Download Heat demand for a building

Principles of HEVAC design
Course
Spring term 2015
Heat loss calculations in buildings
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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
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Introduction
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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)
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Calculation of heat demand in buildings
Heat and energy demand are related to each other in buildings
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The procedure starts always by calculating heat demand for every
single room or space in a building
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Normally any internal or external heat gains are not taken into
account
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The heat demand for a building is got by calculating together all
room and space heat demands and heat demand of air handling
units
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By calculations can be chosen correct heating devices (boilers, heat
exchangers, pipes, heat emitters etc.)
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Heat demand
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Calculations are made for the design conditions
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The design conditions are the “worst” case, when heating is
needed, usually the minimum outdoor temp.
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In all other conditions the heat output of the heating system
is controlled/reduced to those (other) outdoor and indoor
conditions
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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
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Dimensioning of heat production for a building is based on
these calculations
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Heat demand for a building
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The heat demand for a building is calculated according to
Figure 2
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η (eta) is the efficiency of each part in the heating system
there are losses in every parts of the heating system like
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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
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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
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Heat demand is the same as heat losses, only the perspective is
different
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Heat losses through building envelope (fabric)
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These are called normally as heat losses by conduction
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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)
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Heat loss calculations
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Thermal transmittance of a building component is U
(W/m2,K)
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D3 (NBC) gives the reference values for thermal
transmittance, mostly these are used in real buildings
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Usually the constructor engineers calculates the U-values
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New values in the beginning of the 2010
 windows and doors has been especially improved
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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)
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Length of joints between building components
 walls, floors, roofs, windows, doors
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Heat loss calculations
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Designing temperatures
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Indoor air temperature Ts from D2 year 2010 (or Indoor air
classification 2008)
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Outdoor air temperature Tu,mit, 4 different weather zones in Finland
(Appendix 1 in D3)
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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
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Heat loss calculations
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Heat losses through leakage air (infiltration air)
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there is always uncontrolled ventilation through building envelope
leakage air flow is not considered as a part of ventilation air flow
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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)
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Heat demand is calculated by general formula (Formula 2.12)
 in D5 with (formula 2.13)
 in general with (Formula 2.15)
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Heat demand for leakage air is calculated for rooms with at least one
building component against outdoor air or ground under the
building
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Heat loss calculations
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Heat losses through leakage air (infiltration air)
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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)
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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
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Heat loss calculations
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Heat losses of ventilation
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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
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the air flow rates which are used are got from the ventilation design
drawings
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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
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Heat loss calculations
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Heat losses of ventilation
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The power (effect, heat output) of the heat recovery
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TtLTO is calculated with
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or with exhaust air temp ratio
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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
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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.)
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Heat loss calculations
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Heat losses (or heat demand) of hot domestic water
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Usually the hot domestic water (HDW) is heated in one place for the
whole building (a centralized heating and distribution system)
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in D5 is given a general way
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The dimensioning flow of HDW is got by D1
Temperature difference is normally 50 °C
The required temp. of HDW is +58 °C
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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
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Heat loss calculations
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Heat losses (or heat demand) of hot domestic water
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The magnitude of the heat demand for HDW is so high that a HDW
storage is needed in all systems except district heated buildings
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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
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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)
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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)
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