Frequently Asked Questions

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

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.

FAQs – Cost

How much does it cost?


The answer to this question depends on what units you need in the space. Take a look at our design tool and then give us a call to discuss your options.

Are naturally ventilated buildings more expensive to construct?


No. The costs and challenges of ventilation vary from project to project, but natural ventilation can usually be a cost-effective option.

FAQs – Control

How are the systems controlled?


The system is automatically controlled based on internal temperature and CO2 and external temperature. User control is minimal, an on/off/test switch which is usually always kept in the on position except for maintenance. If low-level windows are manually operated then a blue/red LED indicator provides guidance as to when to open and close the windows, but the users still have control.

The e-stack roof based system operates a Summer/Winter mode driven by an outside air temperature threshold, switching automatically between upwards displacement ventilation and mixing ventilation.

Can I change the set points?


Set points can be changed by the Breathing Buildings engineer when we attend site to perform final inspection / commissioning of the equipment.

User adjustable set points are optionally available, however we find our set point values (informed by years of real-project experience) are the most sensible choices to meet the majority of project needs.

Does your system control the heating?


Our system can control the heating if a complex heating control system is required in order for our ventilation to work efficiently. However it is usually sufficient for heating to be controlled on a thermostat linked to room temperature.

How do we explain to the users what it is and how to use it?


Breathing Buildings ensures that users understand the system by performing a client demonstration once the building is occupied.

We provide an operations manual for all our installations that help to understand the system and how it operates. If requested we can also provide a single sheet overview of how our system works, for distribution to the users of the space.

It is important from the outset that users understand that the system is not air conditioning and therefore does not guarantee internal temperatures.

Isn’t mechanical ventilated buildings always going to be more tightly controlled that a naturally ventilated one?


No. Often mechanical ventilation systems are not as accurately controlled as people believe. For example, the local area around a mechanical system’s delivery duct is susceptible to temperature gradients in the same way as an opening on the facade of a building. Issues of temperature stratification within a building, conflicting perceptions of thermal comfort, and air quality issues often arise with mechanically ventilated systems, especially in larger spaces with heat loads at multiple levels.

Manually controlled, Natural ventilation typically also has significant problems with these issues. The challenges of temperature variations etc. are reduced dramatically when a controlled natural ventilation system is used. The optimum solution is usually a hybrid control – part manual, part automated, natural ventilation design.

I’ve used natural ventilation before, but the user thinks that it is aircon and complain when it’s not 21ºC inside on a hot summer day! How do I explain the benefits and limitations to them?


The benefits of natural ventilation are prominent in the reduced energy consumption of the heating, cooling and ventilation systems. Of course the conditions inside the building aren’t going to be completely independent of those outside, and the occupants of the building should be made aware of this as early as possible.

When the building has lots of exposed thermal mass, such as concrete or food stock in supermarkets, we can use the night cool function to keep the temperature down. With a lightweight construction when it is hot outside a building with no cooling is likely to warm up too whether naturally or mechanically ventilated.

What communication protocols does the e-stack system use to talk to the BMS?


If the e-stack system is required to talk to the BMS then it can either be hard wired (usually in smaller projects) or we can communicate via MODBUS links.

This is dependent on the product type and application.  Usually BMS integration is carried out digitally using Modbus RTU with an agreed points list.  When integrating to TREND controllers, BMS providers typically use a Synapsys sip or Tridium Jace to act as a gateway.  However, some of our products can use BACnet MSTP which simplifies the “discovery” and interoperability of available data.

An alternative to digital data transfer is to use hard wire “analogue” integration, but this method limits the complexity of data that can be transferred and is reliant on the BMS system having I/O modules to receive data which can be expensive.  For these reasons, this method is normally only used for smaller schemes.

FAQs – Regulations

Does Natural Ventilation get me BREEAM points?


Yes, the inclusion of natural ventilation alone will directly help you gain 1 BREEAM point. However you can gain many more points indirectly from natural ventilation.

Here are just some of the hidden benefits of natural ventilation:

  • A building designed to incorporate a natural ventilation strategy will typically be more energy efficient so there will be CO2 savings.
  • Naturally ventilated buildings tend to be shallower plan, as you need a certain proximity to windows/openings, so the
  • building will tend to have better day lighting and lower lighting requirements.
  • The building can use free cooling and reduce the use of refrigerants

These are some of the areas where additional BREEAM points can be obtained:

  • Day lighting
  • View out
  • Reduction in CO2 emissions
  • Free cooling
  • Indoor air quality
  • Thermal comfort
  • Thermal zoning
  • Refrigerant GWP – building Services

Does Natural Ventilation meet BB101?


Yes. A well controlled naturally ventilated building can meet BB101 whether that is to provide occupants with fresh air or to minimise overheating in the summer.

BB101 is the regulation for ventilating school buildings. Much of the concern around natural ventilation arises when it is cold outside and people shut off the system to avoid cold draughts, obviously our unique, natural mixing ventilation strategy avoids this problem.

