What Can Suck Smoke Back Down the Chimney? – Building Depressurization as an Invisible Hazard

A well-insulated modern home with high-performance windows is ideal in many ways: low heating demand, no drafts, quiet. That same airtightness, however, brings a rarely discussed yet life-threatening problem. In old, drafty houses, so much air leaked in and out through wall gaps and window edges that the various extraction devices barely “noticed” one another. In today’s practically airtight buildings, by contrast, every cubic meter of extracted air wants to be replaced from somewhere — and if there is no controlled supply, the house takes the path of least resistance: the chimney. When that happens, the chimney designed to vent smoke starts “pulling” inward instead of outward, and combustion gases flow into the living space. This phenomenon is reverse draft (backdrafting), and its cause is almost always the same: the building’s internal depressurization, i.e. negative pressure.

Why is backflowing flue gas dangerous at all?

It’s important to clarify that “smoke” as such is not the main danger. The most insidious component of the flue gas produced by incomplete combustion is carbon monoxide (CO): a colorless, odorless, tasteless gas that binds to the hemoglobin in your blood orders of magnitude more strongly than the oxygen it displaces. This is exactly why the real risk in what people casually call “CO₂ backflow” is not carbon dioxide but carbon monoxide: high concentrations of carbon dioxide cause headaches and a feeling of suffocation, whereas carbon monoxide kills unnoticed, even during sleep. So when a chimney sucks flue gas back in, the occupant gets no smelly warning — they get a poison that no human sense can detect. That is what makes depressurization not a comfort question but a life-safety one.

The risk of backflow exists with those heating and combustion appliances that draw the air for their own combustion from the living space and rely on natural buoyancy (chimney draft) to vent the flue gas up the chimney. These are called open-flue, natural-draft appliances: the classic fireplace and masonry stove, the open-flue (chimney-connected) gas water heater and many older gas boilers, as well as some gas balanced-flue units. Room-sealed, fan-assisted (turbo) appliances, by contrast, draw air in from outside and force the flue gas out under pressure, so they are far less sensitive to depressurization.

The classic case: a fireplace and the kitchen extractor hood

The best-known, textbook example is the coexistence of a fireplace and a ducted (rather than recirculating) extractor hood vented to the outside. A powerful kitchen hood can move air in the 600–1,000 m³/hour range. When that volume is expelled from the living space, the same amount of air needs to come in — and if there is no controlled route for it, the house goes into negative pressure.

The chimney of a burning fireplace then becomes a convenient “make-up air source”: a few pascals of negative pressure is enough to overcome the chimney’s buoyancy and reverse the draft. At that point the hood draws its make-up air not from outdoors but through the fireplace chimney, passing over the embers — pulling smoke, soot and carbon monoxide into the living room. The phenomenon is especially insidious right after lighting the fire or with dying embers, when the chimney’s own draft is still, or already, weak. The same effect occurs even if the hood and the fireplace are not in the same room but the interior doors are open and air moves freely between spaces.

Depressurization isn’t caused by the extractor hood alone

The kitchen hood is the most obvious culprit, but far from the only one. Negative pressure can be created by anything that moves indoor air outward without replacement: bathroom and toilet fans, central mechanical exhaust (in single-direction systems without heat recovery), a clothes dryer’s condensate hose vented outside, extraction-based drying systems, and even a powerful fireplace or boiler’s own combustion-air demand — if several chimney-connected appliances run at once, they can “compete” for air. A few hundred cubic meters per hour continuously leaving the dwelling is all it takes to make an open-flue appliance’s draft unstable.

The hidden case: an old gas water heater and a hung-out mobile AC hose

The rarely discussed scenario raised at the start becomes truly important here, because those affected typically don’t even suspect the risk. Take an old-style, open-flue, chimney-connected gas water heater that burns air from the bathroom or kitchen and entrusts flue-gas removal to the natural chimney draft. In summer, to fight the heat, the occupant buys a single-hose mobile air conditioner and hangs its hot-air exhaust hose out through a slightly opened window or a window-sealing panel.

This is exactly where the little-known trap lies. A single-hose mobile AC uses indoor air to ventilate its condenser (the heat-rejecting side), and blows that heated air out through the hung-out hose to the outside. In other words, the unit continuously pumps the home’s air out — typically on the order of 300–500 m³/hour — and this has to be replaced from somewhere. In an airtight dwelling this deficit creates precisely the few pascals of negative pressure that reverse the gas water heater’s chimney draft. When that happens, the heater’s flue gas — together with the carbon monoxide it contains — does not leave via the chimney but flows back into the living space. All of it in complete silence, with no smell and no visible sign.

The situation is especially treacherous because the occupant often “feels safe” by running the AC on a low setting. But the cooling setting primarily governs the compressor and the amount of work done on the cold side — the fan that ventilates the condenser operates largely independently of it. In fact, when the unit’s internal temperature rises and the appliance “needs to cool itself,” the condenser fan spins up to remove the heat. That means it extracts the most air from the home — and causes the greatest depressurization — precisely when the user least expects it, since they are running the AC “just quietly, on low.” The volume of extracted air is therefore not proportional to the selected setting, and is at its greatest during the hottest, most heavily loaded moments.

This combination — an old open-flue gas appliance and a single-hose mobile AC in a well-insulated dwelling — is one of the most dangerous yet least recognized MEP traps. The two appliances are harmless individually, but together they form a potentially lethal system.

How can it be prevented? – Make-up air and design

The phenomenon comes with good news too: it is a law of physics that can be understood, calculated and prevented. The key to the solution is not eliminating extraction but providing controlled make-up air and coordinating the appliances in the design.

The single most important principle is securing combustion air: in the room housing an open-flue appliance, controlled entry of fresh air must be ensured — for example through outdoor air inlets, trickle vents, or a direct combustion-air feed. Where this cannot be reliably solved, replacing the appliance with a room-sealed (turbo) unit permanently eliminates the risk, since such a unit draws its combustion air from outside and does not depend on the living space’s pressure.

For the coexistence of a ducted extractor hood and a fireplace, a proven solution is a window-contact or pressure-switch-based interlock that only allows the hood to run when sufficient make-up air is available (an open window or open supply damper). For a mobile AC — if there is also an open-flue gas appliance in the home — a dual-hose (twin-duct) unit is strongly recommended, as it draws outdoor air to cool the condenser and expels it outdoors as well, so it doesn’t pump the home’s air out and doesn’t create negative pressure. Finally, as a cheap but life-saving addition, a carbon monoxide detector should be installed near every open-flue appliance — this is the last line of defense that alerts you even if physical prevention fails for some reason.

Summary

Smoke — or, more precisely, the carbon monoxide hidden within it — being sucked back down the chimney is not a mysterious phenomenon but the consequence of simple pressure conditions: if the house extracts more air than it can replace via a controlled route, the missing air flows in through the path of least resistance, the chimney, bringing the flue gas with it. The list of culprits ranges from the kitchen extractor hood through the bathroom fan to the hung-out-hose mobile AC, and the most dangerous cases arise precisely in the most airtight, most modern dwellings, from the most innocent-looking pairings of appliances. Prevention is always the same: thoughtful MEP design, controlled make-up air, and — where warranted — replacing open-flue appliances. This is an area where a few hours of engineering inspection can literally save lives.