Negative Air Pressure in a Building: Symptoms, Testing and Fixes

Mid-October, the first night below freezing on the island. The next morning the service calls sound different: the lobby door no longer closes on its own, reception complains about an icy draft, and a smell of fryer oil has made it up to the second floor. Nothing is “broken” in the usual sense. What changed is the air balance. You are dealing with negative air pressure in a building, and the first cold snap simply made it visible.

This guide is for the technician who has to find the cause, not just name it. The symptom is common; the diagnosis takes a method, because depressurization almost always has several contributors stacking up.

What does negative air pressure in a building actually mean?

A building is under negative pressure when indoor air sits at a lower pressure than outdoor air. It happens when fans exhaust more air than supply units bring in: the difference leaks in through every crack, door frame and chimney. Good design normally aims for the opposite, a slightly positive building.

The consequences go beyond comfort. Air pulled through the envelope is neither filtered nor heated; it brings dust, moisture and odours along with it. More importantly, it takes the easiest path in, and that can be the flue of a natural-draft appliance, running backwards against the combustion gases.

In Montréal, winter adds a player that summer hides: the stack effect. Warm air rises, pressurizes the top of the building and pulls the bottom negative. In a multi-storey building, the basement boiler room ends up in the most negative zone exactly during heating season. An exhaust imbalance you could live with in September can become a real problem in January.

Tools you’ll need

• Two-channel digital manometer with 0.1 Pa resolution. The pressures involved are tiny: one inch of water column is about 249 Pa, so a few pascals is a few hundredths of an inch, unreadable on a needle gauge.
• Flexible plastic tubing long enough to run under an exterior door or through a cracked window, with the outdoor end shielded from wind.
• Smoke pencil or puffer to see which way air moves at door frames and draft hoods.
• Combustion analyzer with ambient CO reading and flue probe.
• Flow hood or anemometer for exhaust and outdoor air grilles.
• The design air balance, if one exists: the list of exhaust fans, supply units and their airflows.

⚠ Safety: natural-draft appliances come first

Before touching anything, locate every combustion appliance vented to a chimney without a draft inducer: water heaters, atmospheric boilers, older furnaces. Those are the ones depressurization puts at risk. Keep a personal CO monitor on for the whole sequence. If ambient CO rises near an appliance during a test, shut off the fan involved immediately, ventilate, and keep the appliance out of service until the cause is fixed. Our article on combustion air requirements covers the openings that CSA B149.1, in force in Québec, requires around these appliances.

Step 1 — Measure envelope pressure

With the building in its normal operating state, doors and windows closed, run the tube under an exterior door and read the indoor-to-outdoor difference. Repeat on the ground floor, in the basement and in the boiler room. Avoid very windy days, or shield the outdoor end: a gust skews the reading more than any fan. Average over about thirty seconds; most digital manometers can do it.

Write down the time and outdoor temperature too. A reading taken at 15 °C says nothing about how the same building behaves at -20 °C, once the stack effect joins in.

Step 2 — Start the exhaust fans one at a time

This is the core of the tree. First put the building in a baseline state: exhaust fans off, supply units off. Record the pressure. Then start each piece of equipment on its own (kitchen hood, washroom exhausts, dryers, dust collector, makeup air unit, rooftop unit) and log the change it causes.

Step 3 — Branch A: makeup air isn’t keeping up

A big exhaust fan without its matching supply is the most common cause, and the easiest to fix. On the associated makeup air unit, check in order: is it locked out (low-limit trip, failed burner ignition); does the outdoor air damper actually open; are the filters loaded; is the belt slipping? A motorized damper with a dead actuator often sits closed without raising any alarm.

Then check the interlock: can the hood run while the makeup air is off? If so, the problem will come back with the next fault.

Step 4 — Branch B: the airflow balance is wrong

Everything runs and pressure still reads negative? The issue is no longer a part, it’s the arithmetic. Measure actual exhaust and outdoor airflows and compare them to the design balance. The usual suspects: an exhaust fan added during a renovation, a bigger washroom, a hood replaced with a more powerful model, or outdoor air turned down to save on heating. At that point the fix is a full ventilation system balancing job, not a quarter turn on a damper.

Step 5 — Check draft and confirm the fix

Put the building back in worst-case conditions: every exhaust fan on, interior doors set to maximize depressurization around combustion appliances. Fire each natural-draft appliance, confirm with smoke that air is being drawn into the draft hood after a few minutes of operation, then measure chimney draft at the flue. After the correction, re-measure envelope pressure at the same points as in step 1. That reading is your proof the fix worked, and it belongs in the report.

How much negative pressure is too much?

There is no single threshold for every commercial building. A few reference points help interpret a reading:

The principle to remember: the tighter the building and the stronger the exhaust, the faster depressurization climbs. Renovated buildings with new windows and a tightened envelope are therefore the most exposed.

Anjou: the restaurant door that wouldn’t close

A restaurant in a small strip mall in Anjou. The complaint: the front door stays ajar and the host is freezing. On the manometer, with the hood running, the dining room read around -18 Pa; with the hood off, about -2 Pa. Branch A, no question.

On the roof, the makeup air unit had been locked out on ignition failure since the first cold night: its fouled flame rod could no longer prove flame, the low-limit had shut the fan down, and the hood kept pulling on its own. In the back room, the natural-draft water heater was spilling at the draft hood during the first minutes of each cycle.

The fix: cleaning and replacing the flame rod, a full ignition-sequence test, and a proof-of-operation relay from the makeup air unit wired into the hood circuit. Re-test: dining room slightly positive, water heater drafting from start-up. The ventilation and indoor air quality maintenance contract now includes a makeup air unit test before every winter.

A tube, a gauge and a door: where to start tomorrow

On your next call to a building with a door that fights back, take the step 1 reading before anything else and log it on the work order with the outdoor temperature. A number in pascals beats “draft reported.” Québec’s building authority, the RBQ, recommends that owners follow a planned maintenance program for ventilation systems, and a pressure reading fits naturally into it.

For the property manager, those few readings turn a vague complaint into a decision with numbers behind it: fixing a makeup air unit costs one service call, while a spilling water heater puts every occupant at risk. That reading is what Montréal Combustion leaves on file, so whoever comes next knows exactly where to start.