17th August 2020
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Natural Ventilation

At a time when all things’ natural’ seem to have taken centre stage, natural ventilation, in contrast to mechanical methods, exhibits a number of advantages worth considering, including:

  • Significant energy savings
  • Improved indoor air quality
  • Space savings

To capitalise on these advantages and to optimise its effects, it may prove helpful to explore the science behind natural ventilation and to consider how to avoid mistakes during its implementation.
In principle, natural ventilation is easy to define: it is no more complicated than circulating fresh air from outside a building, through its interior in a way that offers a pleasant, superior-quality inner environment, achieved without the use of fans (and therefore saving energy).
Choices in how to deploy natural ventilation vary along a technological continuum. At one end you have simple ventilation systems that require no more than the manual opening and closing of windows. At the other extreme, a high degree of automation takes over, with computer systems taken control of actuators which open and close ventilation on the command of the CPU. Such systems stabilise building temperatures, keeping air quality stable. Somewhere between these extremes, you will find methods which combine high and low tech solutions: computerised controls taking over when it’s expedient to do so for optimum performance.

The argument for

Finding positive reviews of natural ventilation systems isn’t difficult. The London Plan, Building Bulletin 101 and PSBP Baseline Specification all sing natural ventilation praise. The following six key advantages stand out:

  1. Energy Savings: Energy savings of between forty-seven and seventy-nine per cent through completely switching to natural ventilation, or by augmenting mechanical ventilation with natural ventilation (Source: Fraunhofer Institute for Building Physics). The Carbon Trust calculated that naturally ventilated structures made average energy savings of £30,000 a year when compares to industry-standard benchmarks buildings utilising air-conditioning.
  2. Lower costs: Eight case studies demonstrated that natural ventilation and hybrid systems show payback periods under 12 months (Source: Carnegie Mellon University). Further savings of eighty per cent over the lifetime of such systems manifest through reduced cost of capital, operations and maintenance.
  3. Healthier environments: Buildings with controllable windows and natural ventilation were shown to lower health costs by 0.8-1.3% in comparison to systems involving sealed windows and mechanical ventilation. Also, in such cases, absenteeism fell by 3.2%, and symptoms of sick-buildings syndrome fell by an impressive 65% (Source: Carnegie Mellon University).
  4. Improved Productivity: It has also been shown that between 7% and 8% improvements are possible in children’s test scores in schools which had manually operated windows as opposed to schools where fixed windows were the norm (Source: Heschong Mahone).
  5. Higher user satisfaction scores: Further research produced evidence of 77% user satisfaction when considering spaces with natural ventilation. The comparable figure for mechanically ventilated areas was 50%. Carnegie Mellon’s investigation also demonstrated a year on year productivity gain of up to 18%.
  6. Plant space-saving: Natural ventilation frees up overhead space and plant rooms offering the possibility of designing smaller buildings or finding innovative ways to use the extra space.

Perceived difficulties

Where more significant buildings are concerned, natural ventilation is often thought of as more challenging. The perception is that, in larger structures, it is problematic to accurately calculate performance. There are also problems with overheating, draughts, unacceptable noise levels and security. All of these issues can be rigorously mitigated by taken a professional design approach, resulting in higher performance at a cost-effective level.

