Building Science Heat Loss Factors Affecting Heat Loss

Building Science Heat Loss

Factors Affecting Heat Loss • Fabric Heat Loss – by transfer through the external shell of the building • Ventilation Heat Loss – by both purposeful and unintentional changes of air in the building

Factors Affecting Heat Loss • • insulation of the building shell exposed area of the building shell temperature difference - outside and inside air change rate exposure to external climate efficiency of services in the building patterns of use for the building

Fabric Heat Loss • Pf = U A ∆ t • Pf = rate of fabric heat loss = heat energy lost/time (W) • U = U-Value of the element considered (W/m 2 K) • A = area of that element • ∆t = difference between the temperatures assumed for the inside and outside environments (°C)

Ventilation Loss • Pv = c v N V ∆t 3600 • Pv = rate of ventilation loss = heat energy lost/time (W) • cv = volumetric specific heat capacity of air = specific heat capacity (density (J/m 3 K) • N = air infiltration rate for the room (the number of complete air changes per hour, ach) • V = volume of the room (m 3) • ∆t = difference between the inside and outside air temperatures (°C)

Worked Example A simple building is 4 m long by 3 m wide by 2. 5 m high. In the walls there are two windows, each 1 m by 0. 6 m, and there is one large door 1. 75 by 0. 8 m. The construction has the following U-Values in W/m 2 K: windows 5. 6, door 2. 0, walls 2. 5, roof 3. 0, floor 1. 5. The inside environmental or comfort temperature is maintained at 18°C while the outside air temperature is 6°C. The volumetric specific heat capacity of the air is taken to be 1300 J/m K. There are 1. 5 air changes per hour. Calculate the total rate of heat loss for the building under the above conditions.

Worked Example • An actual building can be simplified into this type of ‘shed’ format to get a first estimate of heat losses.

Worked Example - Fabric Heat Loss Element U-Value (W/m 2 K) Area (m 2) Temperature Difference (°C) Rate of Heat Loss (W) Windows 5. 6 1. 2 12 80. 64 Door 2. 0 1. 4 12 33. 6 Walls 2. 5 (35 -2. 6) 12 972 Roof 3. 0 12 12 432 Floor 1. 5 12 12 216 Total rate fabric of heat loss 1734. 24 W • Pf = U A ∆ t • Apply formula to each element - Window: • Pf = 5. 6 x 1. 2 x 12 = 80. 64

Worked Example - Ventilation Heat Loss cv = 1300 J/m 3 K V = 4 x 3 x 2. 5 = 30 m 3 N = 1. 5 h-1 ∆t = 18 - 6 = 12°C Using Pv = cv N V ∆t = 1300 x 1. 5 x 30 x 12 3600 So rate of ventilation loss = 195 W

Worked Example - Combine Losses • Total rate of heat loss = fabric heat loss + ventilation heat loss = 1734. 24 + 195 = 1929. 24 W

Heat Gains for Buildings • A building gains heat as well as losing it. • Both processes usually occur at the same time • In temperate climates such as Britain, overall gains are less than the overall losses although heat gains may still provide useful energy savings • They can be considered under the following two broad categories: • Solar Heat Gains • Casual Heat Gains

Heat Gains for Buildings • Solar Heat Gains – from the sun • Casual Heat Gains – from occupants and equipment in the building

Energy Balance of Buildings • We normally require the temperature inside a building to be kept at a constant comfort level • Heat flowing out of the building needs to be balanced by the same amount of heat energy put into the building • In Europe natural heat gains of a building cannot balance the heat losses, we therefore need to add extra heat energy by using heating services • Even if we need to subtract heat from a building using air conditioning equipment that equipment still uses energy

Energy Balance of Buildings • Shows the balance needed in a building between the heat energy losses, the heat energy gains, and the energy used by the heating and/or cooling equipment. • The balance is broader than just heat energy as the services equipment will have supplied the heating or cooling effects by converting other forms of energy such as gas or electricity