Nygel Humphrey, Product Marketing Manager – Non-Residential at Vent-Axia looks at the importance of good air quality in commercial and public buildings. He explains how on-demand ventilation with energy recovery, offers a far more fuel efficient solution compared to natural ventilation systems.
A 2005 CABE report for the BCO (British Council of Offices) found evidence demonstrating the link between poor workplace design, lower business performance and higher levels of stress experienced by employees. The report entitled, The Impact of Office Design on Business Performance highlighted differences in productivity of 25% reported between comfortable and uncomfortable staff due to basics such as air quality, temperature, overall comfort, noise and lighting.
Meanwhile, carbon dioxide levels in UK schools have frequently been measured as excessive. A 2002 study for the International Journal of Ventilation, ‘Carbon Dioxide and Ventilation Rates in Schools’, found that levels above 4000ppm (parts per million) with ventilation rates of less than 0.5l/s per child were commonplace. According to a 2004 DfES (Department for Education & Schools) report, higher levels of CO2 reduced the attention span of students, adversely affecting their learning ability.
CIBSE recommends a CO2 concentration of no more than 900ppm to achieve a medium quality of air. However, in the design of commercial buildings maintaining CO2 levels is largely a trace element within the overall ventilation strategy. The exception is schools where guidance on CO2 levels is integral in the design and control of the ventilation system.
Building Bulletin 101 specifies limiting CO2 levels within teaching and learning spaces to 1500ppm. It would be helpful to take this guidance and place it at the centre of ventilation strategies in all buildings to set a minimum air quality standard and relate it to the occupation density of the space. To do this effectively, fresh air supply rates must be considered, and, in our view, these should be set at between 10-12l/s.
Fresh air
Regrettably, fresh air supply rates per person in schools and other buildings are often so low that CO2 levels are, in fact, well above this recommended level. For example, last year research from Reading University and University College London revealed that new energy efficient schools were being designed to be more airtight to reduce heat loss, but as a result the schools featured appalling ventilation rates with CO2 levels exceeding targets.
Naturally ventilated buildings are often seen as the most energy efficient answer. Natural ventilation, of course, raises immediate air quality issues, especially in city buildings, where outdoor pollutants are brought into the building from outside. It’s also difficult to isolate external noise in these projects.
However, there are other problems. Building Bulletin 101 requires natural ventilation in schools to achieve an air flow rate daily average of 5l/s per child, a minimum level of 3l/s and a maximum capability of 8l/s with teacher intervention. The inclusion of a minimum level (where, interestingly, no minimum level is specified for mechanically ventilated schools) is a tacit admission that natural ventilation struggles to achieve the daily average and maximum capability levels required for schools. Being able to achieve just the minimum air flow rate level under Building Bulletin 101 will only lead to air quality issues down the line in the classroom.
From the energy efficiency and carbon emissions perspective, natural ventilation is also only an adequate choice. Because it is difficult to put resistance into the system, there is no capability to recover heat or cool in a naturally ventilated building. Instead, energy is needed to provide top-up heating to the fresh air brought into the building in winter or it is simply exhausted to outside when it could be put to other uses in summer.
Demand ventilation
In contrast, mechanical demand ventilation with energy recovery is the comprehensive air quality answer for commercial, leisure and public buildings. Demand ventilation works by responding to the exact ventilation demands of a room, supplying or extracting air only when and to the level it is required. The system is activated according to sophisticated control and sensing options, with no manual intervention required. Sensors communicate with the main unit which, in turn, drives the fan to the required speed to deliver the airflow and respond exactly to individual room conditions.
Since the system is demand controlled, rarely does it operate at full power for any considerable length of time to achieve the desired indoor air quality level. A fan running at 80% speed only uses 50% of the energy, compared to one running at full speed. As a result, on-demand ventilation is much more efficient than intermittent fan operation.
Recent changes to iSBEM will prove helpful to specifiers keen to select on-demand ventilation as the most energy efficient ventilation solution. In the previous version of SBEM, demand-controlled ventilation could be represented but required the calculation of parameter values. In the latest version of the compliance tool, there is for the first time the option for specifiers to select on-demand ventilation as part of their zonal control strategy, coinciding with the formal recognition of this control technology in the latest Building Regulations and the energy benefits it brings, whilst guaranteeing indoor air quality
The latest on-demand ventilation systems, such as, Sentinel Totus Demand Energy Recovery Ventilation (D-ERV), integrate 90% energy recovery into the ventilation system. This type of system extracts the energy from the warm, stale air taken from classroom areas before it is exhausted to outside whilst fresh, incoming air is preheated via the high efficiency plate heat exchanger and supplied into the classroom. The energy recovery process is proven to utilise up to 94% of the heat energy which would otherwise be wasted to outside.
An automatic summer bypass takes advantage of any free cooling available when the ambient temperature is below the room design condition, typical in spring and autumn. The TOTUS system also incorporates interlocks for associated heating and cooling equipment installed to optimise energy recovery potential and eliminate any possibilities of systems conflict. A night-time purge facility takes heat from the room overnight in the summer to reduce start up temperatures and help cut over-heat from non air conditioned spaces, or reduce air conditioning start up loads in air conditioned spaces.
Comparative studies we have taken based on an outside ambient of -4°C and a desired room temperate of 18°C, shows that an air flow rate of 5l/s per child will ensure that CO2 levels will not exceed 1500ppm. However, if natural ventilation is used, the result will be a total energy loss to ventilation of 4.04kW/hr per classroom whereas the equivalent figure for demand ventilation with energy recovery will be just 0.83kW/h. And that’s with the fans running on the mechanical system!
It’s not just in new building developments where this type of solution is attractive. On a recent project in Scotland, we have retrofitted demand ventilation with energy recovery into classrooms where natural ventilation previously existed. The TOTUS system is reducing energy demand so significantly compared to the alternative of natural ventilation and other demand ventilation systems on the market that we have eliminated the original requirement for a parallel upgrade of the building’s boiler systems, helping to cut the time and cost of installation as well as the property’s carbon footprint.