CPD 5 2020: Natural ventilation; the principles, benefits and practicalities

This CPD, sponsored by WindowMaster Control Systems, looks at the advantages of natural ventilation and technology to optimise performance.
DEADLINE TO COMPLETE: 2 July 2020

Introduction:

Natural ventilation has the potential to offer considerable energy savings compared with mechanical systems and offers other advantages including better indoor air quality and savings on plant room space for new and retrofit situations. Given the performance gains on offer, it is important to be fully aware of the principles behind natural ventilation, how to optimise it for best performance and to understand the common pitfalls. This CPD will look at the advantages of natural ventilation, key design considerations and the technology available to maximise the benefits including automated window opening systems.

Automated Windows can provide effective ventilation solutions for small and large scale buildings

What is natural ventilation?

Put simply, natural ventilation is moving fresh, outside air naturally through a building to provide a comfortable and high quality indoor environment without fans, saving energy. Natural ventilation systems range in sophistication from simple manually openable windows to fully automated systems where the building management system monitors internal and external conditions and controls window actuators to keep indoor temperatures and air quality stable. Mixed-mode systems use natural ventilation most of the time and can include night cooling, switching over to mechanical ventilation and cooling only during peak conditions or where it is more energy efficient. This helps reduce the run time and load compared with pure mechanical ventilation, in turn reducing plant size and running costs.

Why use natural ventilation?

There is a wealth of information demonstrating the advantages of natural ventilation and its use is encouraged in a number of ways including the energy reduction hierarchy in the London Plan, Building Bulletin 101 and the PSBP Baseline Specification which includes technical guidelines on ventilation in schools.

The advantages of natural ventilation include:

  • Energy savings: According to the Fraunhofer Institute for Building Physics energy savings of 47-79 % are possible by replacing or supplementing mechanical ventilation with natural ventilation or mixed-mode air conditioning. The Carbon Trust found naturally ventilated buildings saved an average of £30,000 a year on energy compared with industry benchmarks for air-conditioned buildings.
  • Reduced costs: Eight case studies by Carnegie Mellon University showed natural ventilation and mixed-mode systems have a payback of less than a year due to energy saving and productivity benefits.Savings of up to 80% on lifetime costsare also possible because of reduced capital, operation and maintenance costs.
  • Healthier working environments: Carnegie Mellon also demonstrated a building with operable windows and natural ventilation reduced health-related costs by 0.8-1.3% compared with one with sealed windows and mechanical ventilation, a 3.2%reduction in absenteeism and a 65% reduction in symptoms of sick buildings syndrome.
  • Productivity improvements: Building energy consultantHeschong Mahone demonstrated a 7- 8%improvement in test scores for school children in classrooms with operable windows compared with children in classrooms with fixed windows. The research also revealed 77% user satisfaction with naturally ventilated spaces compared with 50% with mechanical ventilation. The Carnegie Mellon research also showed an annual productivity gain of up to 18%.
  • Saving on plant space: Opting for natural ventilation liberates space in ceilings and plant rooms so buildings can either be smaller or enjoy extra usable space.

Perceived challenges of natural ventilation

Natural ventilation is sometimes considered as more challenging to employ in larger buildings because of a perception of difficult to predict performance, overheating issues, and potential problems with draughts, noise and security. However, a good understanding of the principles herein, and common pitfalls, while embracing well-founded design guidance, modelling to prove designs and the latest product technology – can deliver high performance, cost effective and low energy buildings.

Principles of natural ventilation:

Natural ventilation utilises the physics of wind pressure and thermal buoyancy to move air through spaces. The latter is useful in large spaces with high heat gain or where there is no wind.

The CIBSE publication, AM10 ‘natural ventilation in non-domestic buildings’ is a useful source of guidance. It considers the limitations of single-sided ventilation and suggests this is only effective where the ratio of ceiling height to room depth is no more than 2.5. Single-sided ventilation is normally therefore restricted to shallow plan offices with low occupation densities.

Cross-ventilation is the preferred method of ventilating spaces with vents on opposing facades, rooflights, or acoustically treated louvres into corridors, common spaces, or atriums to take advantage of cross and stack ventilation.

Cross-ventilation allows better control of airflow by finely controlling vents in areas of differential pressures. This provides better distribution of incoming fresh air, and removal of stale, warm air more uniformly across spaces.

Night-cooling is an important part of a natural ventilation strategy. Cool night air is introduced into the building in a controlled way via automated vents to reduce the temperature of the fabric and contents of the building. The mass of the building can then absorb some of the excess heat and help stabilise air temperatures the following day by reducing temperature peaks and maintaining comfortable room temperatures for longer.

