Breathing easily

Many buildings currently consume unnecessarily high energy levels ventilating and cooling inhabited spaces, whilst simultaneously producing uncomfortable and unhealthy environments for their inhabitants. Clearly an important factor in energy efficiency is ensuring that buildings are as airtight as possible (current revisions to Part L of the Scottish Building Standards now ensures that air-tightness testing is compulsory) which in turn also increases our attention towards ventilation and cooling. The ‘Build Tight – Ventilate Right’ mantra is becoming evermore vocal, however important lessons must be absorbed from the over-use of mechanical systems and as a consequence of which has seen rising levels of sick building syndromes created by unhealthy indoor environments.

Current issues with MV/NV

The current incentives to avoid mechanical ventilation (MV) and air-conditioning are obvious – both consume high levels of energy, with ventilation alone consuming 5-15% of a buildings running costs. If badly maintained, both systems can also be a source of pollution. That is not to say that natural ventilation (NV) provides an all encompassing solution – poorly designed systems can be very difficult to control, also leading to the introduction of pollution and can be a waste of heat energy. With neither method providing an energy-efficient, clean, controllable and easy to maintain system, a mixed mode or hybrid is becoming the more favoured alternative.

Design Solutions

Passive Stack Ventilation
This approach uses a combination of cross-ventilation, buoyancy and suction enhanced by chimneys or outflows into zones of negative pressure to provide the most effective system of natural ventilation. Cool air is introduced into rooms via opening windows or inlets at low level, and is then heated by solar gains (or other internal heating systems in winter) and gradually rises where the warmed air is extracted through stack chimneys where multiple openings at roof level automatically close on windward and open on leeward sides. Careful consideration must be given to the amount of solar gains provided by glazing, with effective solar shading installed to give sufficient control, and window openings should be designed to ensure that over-ventilation is not an issue. Atria can also perform a similar function to chimneys when using a cross ventilation strategy. Mechanical ventilation does have a part to play when improved air flow requirements are necessary.

Night Cooling
Night cooling is a process which involves harnessing the lower night time external temperature to reduce the temperature of the building fabric, therefore reducing the internal temperature for the following day. Attention however must be given to security and window orientation which can aid lowering the ambient temperature and delay the use of energy consuming cooling equipment.

Culvert systems
Culvert systems exploit the constant temperature of the ground to facilitate buildings with warmer or cooler air which in conjunction with proper controls, solar gains and cross ventilation can also provide internal spaces with a regulated desired temperature. The average temperature of the earth at a depth of 2 metres below ground level is 10°C throughout the year. Air flowing into the ground via a wind tower and distributed to the building via an underground system where the warmed/cooled air can then be introduced and employed to warm or cool rooms. This system can also function in combination with heat recovery.

Conclusions

Reliance neither fully on MV or NV, but the introduction of natural means where possible and restrict where possible the use of mechanical equipment to the role of supporting flow and maintaining comfort levels. Any mechanical ventilation should incorporate lower vent rates and lower specific fan power (i.e. energy required to move a certain quantity of air) to ensure high energy efficiency.

Passive systems of cooling should take precedent over mechanical cooling, again with any mechanical cooling acting as a balancer – any heat should be recoverable too.