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# Thermal: Heat Flow Paths

The thermal behaviour of a building is basically a function of its form (architecture), its construction (materials and workmanship), local macro and micro-climate conditions, and by its use.

A typical building structure consists of many components of different thermal conductances arranged variously in series and parallel. For example, the side of a house may have some regions of cavity brick wall into which are inset some metal window frames fitted with single sheets of glass.

Thus, there will be some parts of the room behind that are separated from the outside by only one layer of glass, in parallel with some parts that have two layers of thin metal (the window frame), in parallel with some parts that have a layer of brick in series with an air gap, another layer of brick and a coating of plaster. The overall thermal performance of the wall will be a function of all of these.

Fortunately, enough is known about various materials to enable the calculation of an overall thermal character for most common building systems so that an overall conductance (or resistance) can be derived. Such values can be calculated for single glazed and double glazed windows, concrete slab floors, suspended wooden floors, walls and so on. These characteristics are usually written as an R-value or a U-Value for each of the various forms of construction and/or structural elements. More complex simulation techniques add a lag and decrement value or a set of response factors to describe the dynamic thermal behaviour of the element.

A whole building can therefore be modelled mathematically by taking all the various components and their areas into account, and by subjecting the hypothetical structure to a dynamic regime of internal energy inputs, external solar loads, outside air temperatures, wind velocities, etc.

Thus, if all the fundamental sources of heat loss and heat gain in a building are properly considered, it is possible to determine quite accurately the resulting internal conditions within it and, more importantly, how comfortable it is likely to be or how much air-conditioning energy will be required to make it so.

### Losses and Gains

It is well understood in the building services industry that commercial office buildings tend to be dominated by the need for cooling, almost regardless of the local climate. This is because offices are usually characterised by a very high volume-to-exposed-surface ratio and the amount of electrical and electronic equipment as well as its occupancy results in more heat being generated inside than is lost to the outside. On the other hand, houses and other domestic buildings tend to be dominated far more by the extremes of local climate as rooms are much smaller compared to the extent of exposed surfaces.

To determine the thermal state of a building, we first add up all the different sources of heat gain within and around it, and then subtract off all the sources heat loss. If the gains are greater than the losses, then the building will gradually heat up - requiring some sort of auxiliary cooling in order to maintain a steady internal temperature. Similarly, if losses are greater than gains, the building will gradually cool down - requiring some auxiliary heating. The amount of auxiliary heating and cooling required depends on the magnitude of the loss/gain differential.

Figure 1 - Sources of heat gain and loss within a building.

Thus, to understand how a building might perform under different conditions, and therefore allow us to come up with energy efficient and comfortable designs, knowledge of all the potential sources of heat and energy flow within a building is essential.

### Energy Flow Paths

Without attempting to be entirely complete, the main energy flow paths in all buildings include the following:

Long-wave radiation refers to heat energy radiated by objects at terrestrial temperatures (below 100°C). All elements both inside and outside a building are always exchanging heat by radiation when they are at different temperatures.

Thus the designer must consider radiant exchanges from adjacent structures, paved areas, asphalt car parks and any other dark-coloured surfaces that face towards the building. In most cases the concern is excessive heat gains as these dark surfaces can sometimes be heated by direct solar radiation to well above 50°C. In colder climates where solar radiation is not as high, the external surfaces of the building tend to be at roughly the same temperature as other adjacent surfaces so heat loss by this mechanism is not normally a concern.

Short wave radiation is basically solar radiation from the Sun in the form of infra-red and visible light. It comprises both sunlight (direct solar radiation) and daylight (diffuse radiation from the sky and reflected off other external surfaces). Window glass is almost completely transparent to this form of energy so window size and orientation as well as shading devices and the absorptance/reflectance of both internal and external surfaces are very important. Depending on the climate, this source of heat gain is often quite desirable, offering the designer an effective and free heating system in winter. In hotter climates, a balance must be struck between unrestricted solar access in winter and complete solar exclusion in summer.

### Air Flow

This refers to both the flow of air into and out of a building and any convection currents that occur within it. Flow in or out of a building occurs by ventilation through vents, open windows and doors, or by infiltration through gaps in the fabric and porous materials.

It is greatly affected by opening sizes, construction detailing and by local wind velocity and direction. Convection currents refer to the movement of internal air due to temperature differentials within it, usually referred to as the 'stack effect' as it was originally noticed in large chimney stacks. This effect is most likely to occur near atria or other voids between floors and acts to transport large amounts of heat upwards. In many buildings the stack effect is used as the primary means of inducing natural ventilation and removing excess heat from spaces. Once again, this effect can be useful at some times and detrimental at others so the designer must consider possible controls.

### Casual Gains

Casual gains refer to heat entering a space from the lighting and equipment systems within it, as well as the occupants themselves. A standard office lighting system may add around 20 W/m² of floor area whilst the average office worker would add around 70 W or heat. Such gains have both a sensible and a latent component. Sensible heat acts to increase the sensorable air temperature of the space whilst latent heat occurs when moisture evaporates, increasing the humidity of the air (and therefore its energy content without necessarily increasing it temperature) and storing it for release later when the air cools slightly and condensation occurs.

### Climate

Whilst heat flows due to climate only really occur through the mechanisms described above, it does have a significant effect on the magnitude of gains or losses and where they come from. The nature of climate adds extra complexity as it establishes a pattern within which the amount and type of flow can change quite dramatically at different times of the year.

For example, in summer large areas of window may be desirable for natural ventilation, however in winter it may be so cold outside that any gains through direct solar radiation are not sufficient to offset losses by conduction through the glass. This requires the designer to carefully consider many options and their effects as the solution may not be as simple as just specifying double glazing.

### HVAC & Other Plant

In most cases, it is the job of the Heating, Ventilation and Air-conditioning (HVAC) system to offset any thermal imbalance in the building so that internal conditions remain stable at comfortable levels. Your job, as the designer, is to ensure that you do not unnecessarily waste energy by carelessly creating too large an imbalance, requiring excessive HVAC systems to compensate. Unfortunately it is never as simple as just pumping in hot air when it is too cold and cold air when it is too hot. Everyone has been to at least one building in which the guy sitting in the summer sun next to the window is quite comfortable whilst those at their desks near the lift core freeze. Such a situation requires quite complex zoning strategies, which many architects are completely oblivious to. They simply deliver their completed design to the services engineer who then tries to make it work. What the architect is not told is that, to actually make it work, many of the air outlets have to have their own little heater so that, when the massive chiller on the roof pumps out air at 12°C to meet the requirements of the window guy, the heaters near the lifts have to heat it back up to 16°C so that those guys don't freeze.

Whilst some VAV and distributed systems overcome such problems, you get my point. Efficient HVAC design rally starts with the architect, whether they know it or not. Many firms are now including the services engineer as a fundamental part of the design team right from the concept stage. However, a good working knowledge of HVAC systems is still important for the designer.

Thermal: Heat Flow
Thermal: Insulation