To achieve greater energy efficiency, today’s buildings are much better insulated and tightly sealed than they used to be. They’re so well designed and have such limited airflow that some ventilation systems, such as a range hood, can cause a new problem: negative pressure. Here’s what it is.
What Is Negative Pressure?
Air entering the house, whether through tiny openings in the building envelope or an open window, creates positive pressure. Conversely, when appliances such as a bathroom fan or a range hood expel air from your property, this creates negative pressure.
When more air is forced out of your home than in, a vacuum occurs. In older, more permeable construction, outside air seeps through window frames, door frames and other entry points throughout the building to compensate for this loss. Apart from increasing your heating and air conditioning costs, this has little effect.
The Potential Hazards of Negative Pressure
However, in new construction, air may end up being drawn in through the attic, the garage, the dryer vent, the bathroom vent or a fireplace chimney. Air travelling “backwards” from these locations can be heavy with undesirable substances: moisture, mould, bacteria, lint, chemical fumes, soot, etc.
This is particularly dangerous in the case of a chimney connected to a slow-burning fireplace or the venting of a gas appliance. The negative pressure could cause a backdraft, and smoke or fumes could be drawn in instead of vented.
Not the kind of air you want to breathe!
High-Performance Range Hoods and Energy Efficient Homes
High-performance range hoods are designed to effectively expel odours, grease, and smoke outside your property. The larger and more powerful your cooking surface (especially if it’s a gas appliance), the more powerful your range hood needs to be. It’s possible that it could exhaust more air than can be brought into the room.
For example, a 1,800 sq. ft. bungalow with 9 ft. ceilings has a total air volume of 16,200 cu. ft. A kitchen hood operating at 600 CFM (i.e. a kitchen hood capable of expelling 600 cu. ft. per minute) could exhaust all indoor air from the entire home in less than 30 minutes.
Even when a property is equipped with an air exchanger system, it’s probably not set up to compensate for air losses of such magnitude.
How to Fix a Negative Pressure Problem?
When installing a powerful range hood—or a moderately powerful one in a tiny kitchen—you could install compensation systems that pull in outside air to balance the loss. However, this can be a costly solution—sometimes more so than the range hood itself! – Instead, we recommend a simple and cheap alternative.
When operating your range hood at high speed, open a window slightly. The air exhausted by the appliance will automatically be replaced by outside air from the opening, ensuring pressure balance in your home.