The science of natural ventilation

Natural ventilation leverages the science of wind pressure and thermo-dynamics to circulate air in space. The thermo-dynamic aspects of the issue prove especially useful in more significant areas where there is high heat gain or in situations where the wind is absent.
Professional guidance can be found in the CIBSE publication, AM10′ natural ventilation in non-domestic buildings”. This document examines the limitations of one-sided ventilation only if the ratio of the ceiling height to the depth of the room is only 2.5. For this reason, one-sided ventilation is usually restricted to shallow-plan offices with low occupation.
A method known as cross-ventilation is the preferred option when ventilating spaces characterised by having vents on opposing faces, roof-lights, louvres into corridors, shared spaces, or atriums to take advantage of cross and stack ventilation. Cross-ventilation allows better control of airflow through the excellent control of vents in areas of differential pressures. This improves the distribution of incoming fresh air and removes stale, warm air more uniformly across spaces.
Key to natural ventilation strategy is night-cooling. At night, the cooler night air is offered controlled ingress to the building, using automatic vents to reduce the fabric and content temperatures of the building. This permits the building’s mass to absorb a portion of the excess heat, assisting in the stabilisation of air temperatures during the following day. In effect, lowering temperature highs and keeping rooms at a comfortable temperature for longer.
Heavy-duty construction materials like concrete add to the thermal mass of the building, maximising the potential for temperature stability. In a typical structure of medium mass, night-cooling can make a significant difference. Get the night-cooling strategy right for such a building, and you might expect to lower unwanted peak temperatures as much as 10C and to lower the time rooms record higher temperature by more than 60%.
The downside to night-cooling is that you will need to exercise more care over security. It is highly desirable to have the capacity to accurately set opening limits for vents. And to have some means to feedback vent position data. The highest quality fixings must be employed to keep security levels high at times when vents are open.
Spend a lot of effort into the design stage of your natural ventilation strategy to ensure success. You must also gain a comprehensive understanding of the ergonomics of daily building use to maintain a stable environment and high energy performance.
The thermo-dynamic attributes of the situation can be complex. Ventilation demand varies throughout the day as temperatures and air quality levels rise and fall in the face of continual changes in heat gains and occupancy levels. In addition, forces influencing ventilation rates (wind pressure, wind direction and temperature differentials) vary over the course of a day too.
You need to continually understand current ventilation needs and match them with the external conditions that will impact on their delivery. There is a fine balance to be struck here — over-ventilation causes draughts and unwanted heat loss, but under-ventilation can cause overheating and lower indoor air quality.
The demand for ventilation and external conditions need to be continuously reviewed. Care should be taken to moderate vent opening so as to prevent excessive swings in temperature and air quality to optimise the delivery of an effective indoor climate.

Airflow Management

A thorough scientific understanding of how air flows through an opening is essential to a successful natural ventilation strategy. In a cross-ventilation situation within a context of typical wind pressure, airflow through a window will not be linear (relative to its opening position.) Put another way, a slightly open window exhibits a proportionally larger volume airflow than will the same window when fully open.

Wind tunnel data on windows open to a variety of degrees show that up to 60% of airflow can take place in the first 5% of the opening. The scientific import of this is that for the optimum effect we need to exercise a fine level of control so we can open windows using small by highly accurate adjustments around a baseline of the optimal vent position. We need not bother too much about swings of greater magnitude as these have the potential to negatively impact on comfort and energy performance.
The ability to adjust precisely and in a timely way is critical. System lag in traditional systems using 0-10v type controls do not facilitate the desired fine levels of control needed for an effective method. There is a danger of undesirable vent positions, leading the system trying, and failing on occasions, to adequately compensate. The upshot of this can involve excessive operation and unsynchronised windows. It is the lack of acute control and the absence of window position feedback that creates a risk to system optimisation.
Luckily, a digital solution is available. Hi-tech window actuators with high-sensitivity position control and feedback make window control easier with the ability to modulate tiny, trickle vent positions accurately during winter months.

Controlling Natural Ventilation

Manual

The simplest and cheapest method of controlling natural ventilation is to open and close windows manually. Unfortunately, this introduces human risk factors leaving the effectiveness of the system to the performance of human agents. Psychologically, it is a commonplace for performance to suffer in situations where no single individual has responsibility for window operation. You often find in schools and offices that opening and shutting windows results from feelings of discomfort. And this means that conditions must already have deviated from optimal.

