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Heat flow in buildings

Heat is a form of energy which flows from a hotter to a colder source. Solar radiance is the most important factor in determining heat flow; in summer heat is likely to flow into a building whilst during the winter the flow is reversed. Northern Europe has long winters and short summers whereas the length of the seasons is reversed in southern Europe.

The level of comfort inside a building depends upon the season, time of day and whether the room is being heated or cooled; it also depends upon the level of activity such as working during the day or sleeping at night. In the home the largest amount of energy is used for space heating and it is important to understand the ways in which heat can flow in order to reduce our energy consumption and use energy more efficiently.

Heat transfer

Heat energy can be transferred from a hot to a colder object in three ways

  • Conduction where the heat is physically transferred across a like a wall or window
  • Convection in which the heat is circulated away from the heat source by moving air
  • Radiation in which heat is transferred by heat waves, the sun being the ultimate radiation source

These heat transfer processes are all dependent upon the difference in temperature between the hot and cold source; the greater the temperature difference the faster the heat transfer.


IIlustration of heat transfer processes
The process is of course reversible; in summer the heat flow is inward because it is warmer outside than inside whilst during the winter the flow is outward when it is warmer inside than outside. The objective of good passive architecture is to reduce both sets of heat flow arising from radiation (see Chapter 5).

The role of insulation is to reduce the rate of at which heat flows by conduction. The opening and closing of windows will alter the ventilation. If there are no ventilation vents the windows will need to be open sufficiently to give good air quality but not too far to prevent unnecessary heat loss.


Heat Loss in Buildings

We heat our homes to stay warm, no matter what is the outside temperature. If it gets colder outside, the inner temperature stays the same, but we need more energy to maintain that temperature. This is because heat loss is greater when the temperature difference between the inside and outside is larger so more heat is lost and we need to produce more heat to stay warm inside.

Windows
Windows are responsible for up to a quarter of the total energy loss either through the glass pane directly or by ventilation through any gap between the window frame and the wall. Single panes of glass are very good at allowing heat to be conducted from the hotter to the colder source. The larger the window area, the greater will be the heat transfer; heat loss in winter and solar gain in summer

Single panes of glass can also contribute to condensation during the winter as moisture condenses on the inside of the window.

windows
High levels of condensation can contribute to health problems as these can create conditions in which house mites can flourish which can lead to bronchial illnesses such as asthma or allergies. So reducing heat transfer rates through windows has more than one benefit.

Typical heat loss in a building

Doors
Doors leading to the outside or to a balcony are fewer than windows and so contribute less to heat transfer, however the same principles apply particularly for glass doors.

Walls
Walls are the largest source heat loss because they represent the largest area in contact with the cold air outside. A ‘visible’ example of heat loss is the wall just behind the radiators in your room. The radiator, which is typically located very near the wall, is emitting heat not only towards the free space of the room, but also towards the wall. The wall is then heated and the heat is lost to the outside if the wall is not insulated well enough.

The heat loss is greatest for a detached dwelling which has the largest number of outside walls and least for an apartment in a multi-family dwelling.

Roof and floors
May be responsible for up to 35% of the total energy losses The exact number depends on the type and building and its level of insulation – the loss will be smaller in apartment buildings where most apartments have neighbours with heated flats above and below also and highest in individual homes.


Production and distribution of heat

Heat energy can be produced in various ways of which those using non-renewable sources have the highest environmental impact and those using renewable sources have a low or negligible impact. The various renewable heating methods are described in subsequent chapters.

Heating by combustion
The most common heating source is natural gas which is burned in a boiler. The heat is transferred to air, or more likely water, flowing through the heat exchanger. The heated water is distributed to for example the radiators, where heat is given off by conduction to the air and the heated air then circulates by convection around the room. The cooler water passes back to the boiler where it is reheated.

Similarly, oil, coal or wood, other types of gases or even biomass may be used in other types of boilers which will burn the fuel and heat the water in a similar way. The decision of which fuel to use will depend on its availability and cost. In terms of limiting climate change, it is important to consider the efficiency of combustion and resulting environmental pollution arising from the combustion process. The biggest pollutant is carbon dioxide which is the principal gas responsible for global warming whilst other pollutants which may be harmful include carbon monoxide and nitrous oxides. While wood and biomass are renewable energy sources, oil and gas have been created over millions of years and are being used up faster than they can be created.

Heating using electricity
Electricity is also used for heating homes. Then no chimney is required as the energy flows into the house by wire and is converted to heat through the resistance of an electrical element. A more efficient form of electrical heating is by a heat pump which concentrates the low grade heat energy available in the air, ground or water. Electricity is then only used to concentrate the heat not produce it.

Remember that electricity is only a carrier of energy and is produced somewhere else! Most often, the same fossil fuels mentioned above, are also used to generate electricity, a process which also results in environmental pollution; this time in a central location rather than distributed in individual homes.

