B.2. Calculations of heat losses in compartments - heat demand of compartments<br>

B.2. Calculations of heat losses in compartments - heat demand of compartments

Calculations of building heat demand and compartments heat demand (total heat loses) are based on provisions of norm EN 12831. Total heat losses can also be calculated according to detailed or simplified method (option can be selected).

There are following calculation rules:

- air temperature in compartment is equal to declared value,

- heat losses are calculated in stabilized conditions, constant features of e.g. temperature or thermal parameters of building are assumed.

Calculation methods included in this norm concern mainly buildings, in which:

- compartments are heated to the value of defined constant temperature,

- value of air temperature and operative temperature is the same. As operative temperature it is to understood an arithmetical average value of inner air temperature value and average radiation temperature.

To calculate heat losses in heated compartments, following components should be taken into account:

- heat losses by penetration, which are the heat losses to external environment as a result of heat conduction by outer building surfaces as well as heat conduction between heated areas due to the fact, that it can be another temperature in adjacent heated compartment.

- heat losses for ventilation resulting from heat losses for warming-up of external air on the way of ventilation and infiltration and from ventilation heat losses to other heated and non heated compartments in building.

In calculations of heat losses on the basis of the norm the detailed calculation methods are applied. That are calculations of air temperature in non-heated compartments resulting from heat balance and ventilation air in the building.

The norm enables also in simplified form solving of problem of taking into consideration for heat loss calculations the heat loss for non-heated and heated spaces belonging to other flat or building. Depending on selected type of adjacent room the heat losses are corrected by specific coefficient précised in appendix to the norm.

DETAILED METHOD OF CALCULATION OF HEAT LOSS OF BUILDING – EN 12831

Total normative heat loss by penetration of compartment – detailed method

Heat losses by penetration are calculated as a product of sum of heat loss coefficients by penetration and the temperature difference between temperature in compartment and external air temperature.

Sum of heat loss coefficients includes:

- coefficient of heat loss by penetration between heated compartment and external environment HT,ie,

- coefficient of heat loss by penetration from heated compartment to external environment through non-heated compartments HT,iue.

- stationary coefficient of heat loss by penetration from heated compartment to the ground HT,ig.

- coefficient of heat loss by penetration from heated compartment to other heated compartment with distinctly different temperature value HT,ij.

Heat loss coefficient HT,ie is a product of partition area and heat transfer coefficient of partition. Its value is corrected by factors ek and el taking into consideration weather conditions. Values of these factors are given in domestic annexes. Calculations of heat loss coefficients of partition “U” include a normative addition Δuo and addition taking into consideration the presence of heat leakage bridges in partition.

Heat loss coefficient HT,iue is a product of partition area dividing the heated and non-heated compartment and the heat transfer coefficient U of partition. In the formula there is a temperature reduction coefficient taking into consideration the temperature difference between temperature of non-heated compartment and outer temperature. Temperature in non-heated compartment is calculated on the basis of heat balance and ventilation air balance. Instead of inserting a non-heated compartment we can also declare the type of non-heated compartment on the other side of internal partition. Then the heat losses will be calculated with correction coefficient, depending on selected type of compartment.

Coefficient of heat transfer HT,ig from heated compartment to the ground can be calculated on the basis of detailed guidelines included in norm EN ISO 13370 or of simplified guidelines included in norm 12831. In above mentioned norms have been described following types of partitions having contact with the ground:

- floor plate on the ground: not depressed, depressed under the ground level,

- wall at the ground,

- elevated construction of floor,

- floors on the ground with edge insulation,

- non-heated, underground.

Difference between simplified and detailed method is, that the detailed method is based on precise and complex analytical formulas and in the case of simplified method the formulas are simplified to the product of partition area having contact with the ground, equivalent heat transfer coefficient Uequiv,k and reduction factors. Reduction factors take into consideration annual fluctuations of outer temperature, temperature difference between normative outer temperature and average annual outer temperature as well as influence of underground water if the distance between floor on the ground and water level is smaller than 3m. Value of Uequiv,k is the function of “U” coefficient of partition, characteristic dimension of partition B’ and depression under the ground level.

Characteristic dimension of floor B’ is a product of area of floor on the ground and the half of value of floor perimeter on the ground in specific building. For calculations of floor perimeter on the ground we will take into consideration this section of floor perimeter which has contact with the external environment.

Calculations of Ag and P concern the building, which can include non-heated compartments on the ground. Dimensions of non-heated compartments are also included in calculations providing, they are included in main building block and dimensions of non-heated, added garage are not taken for calculations.

After calculating the characteristic dimension for a building its value can be adapted for compartments. We have here three cases:

- in compartments without external walls separating considered compartment from external environment the value B’ calculated for a building will be taken.

- in compartments with well insulated floors (U<0,5 W/m2K) the value B’ calculated for a building will be taken.

- for all other compartments cases the value B’ is calculated by the program individually on the basis of floor perimeter on the ground and the area of floor on the ground of considered compartment.

Heat loss coefficient HT,ij is a product of area of partition separating heated compartments to various value of inner temperature and heat transfer coefficient “U” of partition. In formula there is a temperature reduction factor taking into consideration the temperature difference between adjacent room and external environment. The influence of heat leakage bridges will be omitted in calculations.

