Heating, ventilation and air conditioning in big buildings | How the system work?

Heating, ventilation and air conditioning (HVAC) systems control the temperature, humidity and quality of air in buildings to a set of chosen conditions. To achieve this, the systems need to transfer heat and moisture into and out of the air as well as control the level of air pollutants, either by directly removing them or by diluting them to acceptable levels.

Heating systems increase the temperature in a space to compensate for heat losses between the internal space and outside. Ventilation systems supply air to the space and extract polluted air from it. Cooling is needed to bring the temperature down in spaces where heat gains have arisen from people, equipment or the sun and are causing discomfort.

How does a HVAC system work?

HVAC system components
Source: The Carbon Trust

HVAC systems vary widely in terms of the individual components that make them up and how they are set up within a building. Most systems contain some common basic components:

  • Boilers (1) produce hot water (or sometimes steam) for distribute to the working space. This is done either by heating coils (2) which heat air as part of the ventilation system, or through hot water pipes to radiators (3);
  • Cooling equipment (4) chills water for pumping to cooling coils (5) . Treated air is then blown over the chilled water coils into the space to be cooled (6) through the ventilation system. As part of the refrigeration cycle in the chiller, heat must also be rejected from the system via a cooling tower or condenser (7);
  • Pumps are used throughout the system to circulate the chilled and hot water to the required areas throughout the building;
  • Stale air is extracted, usually using a fan, via separate ducts and expelled outside (8).

How much energy do HVAC systems use?

Total carbon emissions from energy use in public and commercial buildings 2008.
Source: The Carbon Trust

Heating typically accounts for about half of the energy used in offices. Heating costs rise by about 8% for each 1ºC of overheating.

Five are the major factors that determine the energy use of an HVAC system:

  • The design, layout and operation of the building – this affects how the external environment impacts on internal temperatures and humidity;
  • The required indoor temperature and air quality – more extreme temperatures, greater precision and more refined air quality consume more energy;
  • The heat generated internally by lighting, equipment and people – all of these have an impact on how warm the building is;
  • The design and efficiency of the HVAC plant – provides heat, cooling and moisture control exactly where it is needed in the building;
  • The operating times of the HVAC equipment and ability of the controls – these limit operation to exactly when it is needed.

There are many factors which effect how warm people feel, including:

  • Air temperature
  • Solar radiation
  • Air speed
  • Humidity
  • Activities performed
  • Clothing

All these factors should be taken into consideration when attempting to create a comfortable environment for people to work in, for example ensuring workspaces are shielded from draughts and direct sunlight.

The optimum operating temperature range for a building will depend on the type of activities being performed. Recommended heating temperatures for particular buildings, activities, and processes are provided in the following table.

Recommended heating temperatures for particular buildings.
Source: The Carbon Trust

At DMU temperature range for work spaces (lecture theatres, laboratories and offices, for example) is set between 21⁰C to 25⁰C.

Diagram of ‘dead band’ control indicating recommended temperatures.
Source: Adapted from The Carbon Trust

Based around these optimum temperatures, the DMU Energy Policy requires heating to be provided up to the level of 21⁰C. Cooling is available in some locations only. When the temperature exceeds 28⁰C, cooling can be provided to the level of 25⁰C, where it is available. Adhering to this policy will help ensure energy is used efficiently at DMU for space heating and cooling.

You can help to ensure this policy is followed by monitoring the temperature in your classroom, office or lab. Set the thermostat in the room to 21⁰C during the heating season (October to April) and the A/C controller to 25⁰C during the summer. If there is a thermostatic radiator valve (TRV) in your room, adjust it to a comfortable temperature. If the room is too hot, turn this down rather than opening the window. Keeping the doors and windows closed when the heating or air conditioning is on will further help maintain comfortable temperatures inside and will reduce energy consumption.

Did you notice any un-efficiencies in DMU’s buildings? What are the problems? Do you have suggestions for improvements?

Looking forward to hearing from you!

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