Using heavy-weight materials such as exposed concrete increases the thermal mass of the building and will maximise the temperature stability potential, but even in a typical medium mass building, night-cooling can have a significant impact. With an effective night-cooling strategy, it may be possible to reduce undesirable peak temperatures by up to 10oC and reduce the number of hours rooms are at higher temperature by over 60%.

When automating for night-cooling it is important to consider security. The ability to accurately set maximum opening limits for vents and obtain vent position feedback is important. Solid fixings are also important to ensure high levels of security can be maintained when vents are partially open.

Using high level windows for ventilation offers several advantages including fewer draughts, better security and a reduced risk of occupants trapping their fingers in a closing window. (Photo credit ©edmund sumner)

Operational considerations

Good design and modelling of assumptions is the first requirement of a successful natural ventilation strategy. The second is understanding how the building will be operated on a day to day basis to achieve a stable indoor environment and good energy performance.

Ventilation demand in a space changes during the day as room temperatures and air quality change from variations in heat gains and occupancy levels. At the same time, the forces that influence ventilation rates such as wind pressure, wind direction and temperature differentials will also change during the day.

It is important to understand what the ventilation requirements are at any one time and match it with the external conditions that will impact on delivering that demand. Over-ventilation can result in draughts and excessive heat lost, and under-ventilation can cause overheating and poor indoor air quality.

Ventilation demand and external conditions must be monitored, and vent openings carefully moderated to prevent excessive swings around temperature and air quality setpoints in order to deliver a comfortable indoor climate.

Airflow management though windows

Understanding how air flows through an opening is an important part of a natural ventilation strategy. In a cross-ventilation scenario with typical wind pressure, airflow through a window is not linear relative to its opening position. In other words, a window that is slightly open will exhibit a proportionally larger volume airflow than the same window fully open.

Wind tunnel tests of windows open in various positions under typical wind pressure conditions reveal that up to 60% of the airflow can occur in the first 5% of opening. This means it is important to be able to control window opening through small accurate adjustments modulating around the ideal vent position, and not larger oscillating swings of opening and closing that can upset comfort and energy performance.

The use of traditional timed and 0-10v type controls can make it difficult to achieve sufficiently fine levels of control due to system lag and larger increments of opening. This can lead to incorrect opening vent positions with the result the system is constantly trying to compensate, leading to excessive operation and windows going out of sync. A lack of acute control and window position feedback means the results delivered can be hit and miss and limits the potential for system optimisation.

Intelligent window actuators with accurate percentage position control and position feedback via digital two-way communication makes it far easier to accurately control window opening positions and be able to modulate very small, trickle vent positions accurately during winter.

Natural ventilation control

Manual control

The most basic way of controlling natural ventilation is by manually opening and closing windows. Cheap and easy to deliver, the disadvantage is that building performance is only as good as whoever is responsible for controlling the windows. Particularly in shared spaces where no individual is responsible for operating windows, there is plenty of evidence demonstrating that comfort and building performance regularly suffers as a result. Frequently in offices or schools – opening or closing windows is a reaction normally prompted by discomfort which is a sign that conditions have already deviated from the ideal and may be difficult to recover.

Manual control with traffic light systems

A step toward a more sophisticated approach is to combine manual control with a traffic light warning system linked to air and temperature sensors which indicate when windows should be opened or closed, but not by how much or for how long.

Research into the use of these systems in schools suggest that changing conditions in a densely occupied space could require as many as 40 changes to vent positions per day to maintain optimum room conditions. A classroom with four or more opening windows may therefore require in excess of 160 window interactions in a day. The result is commonly that the sensors are ignored and windows likely to stay open or closed for longer than that required to achieve ideal performance.

A big disadvantage of manually operated windows is also the inability to take advantage of the benefits of night-cooling.

Intelligent automation – the basics

Automated natural ventilation has the potential to significantly improve performance if properly considered at an early stage and as part of a holistic design approach. A study at Untermoos School in Switzerland compared similar classrooms with and without window automation. This showed rooms with automation achieved acceptable temperatures for three times the amount of time of those with manual windows, as well as 50% more time with good air quality levels, and 15% energy saving.

