How does a negative pressure test determine if my Toronto home needs a make-up air system?

A negative pressure test measures how much depressurization your home experiences when exhaust systems run — if the house goes more than 5 Pascals negative relative to outdoors, you likely have a make-up air problem that Ontario Building Code may already require you to address.

Here's what's actually happening: every exhaust fan, range hood, dryer vent, and combustion appliance in your home moves air from inside to outside. That air has to be replaced from somewhere. In a leaky older home, replacement air infiltrates through cracks, gaps, and unsealed penetrations without much resistance. In a tighter modern home — or any home with a powerful range hood — the building can't pull in enough replacement air fast enough, and the interior goes negative relative to outdoor air pressure.

What the Test Actually Measures

A technician uses a digital manometer (a pressure gauge accurate to fractions of a Pascal) with a hose run through a window or door to measure the pressure difference between inside and outside. They run your exhaust systems one at a time — bathroom fans, range hood, dryer, HRV or ERV exhaust — and record the depressurization reading at each step. A well-balanced home stays within 2-3 Pascals negative under normal operation. Readings of 5 Pascals or more indicate a make-up air deficiency. Readings above 10-15 Pascals are a serious combustion safety concern.

The combustion safety piece is what makes this test critical in Toronto homes. When your house goes significantly negative, it creates a pressure gradient that can backdraft your gas furnace, water heater, or fireplace — pulling combustion gases including carbon monoxide back down the flue and into the living space instead of exhausting them outside. TSSA takes this seriously, and it's why Ontario Building Code Part 6 requires make-up air when total exhaust capacity exceeds 75 litres per second (approximately 159 CFM). Many GTA homeowners installing 600-1,200 CFM range hoods for kitchen renovations trigger this threshold without realizing it.

GTA-Specific Context

Toronto's housing stock makes this test particularly relevant. Post-war homes in Scarborough, North York, and Etobicoke were built loosely enough that natural infiltration handled make-up air passively for decades. But as homeowners add spray foam insulation, new windows, and weatherstripping, those same homes become tight enough that a powerful range hood creates dangerous depressurization. Newer homes in Vaughan, Markham, and Brampton built to higher airtightness standards are even more vulnerable — they're designed to be tight, which is great for energy efficiency but means make-up air must be mechanically supplied.

The test is also valuable before and after major renovations. If you're planning a kitchen renovation with a high-CFM range hood, a pre-renovation pressure test establishes your baseline. If you're adding an HRV or ERV (which both exhaust and supply air), the test confirms whether the unit is properly balanced.

Practical Tips

A blower door test (used for energy audits) measures overall airtightness but is different from a combustion safety depressurization test — make sure you're getting the right test for your concern. Ask specifically for a worst-case depressurization test following TSSA guidelines, which involves closing all windows, running all exhaust fans simultaneously, and measuring the result.

If your home fails, the solution is a make-up air system — a dedicated fresh air intake duct, typically with a motorized damper that opens when exhaust runs, and a heating element or connection to the furnace plenum to temper incoming winter air so you're not blasting -20°C air into the house. Installed cost in the GTA runs $2,000-$5,000 depending on complexity, and the system must be interlocked with your exhaust equipment under OBC requirements.

This work requires a TSSA-registered contractor for any gas connections involved in tempering the incoming air, and an ESA-licensed electrician for the interlock controls and damper wiring.

Toronto Ductwork can match you with a local ventilation contractor who can perform the depressurization test and design a make-up air system if needed — find professionals through the Toronto Construction Network at torontoconstructionnetwork.com.