How does BB93 apply?


Table 1 in BB93 (2015) gives upper limits for indoor ambient noise level in occupied spaces based on LAeq,30mins. For a new-build classroom this is 35dB. If the space is naturally ventilated, controlling CO2 levels to 1500ppm occupied daily average, then the maximum allowable noise level is increased by 5dB to 40dB. Assuming 10-15dB of attenuation through a partially open window this means that natural ventilation is definitely achievable with external noise levels up to 50-55dB.

In addition to the above relaxation, internal noise levels can increase to 55dB for the hottest 200 hours of the year in order to prevent overheating. Breathing Buildings can model overheating with windows open for only the hottest 200 hours a year to ensure compliance with both BB93 and the overheating criteria. This allowance extends the range of sites that have the potential to be naturally ventilated up to external noise levels of 65-70dB.

For information on acoustic attenuation which can be provided with Breathing Buildings’ products, please visit Acoustic Attenuation.

All Breathing Buildings’ equipment is compliant with the BB93 requirements for noise generated by ventilation equipment.

How does the e-stack system comply with BB101 minimum guaranteed ventilation rates in winter, and minimise energy consumption?


The system is sized to meet minimum average ventilation rates of 3l/s/person in winter. The system does this by monitoring to CO2 levels in the room and feeding the signals back to our controller which in turn alters the damper position to admit the correct amount of fresh air into the room. The temperature of the incoming air is also measured to ensure that the fans run at a suitable speed to achieve the correct mixing ratios with internal classroom air.

The system minimises heating energy consumption because no preheating is applied to the incoming air, because it enters as a mixed airstream at high level.

How does the e-stack system comply with BB101 summer overheating?


Breathing Buildings carry out their own thermal modelling of spaces taking into account solar gain all other casual gains in the space, and using the closest CIBSE test reference year (TRY) weather file to predict summertime temperatures and ensure sufficient ventilation area is provided to prevent summertime overheating in accordance with BB101.

What about air leakage testing, surely putting lots of big holes in the building will create issues?


The dampers which are integral to our R, S and F-series units have been tested for air leakage. The high performance variable control dampers provide air leakage at 25% lower levels than current building regulations of 5m3/hr/m2.

FAQs – Installations

Are the units visible within a naturally ventilated buildings?


The e-stack unit itself can be either hidden within a ceiling void or bulkhead, or visible to the space. If visible to the space, the units can be painted to any standard RAL colour.

Can the system be connected to a BMS (Building Management System)?


The system has its own controls hardware and software, which allows it to operate as a stand-alone system without BMS input. However, various degrees of interlinking to BMS systems can be realised. Our controllers can include optional MODBUS interface units to allow communication with other systems. This allows certain setpoints to be adjusted from the BMS system, as well as other facilities such as monitoring and logging via the BMS.

Can you supply a maintenance service?


We are able to provide a maintenance service. For further details please contact us on 01223 450060.

Do your roof terminals allow water to penetrate through the blades?


Although no louvre system can be guaranteed to be 100% watertight under all weather conditions, our louvres are very high performance in terms of rain rejection. The louvre blades have been tested in simulated rainfall of 75 litres/hr (~3inches of rainfall in an hour) with wind speeds of 13m/s (~30mph) towards the louvre blades. The e-stack system operates with air inlet velocities typically under 1m/s when it acts as an air inflow pathway. Water repellent efficiency is described as a percentage of the incident rainfall.

Weather performance specifications are as follows for our louvre systems with insect screens fitted.

Double-bladed louvre system


Triple-bladed louvre system

The triple-bladed louvre system provides a higher level of weather performance, providing 100.0% (to the nearest 0.1%) rain rejection for air inlet velocities of up to 2.0m/s, under the same test conditions.

Does it need seasonal commissioning?


Once the units are installed, we attend site to install and test software to the system. This automatically adjusts the operation of the system seasonally and has been developed by drawing on our years of experience in natural ventilation.

However, every building behaves slightly differently, as a result of differences in construction, layout, location, orientation and operation. This can necessitate some initial alteration of setpoints during the first year of operation. We can provide options for seasonal performance evaluation and adjustment as required by the customer and/or end user.

What wiring do I need?


The system needs a single phase 230V AC mains supply and requires site wiring to local peripheral devices (sensors, switches etc.) Typical wiring details can be supplied on request.

Who is best placed to install these systems? How do they know what to do?


Installation is normally carried out by the mechanical and electrical subcontractor. Breathing Buildings supply an information pack describing the requirements for installation and provide CAD drawings of the equipment.

If required we will be happy to quote for the installation of our system. This will require appropriate equipment to be provided on site and can only be completed once all sensors have been wired into place.

FAQs – Terms and Conditions

When you are buying equipment from Breathing Buildings we use our standard terms and conditions as per the attached document. If you have any questions about these terms please contact us.

If you ask us to install the equipment we will use the attached terms to supply and a separate contract for the installation.

Breathing Buildings