Temperature and Heat Loss

1. Temperature and Heat Loss • The following topics are covered in this presentation: • Calculation of U-values • Heat loss from the building envelope • Use of U-values in heat loss calculations

2. Temperature and Heat Loss 1. Calculate the U-value of a) single-glazed window b) double-glazed window. Given: Thickness of glass = 4 mm Thermal conductivity of glass = 1.02 W/mK Thermal resistance of surfaces: 0.12 (inside); 0.06 (outside) Thermal resistance of 20 mm wide airspace = 0.18 m2K/W Solution: The total thermal resistance of a component is given by: Rtotal= Rsi + Rmaterials + Rairspace + Rso where Rtotal is the total thermal resistance Rsi is the thermal resistance of the inside surface Rmaterials is the thermal resistance of the materials Rairspace is the thermal resistance of the cavity Rso is the thermal resistance of the outside surface

3. Temperature and Heat Loss • Single-glazed window: • Rtotal = Rsi + Rglass + Rso (no airspace in this case) = 0.1839 m2K/W U-value = 1/R = 5.44 W/m2K

4. Temperature and Heat Loss b) Double-glazed window: Rtotal = Rsi + Rglass + Rairspace + Rglass + Rso = 0.12 + 0.0039 + 0.18 + 0.0039 + 0.06 = 0.3678 U-value = 1/Rtotal = 2.72 W/m2K

5. Temperature and Heat Loss 25% Wall – 35% W 10% D 15% Floor – 15% The figure above shows the typical values of heat loss from an un-insulated house.

6. Temperature and Heat Loss 1. HEAT LOSS FROM A BUILDING: Heat loss from a building is determined in two parts; the first part is the FABRIC HEAT LOSS and the second is called HEAT LOSS BY VENTILATION. Fabric Heat Loss: The unit of U-value is: W/m2K This can be written as U = W/m2K After transposition, W = U × m2× K Replacing m2 by A (area) and K by T (temperature difference): W = U × A × T where W is the heat loss in Watts. For determining the heat loss from an element/component we need to know its U-value, the surface area and the difference between the inside and outside temperatures.

7. Temperature and Heat Loss 2. The heat loss due to ventilation occurs when the warm air of a room is replaced by cold, but fresh, air from outside. Rate of heat loss due to ventilation = where Cv is the volumetric specific heat capacity of air V is the volume of the room N is the number of air changes per hour T is the difference between inside and outside air temperatures Total rate of heat loss = Fabric heat loss + Ventilation heat loss

8. Temperature and Heat Loss 1. EXAMPLE: Find the total heat loss from a room; the U-values and surface areas of the different elements are given in the Table. The inside and outside air temperatures are 20 ºC and 0 ºC respectively. The ground temperature is 5 ºC. Take Cv = 1212 J/m3K; N = 2 per hour; V = 150 m3 Fabric heat loss: Element U-value Area Walls 0.35 68.0 Doors 2.2 3.3 Windows 1.9 11.7 Floor 0.25 55.0 Roof 0.25 55.0 Temp. Diff. 20 20 20 15 20 Rate of heat loss 0.35x68x20 = 476.0 145.2 444.6 206.3 275.0 Total = 1547.1 W

9. Temperature and Heat Loss 2. Heat loss due to ventilation: Heat loss = = 2020.0 W Total Heat Loss = 1547.1 + 2020.0 = 3567.1 W or 3567.1 J/s (1W = 1J/s)