Direct heating using the sun
This may be done by passive means such as conduction or convection (Trombe wall) or actively through solar thermal collectors. As air or water is heated directly, the only environmental impact is that of distributing the heat around the house.

Heat distribution
In our homes heat is most often distributed through radiators which are frequently located under the windows. The radiators are interconnected by small pipes which are sometimes visible and sometimes hidden in the walls and floors in which warm water is circulating. Other ways of heating include venting hot air through ducts and circulating water through pipes located beneath the floor.

The boiler itself may be a small box located somewhere in your house or flat. Alternatively it may be a larger boiler, located in your neighbourhood or in your apartment block, which allows heat to be distributed to all homes around you.


Reducing the heat loss

To reduce the heat loss will depend upon the age of the dwelling as newer buildings will have a much higher standard of insulation than older dwellings. Some aspects of reducing heat loss are easy but others will require the help of specialists.

Not all buildings have the same heat insulation standard; older houses throughout the EU tend to have less insulation and newer houses have much more. For example prior to 1965 there was no formal requirement to install any insulation in UK homes but today it is recommended to have 300 mm minimum insulation in the gap between the ceiling and the roof .

Windows
To reduce heat loss it is possible to replace a single glass pane with two (or even three) glass panes within the same window frame. To prevent convection of heat between the panes, the space between is either partially evacuated or filled with an inert gas like argon. Whether it is necessary to replace the window frame will depend upon the type and condition of the frame and how well it is sealed into the opening in the wall.

Walls
The highest thermal energy savings can be reached by insulating the walls. If the wall has a cavity between the outer and inner wall then it can be insulated at any time. If there is no cavity then the walls can be insulated internally or externally. External insulation is not easy as it also requires weather proofing and will generally require installation by a technical specialist.

Special reflection foils made of aluminium can be stuck to the wall behind the radiators so that they are not visible. Heat will be reflected from the radiator back into the room minimising the heat to be lost through the walls.

Roof and floor
A variety of insulating materials exist for both roofs and floors. If there is access to the loft space between the ceiling and the roof, an inspection will determine the level of insulation. If this is insufficient to prevent heat loss (now 300 mm in UK), it is generally possible to add additional insulation at relatively low cost for which a grant or subsidy might well be available. If there is no loft, then insulation can only be added beneath the ceiling if height allows or on the roof if it is flat.

It is generally not possible to check whether there is any insulation under the floor unless the house has wooden sprung floors. Only new houses are likely to have insulation under the floor. Floor carpets do provide some level of insulation.

Thermostats
Very often the same temperature is maintained in all parts of the home. But there is no need to keep all the rooms equally warm – the living spaces are the most important whilst the bedrooms can be cooler. Sometimes it may be desirable to have a higher temperature, for example in the bath room, and this is often done by a local electric heater. Contrary, higher temperatures are sometimes reached in the kitchen due to the heat coming from the cooking process.

One of the easiest ways of saving energy is to regulate the temperature! This can be done by installing thermostatic valves on radiators so it is possible to vary the temperatures in each room. With a programmable thermostatic regulator, the temperature can be reduced when the family is out of the home, at work, school or on holiday. Setting the temperature down by 1 degree saves about 6% of heat energy! A room which is not used for longer times may be heated only to 16C rather than 20C for normal occupancy.


Efficient heating

The heat loss of a home depends upon a number of factors including –

  • type of dwelling such as detached, semi-detached or attached or whether it is an apartment
  • number of stories
  • condition of the fabric such as doors and windows
  • level of insulation

Adding insulation
The average life time of a dwelling in the EU is now more than 100 years. Over the years the building standards have increased in all countries so that the older the dwelling, the poorer the insulation standard. Increasing the level of insulation to that closer to today’s standards will improve the fabric of the dwelling, extend its life and reduce any condensation that might be present. In this way, savings can be made in terms of energy, money and the environment. A grant may be available to insulate your home so it is worth checking with your local energy advice centre or utility.

Servicing
If it is more than two years since your heating system was checked, it is worth arranging a service visit to minimise your heating demand and save the environment. Servicing your system will also allow the service engineer to undertake a ‘health’ check to ensure your system is operating at optimum efficiency.

To ensure good heat transfer from your radiators, the neutrality of the central heating water (pH level) should also be checked at the same time. If this level is not neutral then corrosion will likely have occurred between the water and any metal parts such as the radiators which will result in the build up of deposits on the inside of your radiators. This deposit will reduce heat transfer, increase heating bills and reduce life of your central heating system. To prevent such corrosion occurring, an inhibitor and descaler (such as Vexo X-P010) should then be added.

Replacing heating system
If your system is more than 15 years old, it is worth asking the service engineer about the condition of the system, the availability of spare parts and the cost of replacing the existing system with a newer and more efficient heating system. As additional insulation may have been added since the existing boiler was installed, it is desirable to recalculate the heat loss so that the heating system can be correctly sized.

 

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