Manual (with ‘traffic lights’)

A more intelligent approach would be to pair a manual control system with a ‘traffic light’ system communicating with air and temperature sensors capable of detecting when windows are to be opened or closed.
Research into schools reveals that to be able to vary conditions in the sort of densely occupied space that most schools represent might necessitate up to 40 changes to vent positions every day to keep temperatures within optimum parameters. This may seem manageable, but considering a typical class with four windows might require 160 daily window adjustments, the situation looks a little more daunting. In practice, sensors tend to be ignored, and windows are likely to stay open or closed for longer than necessary for optimal performance. A further disadvantage of manual window systems is their lack of capacity to take advantage of night-cooling.

Intelligent Systems

Build an automated natural ventilation system after careful due diligence and planning, and there’s a good chance of a significant performance improvement. Swiss studies have shown that rooms with automated windows held acceptable temperatures for three times the length of time demonstrated by rooms with manual windows. The advantages didn’t end there, though: The automated rooms managed 50% more hours with good air quality, and at a 15% energy saving.
A robust specification for a window control system is critical to success. In the simplest case, automated window systems are fully open or fully closed. There are no positions between these extremes. You could design a simple system with incremental control, say by adding the capacity for windows to be half-open, but to really achieve the finer levels of control typical of a high-performance natural ventilation system there are two main considerations:
It sounds obvious, but it is essential that a programmable building management system (BMS) is programmed by specialists and experts in the field of natural ventilation which have experience with sophisticated control regimes.
Compared to natural ventilation systems, many common BMS applications just have basic requirements, with simple on or off settings. But, a successful natural ventilation strategy demands a more sophisticated degree of control and the ability to manipulate larger numbers of concurrent variables such as room and outdoor temperature, internal carbon dioxide levels, wind speed and direction in relation to the capacity of ventilation openings. Understanding how to optimise vent positions can be achieved only by those with the skills and experience of complex control algorithms. You want to aspire to a ‘measure twice, cut once’ approach. Get the system designed right first time and avoid post-installation ad hoc adjustments arising from user complaints. Any system specification must guarantee fine levels of control of the vents, in accordance with all variables influencing performance. Specifications should also include clearly stated operational expectations.
There should be an insistence that window actuators and related onboard technology must be able to support percentage positional commands via BacNet or similar network communications, as well as position feedback to confirm the expected fine degree of control.
Some automated systems assembled from a variety of third-party components may complicate design and installation. Another way is to install a packaged system a solution provider who can supply all or most of the parts you need. This optimises integration and performance of the system, and shortens the chain of responsibility, raising the chances of timely delivery.

Design Factors

The design of facades for automated natural ventilation takes many forms. Automatic high-level top-hung outward opening windows work particularly well because they offer a higher degree of flexibility for automation as well as larger opening areas.

Other Benefits

In winter, colder outside air enters through small controlled openings to mix high in the room, reducing draughts that can arise from side hung or low-level windows. From a health and safety point of view, the risk of fingers being trapped is virtually eliminated. Small high up apertures for night-cooling are also far less of a security risk, especially compared to openings at ground floor level.
The proposed high-level automated windows cover most eventualities, although they may be augmented by low-level manual windows where there is a need for a larger ventilation area. And to give users a sense of control over their environment—an important consideration—consider including a manual override of automated high-level windows via a keypad.

Choosing the right actuator

Window and window positioning specifications determine optimal actuator size. Critically, selection should be made on the basis of the orientation and weight of the opening section as well as the extent to which it needs to open. The larger the load and the bigger the opening, the bigger the actuator needed. Consider cable locations early in the design process. Wider windows may require more than one actuator.
24V DC actuators with additional intelligence features are to be preferred.

Controls

Local power supply units generally need a 13a mains power supply and are capable of controlling up to 20 actuators in as many as 10 independent control groups. This arrangement facilitates one controller, servicing multiple windows in multiple zones.
Consider having temperature and CO2 sensors in every room. This is the best practice. The BMS system typically uses a weather station communicating with room sensors and fed seasonal information to determine best vent positions at all times.

Chris Nicholls
Commercial Director

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