By Jonathon Hunter Hill – Sector Manager for Education, SAV Systems.
Inflation is at a 40-year high. The price of energy has doubled. Minimising heat loss in schools should be the name of the game. But schools present a particular challenge: 60 m2 rooms with 32 occupants, and a requirement to achieve a daily average CO2 concentration of less than 1000 ppm, all whilst maintain good indoor air quality and thermal comfort.
When energy was plentiful and cheap, the simple solution (although negating thermal comfort) was to open the windows and flood rooms with fresh air. When our buildings were not airtight, this natural ventilation methodology made sense. But minimising heat loss through the building fabric is one of the first steps any building operator must take.
To maintain good indoor air quality in classrooms, we need approximately 6 air changes per hour. Each one of those air changes would be accompanied by a great deal of heat. In a year, a natural ventilated classroom designed to meet the criteria of BB101 could use as much as 5,000 kWh/year of energy to ventilate and heat the room. In moving towards net zero, this has to be cut.
The Passivhaus Trust, in their own research, has found that mechanical ventilation with heat recovery (MVHR) not only uses the least energy in comparison to other options, but also achieve the best indoor air quality. In a typical classroom, the energy consumption for heating and ventilating the room with decentralized MVHR should be approximately 500 kWh/year – a 90% saving.
How does MVHR achieve such good indoor air quality? A heat exchanger is designed in such a way that the air pathways between the supply air to the room and extract air from the room do not interact. There is no air recirculation or recycling because heat is recovered through the heat exchanger. As such, CO2-laden air is replaced with fresh air, instead of recirculating that air to conserve the heat.
Decentralised MVHR is always demand controlled, meaning that fan speed is linked to a measure of indoor air quality – typically room CO2 concentration. Consequently, indoor air quality can be closely controlled whilst minimising heat loss, and maximising thermal comfort.