Designing for Covid-19 and Wildfire Season

How do you design building systems for both smoke from the autumn wildfire season and now, a potential Covid-19 outbreak?

The challenge presented here is unique as the two goals are diametrically opposed. The first entails keeping contaminants out of a building, where the latter entails purging contaminants from a building.

Covid-19 transmission

Current guidance suggests one of the primary ways to minimize transmission of Covid-19 inside a space is an increase in both ventilation rate and providing this through predominantly full outside air. The key is to balance this with the energy consumption of the systems, to ensure that during a wildfire season, when full outside air is not feasible, the strategy is robust to minimize transmission and not compromise internal conditions.

Wildfire season considerations

Current guidance for the wildfire season is to use a large degree of recirculation for ventilation, which works against current Covid-19 guidance. Therefore new build and refurbished buildings will be required to take this into account to provide an energy and cost effective solution to maintain low Covid-19 risk. Obstacles to maintaining a healthy internal environment:

• Clogged AHU filters
• Fresh air polluted by smoke

Published guidelines for reoccupying buildings safely

• ASHRAE Position Document on Infectious Aerosols
• BCO Briefing Note 2020 – Thoughts on Office Design and Operation After Covid-19
• CIBSE Covid-19 Emerging from Lockdown, Safely Re-Occupying Buildings
• REHVA Covid-19 Guidance Document – How to Operate and Use Building Services in Order to Prevent the Spread of the Coronavirus Disease (Covid-19) Virus (SARS-CoV-2) in Workplaces

Anatomy of a typical system

Evaluation process

References: Ai, Z.T. Huang, T, Melikov, A.K. 2019. Airborne transmission of exhaled droplet nuclei between occupants in a room with horizontal air distribution. Building and Environment. 163(1)Zhang, N. Huang, H. Su, B. Ma, X. Li, Y. 2018. A human behaviour integrated hierarchical model of airborne disease transmission in a large city, Building and Environment. 127(1)He, Q. Niu, J. Gao, N. Zhu, T. Wu, J. 2011. CFD study of exhaled droplet transmission between occupants under different ventilation strategies in a typical office room. Building and Environment. 46(1)

Possible Interventions and Example Scenarios

Unavoidable Recirculation of Central Air

Where unavoidable recirculation occurs consider air scrubbers on the exhaust air and high grade MERV 15 / HEPA filters.

Mixed Mode Ventilation

A mixed mode ventilation system is suitable for natural ventilation for the majority of the time. In the event of poor air quality due to wildfires, a local MVHR system will provide minimum fresh air.

Local Modifications to Centralized System

Where only local modifications are possible to a centralized system. Consider in room modifications such as forming discrete bulkheads to allow horizontal air introduction.

Displacement Ventilation / UFAD

Displacement ventilation or under floor air distribution introduces air at low level with a unidirectional air path to high level exhaust. This washes clean air over the occupants and extracts contaminants at the ceiling.