Air Conditioning | Units | Installation | Air Conditioner

Air Conditioning and Ventilation

Ventilating buildings is an important part of forming comfortable spaces for work and living. Enhanced building regulations have improved U-values but convective heat losses have increased. Energy consumption has risen significantly due to often unnecessary increase in the use of air conditioning in many buildings. It has also been realised how much energy can be lost as a result of uncontrolled air passing through leaky buildings. A vital part of improving building energy performance is getting the issues right at the design stage. Ventilation and air conditioning systems come in many types but whether natural, mechanical or a combination of the two, they need to have a good degree of local control and meet health, comfort and cooling needs. A crucial factor in energy efficiency is defining the optimum ventilation rate as conditions such as excessive fresh air will cause excessive energy consumption while an inadequate amount of fresh air will see a rapid decline in internal air quality. 5 litres/sec per person is the minimum required rate though the recommended rate is 8 litres/sec per person. An advised strategy is to avoid smoking in buildings or provide separately ventilated rooms for smokers as cigarette smoking can double or quadruple this requirement.

AIr Conditioning - Build tight, ventilate right!

The greatest energy loss from buildings is often from ventilation though the importance of this has become more apparent as conduction losses have been reduced with improved levels of insulation. Therefore it is important that a tight envelope is developed from the design stage to minimise uncontrolled air infiltration. A common rule for both mechanical and naturally ventilated buildings is “Build tight, ventilate right”. To minimise air leakage and infiltration, attention to detailing and sealing is paramount. The common points of infiltration include gaps around the perimeter of doors and windows, the areas where door and window frames meet a wall and the junctions of walls, ceilings and floors. Often the blame lies with poor workmanship, especially in service ducts. Very large energy losses in existing buildings and some recently built buildings are often through uncontrolled ventilation and are worse than the minimum standards which are often easily achievable. Controlled ventilation is essential in tight buildings as they can have problems unique to them such as a total lack of ventilation can lead to mould growth and other similar problems.

A move to a mechanical ventilation or air conditioning system

If possible, it is advisable to use natural ventilation rather than instantly opting for Air Con solutions which rely on mechanical systems. Buildings that are naturally ventilated tend to use less energy than those with mechanical based ventilation and air conditioning. Driving force for air movement is generated by a stack effect which is achieved by using air passages at different heights and wind effects. This is often achieved by ventilating from at least two facades. Buildings that rely on natural ventilation generally provide advantages such as lower energy consumption leading to lower running costs, the transport energy associated with fans is reduced, the risk of possible plant noise is reduced and capital and maintenance costs are reduced. High levels of noise or pollutants and other external environmental conditions which may limit or preclude natural ventilation usage need to be identified at an early stage and minimised where it is possible. If windows have been kept shut to keep out noise, fumes, dirt or smoke, it may mean a move to a mechanical ventilation or air conditioning systems which will result in higher energy consumption. A most important factor in successful natural ventilation strategies is the minimisation of internal heat gains. When trying to retain a natural ventilation solution, efficient lighting, office equipment and solar shading are all beneficial, with 40W/m2 generally regarded as excessive and subject to assessment previous to the selection of a ventilation strategy. The need for mechanical cooling can also abe avoided by using night air to cool the building fabric. Incidentally, the building fabric can absorb the incidental gains during the occupied hours and act as a heat sink which can be used as a thermal store.

Many buildings have air conditioning installed simply as it is seen as the best solution or for the reason of higher rental value that comes with that. Mechanical ventilation and air conditioning are energy intensive processes and factors such as noise, pollution, excessive heat gains can prevent a passive solution meaning that designers need to consider the next best energy efficient means of providing ventilation. There are a range of mixed mode solutions that can provide intermediate options and involve a variety of ventilation and cooling systems. An example is to rely on ventilation and cooling by opening windows and turning off the air conditioning for parts of the season. Another example is zoning which works by keeping the bulk of the building naturally ventilated while massing all significant heat producing equipment into a space with the appropriate air conditioning.

Full Air Conditioning

When a full air conditioning system is added to a design it can add up to 50 percent to the total running costs for the building and therefore, should be very carefully considered. While full air conditioning involves humidity control, the UK term often refers to systems which simply provide cooling and heating, sometimes referred to as comfort cooling. If it is necessary to use air conditioning then one way to save energy is to avoid humidity control. Often there is little need for tight temperature control and the introduction of flexibility in the original design specification can lead to less energy use.

The fact that mechanical ventilation requires higher air change rates than those in naturally ventilated buildings leads to increased HV AC running and capital costs. One of the largest energy users in offices with air conditioning is air handling and this is often a much greater amount of energy use than chillers. The typical use if air handling in offices is 42-44kWh/m2/yr though best practice installations can roughly half that. Often, systems are over installed and greater than 12W/m2 is excessive while best practice is around 8W/m2. Therefore, sizing the fan is important as it needs to be as close to the real demand as possible to avoid unnecessary costs. The fan power which is a measure of the fans efficiency should be kept as low as possible. Good practice for offices is below 2W/litre/second and with very energy efficient systems, 1W/litre/second can be achieved. Anything above 4W/litre/second can be regarded as having a poor standard of efficiency and many existing systems are oversized with poor fan power. This can be prevented by matching equipment closely to demand and specifying high efficiency fans and motors.

Minimise energy consumed in air conditioning

Measures to minimise energy consumed in air conditioning are included in Building Regulations Part L2. This includes the necessary requirements for refurbished and new buildings as well as existing buildings with an air conditioning or ventilation system is due to be altered or replaced. Showing that the Carbon Performance Rating (CPR) falls within certain limits is a way if achieving compliance in office environments.

Another way of achieving compliancy in offices is meeting ‘Whole Office’ CPR targets. Meeting specific fan power targets in other buildings requires 2W/litre/sec in new buildings, 3W/litre/sec in refurbishments while schools and hospitals comply by following DFEE and NHS guidelines.