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Building Air Balance
11 July 2016

John A. Herbert, Hong Kong
John A. Herbert
Director


Improperly unbalanced HVAC systems in buildings are common, this imbalance can occur when the air flow rate of make-up, or replacement air, into the space is less than the rate of exhaust air, this creates a nett negative (-ve) air pressure in the space.

Equilibrum must be maintained, therefore this nett negative pressure drags replacement air through any available opening, gaps around doors, gaps around windows, and sometimes, the path of lowest resistance is through the drainage system.

Spaces with polluants and odours, such as toilets, shower rooms, refuse rooms, cleaners room, kitchens and the like, should be designed to provide nett negative air pressure to control potential polluants, so they do not migrate to other parts of the building causing nuisance. This can be simply achieved by designing the air flow into room, and or in the adjoining space.

The following video clip shows a typical bathroom room situation, the exhaust fan is switched ON and the bathroom door is closed. First, we discover that the door provides an airtight seal, helped in part by the nett negative pressure.

Without another opening, replacement air is drawn through the floor drain pipe, the tell-tale streamer indicates the direction of the air flow, from the drain pipe into the occupied space!

Building Negative Pressure in action


Air from the bathroom floor drain brings sewer gas, bacteria, and foul odour in to the building. The air flows through the floor drainage pipe because the negative room pressure exceeds the water seal depth a common problem in Hong Kong (SARS 2003).


Therefore, the ventilation air balance is critical to maintain a healthy building, yet it is often overlooked.

In tropical regions it is critcial, where buildings are air conditioned for comfort, nett negative (-ve) pressure must be avoided otherwise hot-humid outdoor air will be drawn in to the space, moisture will condense on the cold surfaces, giving rise to moisture problems and mould.

Kelcroft takes the extra time and effort to review the air balance when commissioned to conducted an energy audit. And in our expereince, we find that most buildings have been modified by either owners or tenants, without providing sufficient make-up air, causing nett negative pressure, and the unwanted side effects causing mould.

About the Author

John A. Herbert is a veteran engineer with more than 30 years engineering experience, educated in the United Kingdom he has worked across Asia for the last two decades engaged by international and local companies. He is a Hong Kong Registered Energy Assessor (REA) and BEAM Professional.


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