HIGH-PERFORMANCE HVAC
FINDING
E C N A BAL
A
t the risk of sounding obsessed with ventilation specifically and HVAC design generally, I will admit that whenever I pull on the front door of a restaurant, I pause to notice if I am pulling against the negative pressure of the kitchen exhaust fan. If there is a big woosh of air, I question whether there has been a proper kitchen exhaust design completed, or if the building owner is just trying to avoid heating and cooling costs of operating a make-up air system. If there is no woosh of air, I imagine either they have a wellbalanced air make up system or that the kitchen workers may be experiencing less than ideal air quality due to inadequate exhaust. It might seem obvious, but I find some building owners or operators and the odd HVAC contractor tend to forget the basic physics that air out always equals air in. Proper HVAC design must consider keeping an appropriate balance across the building enclosures. Consider why maintaining this balance is important: physics dictates that whatever amount of air you need or want to exhaust out to control pollutants from cooking, welding fumes, vehicle exhaust or any other applications, the same amount of air needs to come in to replace it. If there isn’t sufficient replacement air, referred to in codes and standards as “makeup air,” a negative pressure will be induced on the building enclosure itself, and subsequently on the exhaust fan. This has important ramifications: First and most importantly, other appliances in the building susceptible to negative pressures will be impacted. Always consider the combustion safety impacts of large capacity
Gord Cooke
18
M e c h a n i c a l
OVERCOMING BUILDING PRESSURE PRESS The National Building Code of Canada, and all provincial codes, express it this way:
6.2.3.11. Makeup Air In ventilating systems that exhaust air to the outdoors, provision shall be made for the admission of a supply of makeup air in sufficient quantity so that the operation of the exhaust system and other exhaust equipment or combustion equipment is not adversely affected. exhaust fans in buildings. Professional HVAC contractors will recognize the back-drafting potential of natural draft combustion appliances. In the absence of a specification from the manufacturer of the combustion appliance and venting system, a 5 Pascal negative pressure limit should be considered safe. Next, in the absence of a specific make-up air duct or pathway, the building enclosure will be subjected to a negative pressure. The magnitude of that pressure will be dependent on the air leakage characteristics of the building enclosure, that is, how tight is the building? In order to maintain the desired exhaust capacity for the application, the fan system will have
Gord is a professional engineer who has spent 20 years helping builders and HVAC contractors implement innovative technologies into high-performance homes. He has particular expertise in IAQ and airflow management in houses, and can be contacted at gordc@buildingknowledge.ca.
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