathrm {[W / K]}}$ ${\ displaystyle H _ {\ mathrm {T}} = \ Sigma (U _ {\ mathrm {i}} * A _ {\ mathrm {i}} * F _ {\ mathrm {Xi}}) + U _ {\ mathrm {WB} } * A}$ ${\ displaystyle F _ {\ mathrm {Xi}} = (\ upsilon _ {R} - \ upsilon _ {x}) / (\ upsilon _ {R} - \ upsilon _ {e})}$ ( are the temperature correction factors that are dependent on the type of component (floor or exterior wall), the room temperature, standard outside temperature and the adjacent room temperature.)${\ displaystyle F _ {\ mathrm {Xi}}}$ ### Specific transmission heat loss

In the EnEV, this is called the specific transmission heat transfer coefficient .

The transmission heat loss based on the heat-transferring envelope surface A gives the specific transmission heat loss H ' T

${\ displaystyle H '_ {\ mathrm {T}} = H _ {\ mathrm {T}} / A \ qquad \ mathrm {[W / m ^ {2} K]}}$ H ' T is clearly the average heat flow through 1 m 2 of envelope surface with a Kelvin temperature difference (inside to outside). It is therefore the mean U-value of the building envelope. The averaging is weighted according to the area size of an envelope element. A high insulation quality of the envelope surface is at a low value H ' T before.

H ' T indicates the quality of the envelope surface with regard to its thermal insulation properties. The lower H ' T, the better the thermal insulation property.

Because of the still existing influence of the building's compactness (ratio of area to volume), this does not mean that the transmission heat loss per m 2 of usable area of ​​the building is then also low. In the Energy Saving Ordinance 2006 there is a regulation on the maximum values ​​of H ' T for new buildings to be erected (whereby the requirement is strictest and the maximum value is lowest in the case of low compactness, i.e. large A / V )

### Specific transmission heat transfer coefficient according to ISO13789

Caution: In the EnEV H T as transmission heat transfer coefficient referred

${\ displaystyle H _ {\ mathrm {T}} = H _ {\ mathrm {D}} + H _ {\ mathrm {g}} + H _ {\ mathrm {U}} + H _ {\ mathrm {A}} \ qquad \ mathrm {[W / K]}}$ • ${\ displaystyle H _ {\ mathrm {D}}}$ ... direct heat transfer coefficient between the interior and the exterior
• ${\ displaystyle H _ {\ mathrm {g}}}$ ... stationary heat transfer coefficient over the ground
• ${\ displaystyle H _ {\ mathrm {U}}}$ ... transmission heat transfer coefficient through unheated rooms
• ${\ displaystyle H _ {\ mathrm {A}}}$ ... transmission heat transfer coefficient to adjacent buildings

It forms the sum of the transmission losses of all components to the outside air, the ground and through adjacent unheated rooms.

#### Transmission heat loss from the components to outside air

Describes the direct transmission between the interior and the exterior

${\ displaystyle H _ {\ mathrm {D}} = \ sum \ nolimits _ {i} {A_ {i} U_ {i}} + \ sum \ nolimits _ {k} {l_ {k} \ Psi _ {k} } + \ sum \ nolimits _ {j} {\ chi _ {j}} \ qquad \ mathrm {[W / K]}}$ • ${\ displaystyle A_ {i}}$ ... area of ​​the component
• ${\ displaystyle U_ {i}}$ ... heat transfer coefficient
• ${\ displaystyle l_ {k}}$ ... length of the thermal bridge
• ${\ displaystyle \ Psi _ {k}}$ ... length-related heat transfer coefficient of the thermal bridge according to EN ISO 10211
• ${\ displaystyle \ chi _ {k}}$ ... point-related heat transfer coefficient of the thermal bridge

Indicates the heat flow in W of the outer air-touched envelope surface A per Kelvin temperature difference inside-outside.