Heat loss for ventilation – calculations for building and for compartment

In each heated compartment in flat a heat loss for ventilation is calculated. Heat losses for ventilation are proportional to the product of coefficient of heat loss for ventilation HV,i and temperature difference between temperature in compartment θint,I and temperature of external air θe.

Coefficient of heat loss for ventilation Hv,i is proportional to the product of volume of ventilation air stream Vi flowing through heated space and its density and specific heat. Constant value of physical features of air is assumed in calculations.

There are following types of ventilation air:

- minimal stream of air “Vmin” required because of hygienic needs, defined as multiplication factor of air exchanges in compartment “nmin”

- infiltration stream of ventilation air “Vinf” resulting from tightness of building block and presence of windows/ external doors in compartment,

- ventilation air streams flowing into compartment as a result of operation of blow and/or exhaust ventilation. That is the stream of air exhausted mechanically from compartment “Vexh”, stream of air blown mechanically „Vsup” and stream of air flowing into compartment from outside and/or other compartments in a building “Vmech,inf,i”. It is a stream of air flowing as a result of operation of exhaust mechanical ventilation. It occurs, when declared exhaust stream “Vexh” is greater than the blown stream „Vsup” Then, to keep the air balance, through leaks in building construction and/or from other compartments flows external air into compartments.

The values of heat loss for ventilation in compartment depend on declaring of mechanical ventilation in compartment (see annex describing ventilation air balance in compartment).

If a system of mechanical blow ventilation has been declared, then the blown air can have other parameters, than external air, for example when:

- a heat recovery system has been installed

- external air is preliminary warmed up.

As a result, blown air can have temperature “θmech” lower or higher, than temperature in compartment.

Additional heat loss ΦRH resulting from temporary temperature reduction in compartments.

Additional heat loss ΦRH resulting from temporary temperature reduction in compartment results from ability of heat accumulation in building elements of compartment. During temperature reduction in compartment the thermal energy accumulated in building mass is given up to the air in compartment. After specific time the temperature of air and partitions is reduced. To warm-up again the compartment to required temperature, additional heat is required to increase the inner air temperature and cooled partitions to previous temperature value.

The bigger ability of heat accumulation in partitions, the smaller temperature reduction and smaller heat amount required for warming-up the temperature in compartment to previous level.

The norm states an analytical algorithm of additional heat losses ΦRH for compartments, in which temporary temperature reduction is applied. Compartments with temporary temperature reduction require giving of following data:

- effective heat capacity of building elements creating a compartment,

- time of temperature reduction,

- temperature drop in compartment during reduction. This value can be calculated or given. If the User can obtain specific temperature in compartment during reduction due to control features of heating system. If the temperature reduction occurs after cutting off of heat flow (e.g. closing of heater valve), then this value should be declared as calculated by the program.

Temporary temperature reduction in housing buildings that is in most cases night temperature reduction for ca. 8 hours. Such buildings should not have light construction.

In not inhabited buildings there can be a 48 hours weekend temperature reduction or 8 h temp. reduction during the day.

On the basis of entered data a correction coefficient fRH is determined, depending on ventilation parameters during warming-up of compartment, effective mass of compartment, temperature drop in compartment and time of reheating.

Heat loss ΦRH is proportional to the floor area of heated compartment “Ai” and correction coefficient fRH.

Heat demand of compartment

Heat demand of heated compartment is a sum of losses by penetration ΦT,i, for ventilation ΦV,i and additional heat loss ΦRH,i because of temporary temperature reduction.

Heat demand of building

Total heat demand of a building is calculated as a sum of heat losses by penetration ΦT,i, for ventilation ΦV,i and additional heat loss ΦRH,I for all heated compartments in a building.

Dimensions used for calculations

Area of all external building partitions is calculated according to outer dimensions. Area of internal partitions is calculated according to dimensions in axes.

SIMPLIFIED METHOD OF CALCULATION OF HEAT LOSS IN BUILDING – EN 12831

Scope of application of simplified method is given in domestic annexes to the norm. The simplified method can be applied for housing buildings, in which the multiplication factor of air exchanges determined on the basis of pressure difference 50 Pa between building interior and external environment does not exceed n50 ≤ 3 1/h and in which there are more than 3 flats.

Heat loss calculations according to norm are as follows:

1.Total heat loss of heated space is the sum of losses by penetration and for ventilation. This value is corrected by temperature factor in heated compartments to the temperature which is distinctly higher, than temperature in adjacent rooms, for example in bathroom. Required temperature difference is ∆t ≥ 4K.

2. Heat losses by penetration for compartment are proportional to the area of partition, heat transfer coefficient U, temperature difference between compartment interior and external environment. This product is corrected by temperature factor fk depending on selected type of adjacent room, through which occurs the heat flow to external environment. We can select an unknown compartment heated from other building/flat, non-heated compartment and compartment heated to specific temperature.

3. Ventilation heat losses result from declared air streams required by hygienic needs.

4. If in heated compartments is a temporary temperature reduction, then additional heat losses ΦRH are calculated as a product of floor area and correction factor depending on ventilation parameters during warming up of compartment, effective compartment mass, temperature drop in compartment and time of warming up. Data for temporary temperature reduction are given only for complete building.