FAQs – How it Works

What is natural ventilation?


Natural ventilation is a low energy way to provide a flow of fresh air through a building using the natural forces of wind and/or buoyancy.

Modern buildings tend to be well insulated to retain heat in the winter, so to prevent rising CO2 levels you need to provide fresh air. Natural ventilation is a low energy way to provide this fresh air using the natural tendency of hot air to rise and cold air to sink. By understanding how air will move naturally in a space you can provide openings that allow you to control the ventilation without the need to force this using energy intensive mechanical systems.

What is hybrid ventilation?


Hybrid ventilation, also known as mixed-mode ventilation, describes a ventilation system which is neither entirely natural nor entirely mechanical. The system combines both natural and mechanical ventilation solutions to meet the ventilation requirements of the space. A hybrid ventilation strategy means that mechanical fans are only used to help ventilate a space when natural ventilation is not sufficient. This minimises the energy consumption of the ventilation system whilst ensuring sufficient ventilation capacity under a wide range of conditions.

For more information please visit Hybrid Ventilation.

How does the e-stack system work


Breathing Buildings’ e-stack systems use a unique ventilation strategy which varies throughout the year depending on the external environment:

  • In summertime the e-stack system works in conjunction with low level opening windows to effectively ventilate a room naturally.
  • In winter, a mixing ventilation strategy operates in the e-stack system. This strategy eliminates cold draughts without the use of preheating radiators/heating batteries and hence has major energy saving benefits.

The system is electronically controlled, measuring internal and external temperatures and CO2 levels; an LED indicator light on the wall panel indicates to the occupiers whenever windows should be opened or closed, based upon internal and external temperatures.

The system provides fresh air to the space by monitoring the CO2 level and avoids pre-heating of cold outside air. The natural mixing allows a considerable amount of energy saving.

Do I need to have a specific building design?


No, the e-stack system can work in a broad variety of building designs and uses. There are two design solutions for the e-stack system our roof mounted units and an atrium based system.

The roof-mounted solution is suitable whenever a room has direct access to the roof. In this case the ceiling needs to be capable of housing the e-stack unit, which is at least 1600 mm long and 950 mm wide. This is linked to the roof by a shaft and capped with a weather-proof terminal. You can choose between our R-Series and S-Series units in this situation, depending on the room design and occupancy.

Many buildings however are multi-storey and sinking a shaft to the lower floors can use valuable floor space on the higher floors. In this situation we often find that the building incorporates a central space, a perfect natural mixing chamber. In this situation our atrium system is perfect.

We use the central space as a mixing chamber, controlling the temperature in the central space and exchanging air with the individual rooms. In this case the units are installed at the top of the wall of the room. Noise attenuators are provided to preserve the acoustic integrity of the rooms and grills can be installed at the inflow/outflow of the unit. This is a wonderfully flexible system and can be adapted to different building layouts.

Does Breathing Buildings have views on Passivhaus designs?


The Passivhaus building principles such as high levels of insulation and air tightness are properties a building should have. However, the ventilation system used on many Passivhaus schemes is mechanical ventilation with heat recovery, which is specific to German requirements where Passivhaus originated.

In Germany long periods of time there are spent in sub-zero temperatures and buildings are a net heat sink, with no net heat gains. This means dedicated space heating is often required to maintain the desired temperature and it is economical to expend electrical energy to recover as much heat as possible from the outgoing air stream.

In the UK, most buildings are actually a net heat source with net heat gains meaning dedicated heaters are not required to maintain the space at the desired temperature. It is not economically beneficial to extract as much heat as possible from the outgoing air stream, which uses relatively large amounts of electrical energy to drive the air over a heat exchanger.

Does it only work on the top floor?


No, natural ventilation can be used on multi storey buildings. There are multiple ways we can achieve this. You can use our NVHR units, our facade mounted system, our roof mounted system and provide shafts to the lover floors or use our atrium system.

Our NVHR units are a very popular solution, providing single sided ventilation to the room. Each unit has low energy mixing fans to mitigate against cold draughts in winter.

The atrium system requires less floor area than is required for the shafts for a roof solution. Using the buoyancy head and the effective area of openings is possible to design a building to be naturally ventilated almost at every floor.

The difference in pressure between the interior and the exterior of the building varies with the floor levels; the internal pressure variation with the height of the buildings depends on the size of the openings at the high and low level of the buildings and on the heat gain within the space.

How can you provide Natural Ventilation at a noisy site?


Yes, we have helped several clients with strict noise requirements using acoustic attenuation. The attenuation can be provided at low level, by putting acoustic baffles into louvres, form part of the roof terminal, or sit within the shaft between the roof terminal and the room. Often, acoustic attenuation may only be required at high level (i.e. in the roof terminal or shaft) as this is the primary path for ventilation air.