Example : U = 0.51 W / (m 2 · K), heat-transferring envelope area A = 310 m 2H T = 0.51 W / (m 2 · K) · 310 m 2 = 158 W / K

### Specific transmission heat loss according to the enclosed space

${\ displaystyle H _ {\ mathrm {T}} = H '_ {\ mathrm {T}} \ cdot A / V \ qquad \ mathrm {[W / (m ^ {3} \ cdot K)]}}$ indicates the heat flow heat loss in watts of the entire envelope area A per degree of temperature difference inside-outside and per m 3 of enclosed space. It is a building parameter that not only includes the insulation quality of the envelope surface, but also the compactness of the building. A low value also means a low TWV per m 2 of usable area, with the conversion being based on the floor height. This building parameter best characterizes how well 1 m 2 of living space is protected against heat loss through transmission heat loss; the lower it is, the better

Example : H ' T = 0.51 W / (m 2 K), A / V = 0.71 1 / m ⇒ H T A / V = ​​0.51 W / (m 2 K) 0, 71 [1 / m] = 0.362 W / K per m 3 of enclosed space ⇒ approx. 0.12 W / K per m 2 of usable area

### Transmission heat loss with a given temperature difference inside – outside

${\ displaystyle {\ dot {Q}} _ {\ mathrm {T}} = H '_ {\ mathrm {T}} \ cdot \ Delta T \ qquad \ mathrm {[W]}}$ indicates the energy flow, transmission heat loss in watts of the entire envelope surface A , which is the heat output of the building through transmission at a given temperature difference inside-outside. This is a performance specification and to maintain the internal temperature, this value must be balanced by the same heating output (actually required heating output higher by ventilation heat loss and possibly required heating output and lower by solar and internal energy gains per unit of time)

Example : H T = 200 W / K, cold winter day with T a = −10 C, T i = 20 ° C ⇒ Δ T = 30 K ⇒ transmission heat loss = 200 W / K 30 K = 6 kW

### Transmission heat loss over a period of time (with a given time profile of the temperature difference inside-outside)

${\ displaystyle Q _ {\ mathrm {T}} = H '_ {\ mathrm {T}} \ cdot \ Sigma (\ Delta T \ cdot \ Delta t) \ qquad \ mathrm {[kWh]}}$ indicates the total amount of heat flowing away through transmission in kWh in the specified period. It is determined by the building parameter H T and the number of heating degree hours of the specified period.

Example 1 : H T = 200 W / K, 24 h cold winter day with T a = −10 ° C as the daily mean, T i = 20 ° C ⇒ (Δ T = 30 K) ⇒ Q T = 200 W / K · 24 h 30 K = 144 kWh
In other words, the total transmission heat loss of the building envelope on the cold winter day with the daily mean temperature of −10 ° C is 144 kWh, this heat loss and the additional heat loss from ventilation must be compensated for by the corresponding amount of heating heat so that the building's internal temperature is kept at 20.
Example 2 : H T = 200 W / K, entire heating period with 80,000 heating degree hours ⇒ Q T = 200 W / K 80,000 Kh = 16,000 kWh
This means that the total transmission heat loss of the building envelope in a heating period with 80,000 heating degree hours is 16,000 kWh.This heat loss and the additional ventilation heat loss (reduced by the sum of the solar and internal energy gains) must be compensated for by the corresponding amount of heating heat so that the building's internal temperature of 20 ° C is maintained at all times.

## Transmission heat loss of a building

The individual parts of the envelope surface of a heated building, through which heat escapes to the outside through thermal conduction , are:

• External walls (against outside air or soil), with separate consideration of windows, external doors and the like
• Roof, with separate consideration of roof windows and the like
• top floor ceiling against unheated rooms
• Cellar ceiling against unheated cellar rooms or floor slab
• Floor ceiling against outside air downwards (for example when driving through)
• Walls to other heated or unheated buildings or parts of buildings (for example in semi-detached and terraced houses)

The transmission heat loss of an element of the envelope surface (component of the envelope surface) depends on the heat transfer coefficient (= U-value of the element) and its area.

The heat storage of the building mass (building materials, stored moisture) as well as any heat input from outside through solar radiation and atmospheric counter radiation must also be taken into account in the calculation .

### calculation

Specific heat flow through the envelope surface element: The product of U-value and area gives the heat flow through the envelope surface element at a temperature difference of 1 Kelvin between the inside and outside temperature, measured in watts per Kelvin (W / K).

Specific heat flow through the entire envelope: summation of the individual products over the entire envelope results in the heat flow through the entire envelope with a temperature difference of 1 Kelvin between the inside and outside temperature at the envelope.

