Revisiting the design of domestic exhaust systems during COVID-19
By Ir Dr Eddy W T LAU

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The COVID-19 pandemic has prompted the need for rigorous research on building designs to reduce the risks of disease transmission in high-density urban environments, especially within high-rise residential buildings. While numerous technical articles have been written on the enhancement of drainage systems, relatively less research work has been conducted about problems caused by domestic mechanical ventilation systems. This article aims to review the common design problems of domestic bathroom and kitchen ventilation systems in Hong Kong and suggests a practical solution to resolve these problems.


The common problems

Taking public housing as an example, Figure 1 shows the layout of typical residential flats in Hong Kong. Two problems exist with this design:


1) The kitchen is usually equipped with a window exhaust fan and a stove exhaust hood. Unlike the toilet door, however, the kitchen door is a fire-rated door without any ventilation louvre. When the kitchen exhaust fan and/or hood is switched on, excessive noise is generated by air passing through the narrow door gaps. To solve this problem, a user would open the kitchen window to allow fresh air to flow in from the re-entrant.


2) The toilet window is normally very close to the kitchen window. The distance ‘a’ in Figure 1 is just 1.5 m. Therefore, if the kitchen window is opened to serve as the makeup air intake, there is a chance that foul air from the toilet would migrate into the kitchen when the kitchen exhaust system is operating. Such an incident was actually reported to have occurred at Oi Fai House in Yau Oi Estate in early April 2021. The problem in that incident was worsened by a wall opening cut between the toilet and the kitchen. In January 2022, multiple cases of horizontal and vertical spread of COVID-19 occurred in both private and public housing estates.


Self Photos / Files - Photo 1Figure 1: Typical residential flat design and simulated exhaust from the toilet


Comparison against international code

To assess whether the Hong Kong design is appropriate, the author has selected the International Mechanical Code (IMC) as the reference standard. The IMC is published by the International Code Council (ICC), a leading global source of model codes and standards and building safety solutions. The codes published by ICC include building codes, fire codes, plumbing codes and so on. These are all compiled through a governmental consensus process and are updated from time to time to include the latest advances in technology. The current version of the IMC is the 2021 edition.


The two relevant clauses in the 2021 IMC regarding the situation are:


1) Code 505.4, which states that exhaust hood systems capable of exhausting in excess of 0.19 m3/s shall be provided with makeup air at a rate equal to the exhaust air rate.


2) Code 501.3.1, which states that the termination point of exhaust outlets shall be located with the following distances: 914 mm (3 ft) from operable openings (i.e. windows) and 3,048 mm (10 ft) from any air intake openings (i.e. mechanical-assisted flow paths).


Regarding clause 1) above, most exhaust hoods being sold in Hong Kong (except the smallest size) have a design exhaust rate higher than 0.19 m3/s. For example, centrifugal motor type exhaust hoods with widths at 710 mm to 900 mm would have a Hi-setting exhaust air flowrate of about 0.21 to 0.27 m3/s, meaning that the provison of makeup air to the kitchen is necessary. Unfortunately, the vast majority of domestic kitchens in Hong Kong do not have makeup air provision.


Regarding clause 2), the kitchen window of Flat A in Figure 1 can be regarded as an air intake opening because the

mechanical exhaust system of the kitchen would help to draw in air from the re-entrant. The distance of ‘a’ is just 1,500 mm. This is very much less than 3,048 mm and would be regarded as non-compliant with the IMC. Figure 1 also shows other distances ‘b’ and ‘c’, which are 2.5 m and 3 m approximately. It can be seen that only ‘c’ is considered as marginally compliant. The non-compliant case of ‘b’ is conditional upon the scenario that the toilet window of Flat B is being used as an air intake opening, that is, a toilet exhaust fan operates while the toilet window is also opened at the same time.


Proposed solution

To address the above problems, the author has modified  the layout of Figure 1 by incorporating a common vent shaftnext to the kitchens and re-orienting the windows of the  toilets so that the toilet exhaust fans will discharge awayfrom the kitchens. This modification is shown in Figure 2 and further explained as follows:


1) The common vent shaft opens to the outdoors at the building roof and possibly also at the base of the building. A permanent opening is provided at the vent shaft (preferably at a low level near the floor) in every kitchen so that when the kitchen exhaust system is operating, ample makeup air would be coming from the vent shaft instead of through the door gaps. This design will thus prevent any noise problem. Two dampers need to be installed at the permanent opening, namely, a gravity-type non-return damper to prevent inter-floor smell spread, as well as a fusible link fire damper to prevent inter-floor fire spread in case a fire occurs.


2) The revised floor plan enables the toilet exhaust to be directed away from all adjacent windows. As a reference, IMC Code 506.3.13.3 allows the distance of exhaust outlet from adjacent air intake openings to be shortened to 1,524 mm (5 ft) where air from the outlet discharges away from such locations. In this case, the sum of distances ‘d’ and ‘e’ is 2,500 mm, which is compliant. Furthermore, the width of the re-entrant, that is, distance ‘f’, has been increased from 2.5 m to 3.6 m, with the width-to-depth ratio improved from 1:2.5 to 1:0.9 at the line of the toilet windows. This will greatly reduce the risk of pollutants becoming trapped in the re-entrant.


Self Photos / Files - Photo 2

Figure 2: Proposed new design and simulated exhaust from the toilet


Concluding remarks

This article has proposed a solution to two common problems in domestic ventilation systems in Hong Kong. Firstly is the lack of makeup air for kitchens due to the use of fire-rated kitchen doors, and secondly is the risk of disease spread due to the close proximity between toilet window and kitchen window. Nevertheless, the solution suggested only addresses the spread in the horizontal direction. In the situation where air containing viruses may be spread from a lower floor to an upper floor, the safeguard still relies on the user closing the window whenever his/her exhaust fan is operating. This can be accomplished by an interlock between a window sensor and the fan power supply. The author estimates that incorporating a projecting building feature, such as a concrete canopy, above the toilet window should also help in reducing such risk. This feature will separate the outgoing air stream from the incoming air stream at two different floor levels. This topic will be a subject of further research.


About the author: Ir Dr Eddy W T Lau, a Fellow of the HKIE, an R.P.E. in the Building Services discipline, and the Head of Green Labelling at Hong Kong Green Building Council.

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