About the CHEPA model

The CHEPA (Classroom HEPA) impact calculator is designed to estimate how a HEPA filter unit could affect air quality and energy use in a classroom. This easy-to-use tool explores the potential benefits of adding a HEPA filter unit to a specific classroom, supporting informed decision-making. It's based on a scientific model developed by our team of researchers co-designed with partners and supported by UKRI funding. You don't need to understand the science to use it, but if you're curious, here we explain what's going on behind the scenes.

How does the calculator work?

The CHEPA impact calculator is built on a scientific model that simulates how air flows within, into and out of a classroom over time. This model, known as a coupled dynamic thermal and indoor air quality model, underpins the results shown by the tool. For those interested in the technical details, a full description of the model is provided in our open-access paper published in Indoor Environments.

  • “Coupled” refers to the fact that thermal conditions (temperature) and indoor air quality may influence each other.
  • “Dynamic” means that conditions such as temperature and air pollutants change over time. The model accounts for changes in indoor and outdoor temperature, people entering and leaving the classroom, and opening windows to regulate the classroom temperature and carbon dioxide levels over time.
  • “Thermal model” refers to how heat flows between classroom air, building fabric and the air outdoors. It considers the behaviours of staff and pupils in the classroom such as window-opening, and how this affects the classroom temperature and energy needs.
  • “Indoor air quality” model tracks carbon dioxide (CO₂), particulate matter (small airborne particles) and respiratory aerosols (from breathing). It models their production, movement, and removal, including by ventilation and filtration.

The CHEPA tool calculates two scenarios for your classroom: one with a HEPA filter unit in place and one without. It shows a side-by-side comparison of the results to help you understand the current condition and potential impact of adding a HEPA filter. Specifically, it estimates how a HEPA filter unit can:

  • reduce particulate matter in the classroom
  • lower levels of respiratory aerosols
  • increase or change energy use

The CHEPA impact calculator solves equations that simulate conditions inside a classroom. It combines your input with a set of standard assumptions to estimate how indoor air quality and energy use might change if a HEPA filter unit is added. The model looks at four main things:

  • Heat and energy: Heat moves between the building and the classroom air, and is carried in and out with the ventilation flows. Heat is generated by people, electrical devices, and the heating system in the classroom. HEPA filter units also use electricity. The tool adds this all up to estimate overall energy use.
  • Carbon dioxide (CO₂) concentrations: CO₂ is naturally released by people when breathing. It builds up indoors and is reduced through ventilation as fresh air enters the classroom.
  • Particulate matter (PM) concentrations: These are small airborne particles that either come in from outside (for example, car emissions) or are stirred up or released by people in the classroom (for example, fibres from clothing). These particles both enter and leave the building through ventilation. When a HEPA filter unit is in use, the particulate matter concentrations in the classroom are reduced by filtration.
  • Respiratory aerosols: These accumulate indoors by people breathing. They settle onto surfaces, are removed by ventilation or are filtered by a HEPA filter unit.

The CHEPA impact calculator can only provide an estimate of the impacts of adding an air filter unit to a classroom. These estimates are based on assumptions about the classroom's usage and performance based on your inputs, and some impacts, such as noise, are not included in the model. The results shown come from simulating a classroom, according to your inputs, for a duration of 500 days both with, and without, an air filter unit. In addition to your inputs, the simulations assume a representative occupancy for the classroom (i.e. a class occupies it on a schedule of school days, with two breaks and a lunchtime during the school day), and use outdoor air quality data from the UK Government's Automatic Urban and Rural Network and weather data sourced from the CIBSE Test Reference Year (TRY) weather files, available at CIBSE Weather Data.

Room size and basic layout

  • The floor area and ceiling height to calculate the overall volume of the room
  • The number of windows which affects how much fresh air can enter through ventilation
  • The number of exterior walls as these influence how much heatthe room gains or loses

Insulation and ventilation

  • Whether the room feels draughty when windows are closed
  • Whether most windows are double glazed
  • How well-insulated the walls are

Heating and equipment

  • The number of radiators or electrical heaters
  • The number of computers, laptops and other large electrical devices typically in use (these can contribute to the heat and energy consumption of the classroom)

People in the classroom

  • The typical number of pupils and staff in the classroom (contributing to carbon dioxide, levels of particulate matter, and are a source of respiratory aerosols)
  • The age group of pupils (which affects heat and carbon dioxide levels)

Air filter details

  • If you're considering a specific HEPA filter unit, you can specify the clean air delivery rate (CADR) - this is recommended but optional
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