Principles of HEVAC design Course Spring term 2015 Heat loss calculations in buildings

Heat transfer Steady state conditions not for dynamic systems in buildings through walls, roofs, floors, windows, doors building structures and U-values National building codes (NBC) in Finland (C3 Regulations) C4 (2003, draft 2012) Thermal Insulations Guidelines D3 (2012) Energy efficiency in buildings D5 (draft 2012) Energy and heat loss calculations in buildings D2 (2012) Indoor Climate and Ventilation in Buildings Guidelines and Regulations

Heat loss calculations and heat demand for heating Introduction Heat loss calculations and heat demand for heating based on National building code D5 aim is to give information so that a student can calculate heat demand at the design outdoor temperature this information is needed for designing heating systems in buildings heating of buildings is one of the main subjects in HEVAC engineering the main purpose of heating is to maintain healthy and comfortable conditions in buildings and usually also to produce hot domestic water heating of supply and leakage air and HDW is also included in the calculations and design (Figure 1)

Calculation of heat demand in buildings Heat and energy demand are related to each other in buildings The procedure starts always by calculating heat demand for every single room or space in a building Normally any internal or external heat gains are not taken into account The heat demand for a building is got by calculating together all room and space heat demands and heat demand of air handling units By calculations can be chosen correct heating devices (boilers, heat exchangers, pipes, heat emitters etc.)

Heat demand Calculations are made for the design conditions The design conditions are the “worst” case, when heating is needed, usually the minimum outdoor temp. In all other conditions the heat output of the heating system is controlled/reduced to those (other) outdoor and indoor conditions The heat demand is calculated for every room or space because this heat demand determines the selection of the heat emitters (radiators, convectors etc.) for those rooms or spaces Dimensioning of heat production for a building is based on these calculations

Heat demand for a building The heat demand for a building is calculated according to Figure 2 η (eta) is the efficiency of each part in the heating system there are losses in every parts of the heating system like losses in production (e.g. boiler system) losses of heat storage (e.g. water tank) losses in heat distribution losses in heat emitters (e.g. under floor heating) losses in control systems If the efficiencies of each part is not known or calculated (D5 Chapter 6) then as the value of η (eta) can be used 0,9 Heat demand is the same as heat losses, only the perspective is different

Heat losses through building envelope (fabric) These are called normally as heat losses by conduction The basic information needed here are the U-values of different building components, the dimensions/areas of each component, length of joints between different parts of building elements and temperature difference between indoors and outdoors (or underground)

Heat loss calculations Thermal transmittance of a building component is U (W/m2,K) D3 (NBC) gives the reference values for thermal transmittance, mostly these are used in real buildings Usually the constructor engineers calculates the U-values New values in the beginning of the 2010 windows and doors has been especially improved Areas of building components walls, floors and roofs (ceilings) according to the internal measures of a space or room (length, width and height) windows and doors according to the measures of the installation apertures (hole) (external measures of the frames) Length of joints between building components walls, floors, roofs, windows, doors

Heat loss calculations Designing temperatures Indoor air temperature Ts from D2 year 2010 (or Indoor air classification 2008) Outdoor air temperature Tu,mit, 4 different weather zones in Finland (Appendix 1 in D3) Under floor temperatures outdoor air temp. when totally against outdoor air crawling space and ventilated = designing temp. (formula 2.10) annual outdoor average temp. – 2 K against the ground, designing temp. (Formula 2.11) annual outdoor average temp. + 2 K

Heat loss calculations Heat losses through leakage air (infiltration air) there is always uncontrolled ventilation through building envelope leakage air flow is not considered as a part of ventilation air flow leakage air flow rates are affected by e.g. the following air tightness of the building envelope the positions of the holes of leakage temp. difference between indoors and outdoors wind conditions on the site the air balance (outdoor and exhaust air flow rates) Heat demand is calculated by general formula (Formula 2.12) in D5 with (formula 2.13) in general with (Formula 2.15) Heat demand for leakage air is calculated for rooms with at least one building component against outdoor air or ground under the building

Heat loss calculations Heat losses through leakage air (infiltration air) The air flow rates are calculated with the coefficient of leakage air flow rate (called n, unit 1/h) and with the internal volume of the calculated room or space (Formula 2.16) Usually is needed the conversion from one unit to another (m3/h -> m3/s or litres/second l/s) value of n depends essentially of the tightness of the building definition of n50 (Figure 2.2) overpressure inside the building is 50 Pa air flow rate with that overpressure and the nleakage air (Formula 2.19) the procedure is n50 q50 qv,leakage air flow rate

Heat loss calculations Heat losses of ventilation The heat demand of ventilation is calculated (Formula 2.22) the density of air the temp. difference between supply air and room temp. air flow rate specific heat capacity of air the air flow rates which are used are got from the ventilation design drawings The heat demand of the air handling unit (AHU) (Formula 2.23) without heat recovery The heat demand in an AHU with the heat recovery Efficiency for heat recovery either for exhaust air or for supply air The efficiency for heat recovery for supply air (Formula 2.30) Values for temp. ratio (efficiency) in different types of heat recovery Ratio between supply air and exhaust air R

Heat loss calculations Heat losses of ventilation The power (effect, heat output) of the heat recovery TtLTO is calculated with or with exhaust air temp ratio And the power for the heating coil of an AHU the design value of the supply air temp. after the heating coil is used usually the heating coil is dimensioned for 5 °C lower temp. after heat recovery to ensure enough heating power For the whole building the heat demand of AHU’s are calculated together If there is only exhaust air system in the building (or in a part of it) all heat losses of ventilation are compensated by the heat emitters in the rooms or spaces (radiators, convectors etc.)

Heat loss calculations Heat losses (or heat demand) of hot domestic water Usually the hot domestic water (HDW) is heated in one place for the whole building (a centralized heating and distribution system) in D5 is given a general way The dimensioning flow of HDW is got by D1 Temperature difference is normally 50 °C The required temp. of HDW is +58 °C The HDW system usually always include the circulation system to avoid delay when using HDW in taps (faucets) The heat demand of circulation is very small when comparing it to the heat demand of HDW In D5 are given values for calculations The magnitude of the heat demand for HDW is so high that usually a HDW storage is needed in all systems except district heated buildings

Heat loss calculations Heat losses (or heat demand) of hot domestic water The magnitude of the heat demand for HDW is so high that a HDW storage is needed in all systems except district heated buildings A building with a boiler system, principles of producing HDW part of the heating power from the boiler and part of the power from the storage D5 guidelines: the storage capacity and the loading capacity are calculated for one day (diurnal, 24 hours) and the heat losses of the storage must be taken into account When calculating the total heat demand of the building, only the loading share of the HDW power is taken into account (at the same time as other heat demands occur) The boiler capacity is calculated with Values for η (eta) for each share can be 0,9 if there is no other information Usually in boiler systems some extra capacity is designed ( e.g. two boilers of 400 kW capacity for total heating demand for 650 kW)