The window control regime and how this is specified is an integral part of a successful automated natural ventilation strategy. At its most basic, automated window systems are either fully open or closed with no intermediate positions. A system featuring rudimentary incremental control may not offer much more control as this could mean either closed, 50% open or fully open. To achieve the fine levels of control needed for a high-performance natural ventilation system there are two key elements to consider:

  1. The building management system (BMS) must be programmed by those with a good understanding of natural ventilation and sophisticated control regimes. Many of the more common BMS applications have more basic requirements compared with natural ventilation control with simple on or off settings. A successful natural ventilation strategy needs a much finer degree of control and the capacity to assimilate multiple concurrent variables of room temperature, outdoor temperature, internal carbon dioxide levels, windspeed and direction in relation to the capacity of ventilation openings. Understanding the resulting optimum vent positions therefore requires knowledge and experience of complex control algorithms to get the system right first time and avoid post installation trial and error adjustments which are often driven by user complaints. The specification should clearly state the system must have the ability for fine levels of control of the vents according to the many variables that will influence overall building performance and include a clear statement of operational expectations.
  • The window actuators and onboard technology must be able to support percentage positional commands via BacNet or other network communications, and position feedback to confirm the fine degree of control expected. They should also have the ability to operate very quietly during automation and more quickly during rain or other functions.

The use of multiple third-party components to build an automated system can complicate design and installation. One alternative is to use a packaged system or matched components from a proprietary solution provider who can supply all or most of the components. This makes it easier to optimise integration and performance of the system and has the benefit of a reduced chain of responsibility to help ensure solutions are delivered as intended. Which approach should be adopted will depend on the project requirements and budget, and specialist input can help navigate the various options and the most appropriate fit.

Design considerations

Facade design for automated natural ventilation can follow many different forms. Experience shows a model approach that offers a cost effective and high performing arrangement which is detailed below:

The atrium at the school of veterinary medicine at the University of Surrey features opening vents at ground and first floor level as part of extensive window automation across the building.

Window type

Automated high-level top hung outward opening windowsare often the preferred arrangement as these offer the most flexibility for automation and larger opening areas.

Other benefits include:

  • In winter cooler outside air can be introduced through small controlled openings and encouraged to mix at high level in the room to reduce direct draughts that can occur from side hung or low-level windows.
  • The risk of occupants trapping fingers is reduced because of the high-level position.
  • Small openings at high level for night-cooling are less vulnerable to security risks, particularly at ground floor level.

High-level automated windows will do most of the work, most of the time, but may also be supplemented by low-level manual windows if a greater ventilation area is needed and to give occupants a sense of control over their environment. Manual override of automated high-level window via a keypad should always be considered for this reason too. Manual override of automated high-level window via a keypad should always be considered for this reason too.

Actuator selection         

Actuator size and positioning will depend on the specifics of the window. Selection is normally based on the orientation and weight of the opening section and how far this needs to open. The bigger the load and opening, the bigger the actuator required although there are compact options including ones that can, depending on the window profile, be integrated into the frame. Although actuators cannot always be concealed, often a good balance can be found using actuator cover profiles that match the window. These can offer discrete aesthetics at high level and provide a good route for cables. Cable locations should also be considered at an early design stage.

Wider windows, typically over 1200mm, may also require more than one actuator to ensure the opening section seals well in the closed position. This depends on the window type and the flex of the window profile as it is pulled tight against the seal. Window and actuator providers can normally advise on this. Multiple actuators normally share a power supply cable so that these can be synchronised to operate without skewing the frame.

Actuators that operate on 24V DC and feature additional intelligence are normally preferred with such features as position control via addressable commands, position feedback and multiple speeds of operation for quieter automation. These are normally powered from a local power supply unit/controller that sits on the controls network.

Controls

Up to four actuators may be grouped locally where these can operate together and should be wired back to local controllers with suitably sized three core cables for the 24v DC power supply and two-way communication.

Local power supply units normally require a 13a mains power supply and may control up to 20 actuators in up to 10 independent control groups, allowing one controller to service multiple windows in multiple zones.

The controllers for an intelligent solution are normally integrated into the BMS network to reduce field cabling and enable BacNet or KNX communication. This allows the BMS to monitor specific vent positions and send accurate positional commands for changes to opening or closing as required. This arrangement also enables the system to gain feedback on position status and detect manual override which is useful for system optimisation and security reasons. 

Each room should include a temperature and CO2 sensor and local override for each bank of windows. The BMS or proprietary control system will normally use a weather station in conjunction with readings from the room sensors and seasonal information to determine optimum vent positions during the day and for secure effective night-cooling.

Taking advantage of specialist guidance early during design can help join up the relevant component factors for different design elements and help get the best performance and value for money according to project aspirations and budgets.

A useful tool for simple free area calculations is available here Free Area & Window Actuator Tool.

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