When is Natural Ventilation, really natural?

Over the last year we have seen a greater acceptance within the building industry for three defined methods of ventilating buildings, these being: Mechanical Ventilation, Natural Ventilation and Hybrid Ventilation (a combined Mechanical and Natural solution). In this article, Monodraught technical director Nick Hopper reviews them, and calls for greater clarity in defining them in order to ensure that clients and consultants understand exactly what is being specified.

Within the latest guidance documents for the Priority School Building Programme, section 2.8.21.3 references the use of natural, mechanical or hybrid ventilation systems as a provision for the PSBP Facilities Output Specification.

However, as a manufacturer of all three types of ventilation system and a company that has embedded the latest building simulation tools within its free design service, Monodraught has seen little clarity in the design criteria for the Hybrid solutions.

This lack of clarity therefore leaves individual consultants deciding: Is it natural? Is it mechanical? Instead of hiding from this discussion we should celebrate that there is an alternative and that it is acceptable to use a combination of equipment and strategies when it is suitable. However, technical information that details items such as the specific fan powers (SFP), control strategies and the acoustic rating of products, are not grey areas and as a manufacturer Monodraught undertakes to make this data is available as standard.

This article sets out the current definitions that exist and offers alternative definitions of the three ventilation methods, associated control strategies and accompanying information that can be used to measure performance and compliance.

CIBSE Knowledge Series: KS17 Indoor Air Quality and Ventilation: Ventilation may be driven by natural forces (natural ventilation) or by the use of a fan (mechanical ventilation). Natural ventilation uses buoyancy and/or wind to provide the driving force(s) for air movement, while in the case of mechanical ventilation air flow is driven by a fan.

EN 15239:2007 : Natural ventilation occurs through leakage paths (infiltration) and openings (ventilation) in the building and relies on pressure differences created by thermal forces (stack effects), wind forces or a combination of the above without the aid of mechanical means.

Mechanical Ventilation (also referred to as active ventilation)
A mechanical ventilation strategy relies upon fans to provide the driving forces for ventilation. Controlled levels of air temperature and/or diffusion of air is used to provide draught free ventilation. Optimisation of energy performance is achieved through a combination of energy efficient components and demand control strategies. Specific Fan Power (SFP) dictates the energy performance of the equipment and is based on the ventilation rates provided given the energy used. Pressure loss within designs can be overcome through greater fan energy.

Monodraught manufactures COOL-PHASE® low energy ventilation and natural cooling systems that use Phase Change Material coupled with an intelligently controlled air handling unit to provide demand control ventilation. This system maximises the effects of night time cooling and allowing the use of intelligent thermal mass to be used throughout the occupied period. Industry leading SFP’s combined with free cooling has enabled clients to achieve dramatic energy savings. Monodraught provides a complete design service which includes full dynamic simulation to show compliance against the relevant building standards.

Natural Ventilation (also referred to as passive ventilation)
A natural ventilation strategy relies upon a combination of thermal stack driven buoyancy and wind driven ventilation paths within a building. Control of openings can be from simple manual controls through to fully automatic intelligent façade or roof mounted systems that control openings in advance of building use. The design of the openings, positioning and control methods minimise draughts and maximise thermal performance. Relatively complex building simulations can be provided using CFD analysis and dynamic building simulation tools. Resistances imposed upon the ventilation paths can have a significant effect on performance. Therefore beware of designs that use simple spread sheets to try and simulate dynamic processes!

Monodraught manufactures WINDCATCHER® natural ventilation systems that utilise wind driven and thermal buoyancy forces to provide ventilation and heat gain dissipation. Integral control strategies maximise night time cooling and indoor air quality throughout the year. Monodraught’s commitment to building simulation can be seen in the advanced component models integrated as part of the IES-VE Pro suite. Further optimisation within the air flows are provided with low resistance, modulating Activlouvre® technology which comes complete with a no-leak guarantee, even under conditions of airborne snow particles such as blizzards which can compromise standard louvre designs.

