When is natural ventilation really natural ventilation?

Monodraught, ventilation, mechanical, natural, hybrid
Natural ventilation is popular in schools. This Monodraught X-Air installation is at Takeley Primary School

While the terms natural ventilation and mechanical ventilation are self-explanatory, the perceptions of hybrid ventilation, a combination of the two, are less well defined. Nick Hopper of Monodraught would like to see greater clarity about the various systems in order to ensure that clients and consultants understand exactly what is being specified.

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).

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 design service, Monodraught has seen little clarity in the design criteria for hybrid solutions.

We believe this lack of clarity can leave individual consultants or clients deciding whether a particular system is natural, mechanical or hybrid.

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.

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 we undertake to make this data available as standard.

Guidance from the CIBSE Knowledge Series: ‘KS17 Indoor air quality and ventilation’: explains that 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 explains that 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.

Monodraught donated Windcatcher X-Air natural ventilation, with internal louvres, for this Children in Need ‘Little miracles’ charity build project.

A mechanical-ventilation (also referred to as active ventilation) strategy relies on fans to provide the driving forces for ventilation. Controlled levels of air temperature and/or diffusion of air are 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 power.

A natural-ventilation strategy (also referred to as passive ventilation) relies on a combination of thermal-stack-driven buoyancy and wind-driven ventilation paths in 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 on ventilation paths can have a significant effect on performance. Therefore beware of designs that use simple spreadsheets to try to simulate dynamic processes!

A hybrid ventilation strategy (also referred to as mixed-mode ventilation) 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 by assisting 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.

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 relies on a fan to provide effective ventilation performance for a higher proportion of occupancy periods, then we believe that this should be considered as a mechanical 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 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.

Within school design guidance 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 differing wind speeds.

Natural ventilation for this Tesco store at Cheetham Hill, Manchester, is provided by 11 Monodraught Windcatcher units in Tesco livery.

To minimise energy use for hybrid ventilation, 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, and thermal-mixing ventilation

At all times the occupants of a room can override the operation of the ventilation equipment for a default time period 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 maximises the use of crossflow 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 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, the boost mode will be enabled during night time to purge the building and cool the fabric of the structure.

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 principles, 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.

Nick Hopper is technical director with Monodraught.

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