How does purging a science room work with Natural Ventilation?


The purge requirement of BB101 requires that science rooms can have a ventilation rate of 10 air changes per hour.

The purging strategy depends on the type of ventilation system in place and is perfectly achievable with a natural ventilation system. When e-stack units are used in the science room, there are two simple methods of purging. If there is direct access to the roof, then the purge will involve extracting air from the room to the exterior; opening low level windows will increase the purge rate further but is not critical if an e-stack R or S-Series unit is being used.

Is it true that you can avoid cold draughts?


Yes it is true.

In winter cool, fresh, external air and warm, internal air are mixed within the unit at ceiling level avoiding cold draughts. The mixed air that reaches occupants is well mixed avoiding cold draughts.

In conventional building design, fresh air is provided through opening windows and needs to be heated as it enters the space to avoid cold draughts for people close to the windows. This doesn’t make sense to us when in most spaces we have more than enough heat being generated to provide the pre-heating naturally.

Is mechanical cooling required?


Sometimes even a well-designed natural ventilation scheme may be unable to maintain internal temperatures within the range required by the client. This can be the case in particularly strictly temperature controlled environments, or if there are exceptionally high internal heat gains.

In such situations, a strategy should be followed that maximises the use of natural ventilation and night cooling, and minimises the number of hours and the power of cooling which must be used.

Breathing Buildings control systems have the capability to interlink with AC cooling systems, offering a hybrid solution which makes best use of natural ventilation whenever possible, but hands over to the mechanical cooling as and when it is needed.

For more info, contact the design office by emailing info@breathingbuildings.com

Is natural ventilation effective on days with no wind?


Yes with the e-stack system it is.

Natural ventilation can be driven in two ways; one is by wind, the second is through buoyancy (the stack effect). As air heats up it becomes less dense so will rise in a space – this is the main principle behind the stack effect.

If you have a room with openings both at high and low level, fresh air will typically enter the space through the low level openings. As it heats up due to the internal heat gains it rises and exits through the high level openings. This strategy is not reliant on wind pressures so will be effective even on days with no wind.

Is natural ventilation just opening windows?


Opening windows can provide occupants with ventilation. However, ventilation is not just about giving people fresh air – it is also about providing comfort for occupants. Comfort for occupants cannot be achieved from opening windows alone. One example of this is opening windows for ventilation in winter; the occupants in the space will soon experience uncomfortable cold draughts. Good design adapted to each individual building, combined with a good control strategy, can help give the occupants a more comfortable environment.

Radiators or underfloor heating which is better for natural ventilation?


Underfloor heating systems normally consume less energy overall than radiators, and the relatively low temperatures required makes them ideal for utilising ground source heat pumps, which are very efficient. The downsides with underfloor systems are that there is high thermal inertia, and underfloor heating is not good at tempering incoming air.

With an e-stack system, the most efficient way of heating a space would be using a skinny radiator with low thermal inertia. This could be switched on immediately prior to occupancy to warm the space, and then switched off, to allow the heat gains from people and lighting to maintain room temperature.

With an underfloor heating system, care must be taken not to overheat the room, which could result in energy being wasted once the heat gains due to occupancy and lighting take effect. The underfloor heating system should only be used to keep the room to the minimum acceptable temperature, and switched off in advance of occupants arriving because the large thermal mass inertia will maintain temperature for some time after the system has been switched off.

Is the e-stack system in winter mixing mode the same as a heat recovery system?


A heat recovery system forces incoming fresh air and outflowing stale air to be passed through a heat exchanger. The fan power used in driving the air through these devices is higher than in an e-stack unit. In very cold climates, or where there are insignificant heat gains within an occupied space, the heat recovered in mechanical ventilation heat recovery systems more than outweighs the fan power used and is a sensible solution for winter.

However, in more temperate climates and in cases where there are reasonably high internal gains, the challenge is to simply use the heat rather than recover it. A mechanical ventilation system with heat recovery is not necessary, and is in fact a higher energy option than the e-stack because the fan power used in the mechanical system is much higher.

What types of buildings does natural ventilation work in?


Natural ventilation can work in any building. However we are pragmatists and in some building types it is not the most efficient form of ventilation.

Natural ventilation can work in any building provided that an appropriate air pathway can be provided from inside the building to outside and that the external air is of suitable quality and temperature. In practice, natural ventilation is most effective in buildings that are shallow plan, so the air does not have to flow too far laterally within the building (or through / around restrictions) before or after reaching occupants.


Breathing Buildings