Hybrid Ventilation (also referred to as Mixed-mode ventilation)
A hybrid ventilation strategy incorporates natural ventilation openings within its design and uses fans to provide boosted levels of ventilation and/or seasonal ventilation strategies. During summer periods, fans can be used to provide supplementary ventilation in times of low wind speeds or high temperatures, to assist air movement. During winter fans can be used to push air around, mixing fresh air and re-circulated room air, to provide a method of air tempering.

Monodraught launched its SOLA-BOOST® version of the Windcatcher in 2005 winning the Interbuild New Product Award in 2006. The system incorporates a solar powered boost fan located at the base of the ventilation system. Perhaps this could be considered the very first mass manufactured Hybrid ventilation system!

Monodraught manufactures Hybrid Thermal Mixing (HTM) systems, a combination of both natural and mechanical ventilation systems that have integral control software to optimise Indoor Air Quality, reduce energy consumption and provide fresh air, mixed with re-circulated air during winter periods.

When designing a Hybrid ventilation system it is important to note how often the mechanical methods of operation will be utilised. Under-sized ventilation opening or openings that provide higher levels of restriction will result in lower natural ventilation use over the year. If a Hybrid system is reliant on a fan to provide an effective ventilation performance for a higher proportion of occupancy periods, then we believe that this should be considered as a Mechanical ventilation system. We believe this lack of clarity has seen certain hybrid systems, which rely on mechanical fans to operate for a large proportion of the year, to slip through energy assessments without their true energy consumption being accounted for.

Likewise to claim that a ventilation system is a natural ventilation system it must therefore have no mechanical parts, such as fans, that are used to enhance, boost or induce an air flow within a building. The CO2 levels for natural ventilation are higher than mechanical ventilation as it is accepted that ventilation levels will vary through natural processes such as varying wind speeds.

As a long standing member of the CIBSE Nat Vent Group, we are seeking clarification from CIBSE on classification and the design criteria associated with these types of systems. Whilst this performance standard is considered we have decided to publish our own control strategy to disseminate this type of information.

Hybrid Ventilation Control Strategy
In order to minimise energy use, we operate a demand control ventilation strategy that leads to decentralised room based equipment with individual room sensing of temperature, air quality and user control.

  • We utilise three modes – Natural Ventilation, Boost Ventilation, Thermal Mixing Ventilation
  • At all times the occupants of the room are able to override the operation of the ventilation equipment with a default time used before the system reverts back to automatic operation.
  • To minimise energy use the system defaults to natural ventilation mode whenever possible. Natural ventilation can be provided through a combination of manual or automatic façade or roof openings and the openings that are integral to the hybrid system. The positioning of openings maximise the use of cross flow ventilation to assist with ventilating deep plan rooms.
  • When external temperatures are below the set point criteria the hybrid system will close automatic natural ventilation openings and indicate to the users to close manual openings. The hybrid system will automatically mix cold fresh air with warm internal room air. Modulating volume control dampers and varying fan speeds are used to ensure that the ventilation air provided is above 15°C and that optimum room conditions are met. Grille diffusers further ensure that the ventilation air provided does not create uncomfortable draughts.
  • When internal room conditions exceed the room set points (either CO2 set point or room temperature) and the external temperature is above the set point, the hybrid system will enable boost mode in order to vary the fan speed to assist natural ventilation.
  • During summer time the boost mode will be enabled during night time to purge the building and cool the fabric of the structure.

Set Points

  • Low External Temp < 16°C
  • High Internal Temp >26°C
  • High Internal CO2 >1400ppm
  • Low Internal CO2 <1200ppm
  • Minimum Mixed Air Temp >15°C

Climate change is acknowledged as one of the biggest threats to our environment, so it is important that efforts to reduce emissions focus on low carbon, energy-saving equipment that can help the UK achieve its targets for reducing carbon emissions. For many years natural ventilation has been the innovative technology that meets these criteria, and it is widely recognised as the most reliable and effective means of harnessing the wind’s potential as a renewable energy source.

These ancient principals, when applied to modern, zero-carbon buildings can keep 21st Century populations cool in hot weather, so we owe it to the specifiers of modern buildings to make sure they are not misinformed by misleading definitions.