Transmission heat loss: The multiplication with the temperature difference inside-outside then provides the transmission heat loss = heat flow through the entire envelope surface measured in W or kW at the given temperature difference.

When calculating the heating load, this calculation must first be made for each room, and only then can it be added up to the heating load of the building. However, since not every part of the enveloping surface borders on the outside air (e.g. basement ceiling, top floor ceiling and walls to other buildings) and the temperature difference inside-outside is lower, the calculated heat dissipation would be too high. The correction can be made in such a way that when the product is formed, the U-value times the area of ​​a component is additionally multiplied by a correction factor ≤ 1, which takes into account the lower temperature difference on this component. The sum of the various heat flows of the entire envelope surface can then be multiplied by the difference between the inside and outside temperature in order to calculate the (current) transmission heat loss of the building envelope at the (current) temperature difference.

### Annual transmission heat loss

The annual transmission heat loss of a building is an amount of energy given in kWh. In addition to the thermal insulation properties of the heat-transferring envelope, it is determined by the location of the building (geographical, altitude, wind disposition ...) and the user behavior of its residents (internal temperatures of the various rooms such as bathroom, living room, bedroom).

The determination of the annual transmission heat loss can be imagined as follows:

The entire year is divided into small time intervals, for example hourly or daily intervals, and for each time interval the amount of heat flowing out of it is determined as the product of three variables:

• Specific transmission heat loss H T in W / K (constant building parameter)
• Temperature difference inside-outside in K (averaged over the time interval)
• Size of the time interval in h

Summation over all time intervals provides the annual transmission heat loss of the building, specified in kWh / a (the reverse energy flows from outside to inside, as soon as the outside temperature exceeds the inside temperature, are not taken into account in the summation). Since the building parameter H T is a constant (it is unchanged for all time intervals), the summation can only be carried out using the products:

Temperature difference inside-outside Δ T times time interval Δ t

executed and the result multiplied by H T. This summation value of the temperature difference inside-outside over all time intervals of the heating season is called heating degree hours or heating degree days depending on the time interval . Then the annual transmission heat loss of a building results as the product of H T in W / K and the heating degree hours in Kh.

The dependence of this annual transmission heat loss of a building on the building location and user behavior is based on the different value of the heating degree hours depending on the building location (outside temperature according to the microclimate!) And depending on user behavior (inside temperature!). Assuming standardized user behavior (average constant indoor temperature, for example = 19 ° C in all rooms) and selecting a specific geographical location, a specific sum of "heating degree hours" results for the heating period. The value is not entirely clear, as the indoor temperature is neither can be assumed to be constant in all rooms of the building over the entire period of the heating season. The EnEV is based on 66,000 heating degree hours per heating period.

The annual transmission heat loss based on 1 m² of living space is a clear value for the apartment user, more clear than the specific transmission heat loss H ' T based on 1 m² of enveloping area , and is also more directly related to the heating requirement. In earlier thermal insulation ordinances it was also used as a prescribed limit value depending on the degree of compactness A / V. Today, however, the EnEV uses the pure building parameter "transmission heat transfer coefficient" (equal to the mean U-value of the building envelope) as a limit value in the regulation for limiting the transmission heat loss.

## Energy Saving Ordinance EnEV (Germany)

When calculating in accordance with the EnEV , the legislator specifies maximum values ​​for the area-related specific transmission heat loss H ' T [W / (m² · K)] for residential buildings . For non-residential buildings there are no requirements for the transmission heat loss - here the average heat transfer coefficient is used as a condition. Unless it is a public building within the meaning of the EEWärmeG. Then the H ' T value is used.

Note: Further requirements for the energetic quality of buildings can be found in the EnEV and the EEWärmeG .

## Individual evidence

1. Duzia, Thomas; Bogusch, Norbert: Basic knowledge of building physics: [Basics of heat and moisture protection]. 2. actual Edition Berlin: Fraunhofer IRB Verlag, 2014. p. 56.
2. DIN EN ISO 13789: 2008-06 section 4.1
3. Duzia, Thomas; Bogusch, Norbert: Basic knowledge of building physics: [Basics of heat and moisture protection]. 2. actual Edition Berlin: Fraunhofer IRB Verlag, 2014. p. 55.
4. EnEV2009 Annex 2, Table 2 (PDF file; 183 kB)