Thursday, April 11, 2024

Mechanical Ventilation Can Cut Carbon Emissions in Schools by up to 40%

The average UK primary school is said to consume almost 120 kWh per m2 per year. The Carbon Trust estimates that over 50% of that energy use is by heating. By targeting space heating consumption in schools, we could see a substantial reduction in carbon emissions.

In ventilating classrooms air must be extracted from the room and replaced with fresh air. In the UK, ventilation has largely been managed by two factors: the building fabric and openable windows. Considering the building fabric, many buildings in the past were built with a poor air tightness. As such, regular exchange of air was possible through gaps in the fabric. Newer buildings, however, are required to meet a certain level of air tightness which prevents this natural exchange of air through the fabric. The result is a poor level of background ventilation.

Traditionally, to increase or decrease the ventilation rate in a building, one opened or closed the windows. However, there is one major issue with both this, and ventilation through the building fabric: heat loss. Consequently, space heating and ventilation are intrinsically interlinked.

Finding a balance between efficient room heating and proper ventilation is a delicate process. BB101: Guidelines on ventilation, thermal comfort and indoor air quality in schools stipulates that daily average CO₂ levels in mechanically ventilated classrooms should not exceed 1,000 ppm. To achieve this the air must be changed multiple times per hour. This air must be supplied at close to room temperature to avoid draughts. When the external temperature is lower than the room temperature, the incoming air must be warmed up, requiring a source of heat energy.  

Each classroom has a base heating load needed to maintain a certain room temperature. Classroom occupants negate some of the load due to their own heat output, typically at 70 W per person. This presents a challenge: how can we ventilate classrooms without losing heat? The solution is heat recovery.

As the name suggests, the driving principle of heat recovery is that heat is recovered from the air. The process is simple; stale air containing heat is extracted from a room. This air is passed through a heat exchanger, where it transfers its heat into fresh air from outside. This fresh air is therefore warmed up before entering the room with the energy from the room with an efficiency of about 90%. Consequently, up to 90% of the space heating from the room is recovered and won’t need to be made up from other sources. This type of equipment takes the form of mechanical ventilation with heat recovery or MVHR.

BB101 actually alludes to the difference in energy consumption between MVHR and natural ventilation commenting that, “systems with low initial capital costs may have unaffordable running costs.” (BB101 pg. 62, Life cycle and maintenance), often in the form of supplementary heating.

An alternative method of retaining heat in a room is recirculation of air. Whilst this has a relatively lower energy cost, it creates one major issue particularly relevant to today’s climate: recirculated air is contaminated air. Therefore, by recirculating air in a room you are increasing the risk of transmission of airborne diseases such as the SARS-CoV-2 virus. Furthermore, BB101 requires that the daily average carbon dioxide (CO2) level in a classroom must be below 1,000 ppm. CO2 levels above 1,000 ppm have been proven to have a dramatic effect on one’s ability to perform numerous cognitive tasks. Therefore, maintaining a low room-CO2 level is paramount in the learning environment.

Heat recovery ventilation can eradicate the issue of recirculation. MVHR units are equipped with a pair of fans, filters, and a heat exchanger. To maintain a good level of indoor air quality, the entire volume of air in the room must be changed 4 to 6 times every hour. In doing so, harmful pollutants found outside are also removed, whilst extracting potentially dangerous pollutants from inside. By extracting the room air and recovering up to 90% of the energy contained within, both energy consumption and indoor air quality will be dramatically improved.

To quantify the benefits of MVHR over natural ventilation or openable windows, a classroom typically requires about 3 kW of space heating to maintain the room temperature. Some of this may come from internal gains of occupants. To maintain good indoor air quality, this air will have to be extracted from the classroom 4 to 6 times every hour as previously stated. When using natural ventilation or openable windows, the 3 kW of energy in the room is able to flow out. As such, 3 kW of space heating would be required to return to the base level of heating. With MVHR, up to 90% of this energy is recovered, which is approximately 2.7 kW. To again achieve the 3 kW base level of heating, an input only 0.3 kW would be required. The result of using MVHR, therefore, is a much lower energy bill.

Much of Europe has been on board with using MVHR for some time now. Indeed, in all Scandinavian schools it is compulsory to use MVHR. These countries typically set a minimum heat recovery rate of 70-80% and have limits on specific fan power, which are improved year-on-year. Demand controlled ventilation is required in a number of European countries to prevent unnecessary use of energy when rooms are not occupied. We are fortunate in the UK to be able to benefit from the leaps in technology that these countries have been required by law to make.  For us SAV Systems that is exemplified in AirMaster Smart Mechanical Ventilation.

Author: Jonathon Hunter Hill, Sector Manager for Education, SAV Systems.

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