How a House Pulls Flue Gas Back Down a Vent

Why this matters

A natural-draft water heater or furnace sends its exhaust up the vent for exactly one reason: the hot gas is lighter than the air around it. That push is small. Anything in the house that makes the appliance's room more negative than the vent can push against turns the vent around, and the flue gas, carrying carbon monoxide when combustion goes bad, comes out into the basement instead. The appliance is usually fine. The vent is usually fine. What changed is the house: a tighter shell, a bigger fan, a door, a duct leak. That is why the trade most likely to cause a backdraft is one that never touched the appliance.

Safety first

If a CO alarm is sounding, or anyone in the house has a headache, nausea, dizziness or confusion that eases outdoors, stop. Everyone leaves the house, call 911 or the fire department from outside, and nobody goes back in until responders say so. If there is a gas odor, leave without operating switches, lights or phones inside and call the gas utility from outdoors. Do not try to confirm spillage by running the house to worst case yourself; that is a deliberate attempt to make the appliance spill, and it belongs to a qualified combustion-safety or gas tech with a CO analyzer and the authority to shut the appliance off.

How a natural draft vent works

An atmospheric appliance burns gas in an open combustion chamber, using air from the room. The hot products rise into a draft hood (on a water heater, the cone on top) or a draft diverter (on an older furnace). The hood is deliberately open at the bottom: room air, called dilution air, is drawn in through that gap along with the flue gas, and the mixture rises up the vent. The open gap also lets a downdraft escape harmlessly instead of blowing out the burner.

The push up the vent comes from the weight difference between the column of hot gas inside it and a column of cooler air the same height outside. Once the vent is warm, the draft that push creates at the hood is only a few pascals. At a cold start, with the vent full of room-temperature or colder air, it is close to zero until the vent warms up. For how draft is generated and what weakens it (vent size, height, a cold exterior chimney, long connectors), see How Draft Actually Works. This card is about the other side of the contest: what the house does to the room the appliance sits in.

Spillage and backdrafting

Two words that get used loosely:

• Spillage is flue gas coming out of the draft hood's relief opening into the room instead of going up the vent. It can be brief, at a cold start, or continuous.
• Backdrafting is air flowing down the vent into the room. When the burner is off, a backdrafting vent just brings in cold outdoor air. When the burner lights against a backdrafting vent, the flue gas has nowhere to go but out of the hood, so it spills.

Spillage is the dangerous one, and it gets worse on itself. Flue gas spilled into a small room mixes with the air the burner is drawing in. The burner now has less oxygen to work with, combustion gets worse, and the carbon monoxide in the exhaust rises. A spilling appliance in a tight, closed room can turn a modest CO problem into a serious one.

   basement section, house running negative

            outdoors (higher pressure)
                   |  |
                   |  | <- air pulled DOWN the vent
                   |vv|
   ================|  |============ main floor
                   |  |       dryer + bath fan
                   |  |       pull house negative
                 __|__|__
   flue gas  <- /  hood  \ ->  flue gas spills
   into room    | WATER  |     out of the hood
                | HEATER |
                |_burner_|  <- burner draws room air,
                               now mixed with spilled
   rim joist leaks -> in       flue gas
   basement (most negative point in winter)

The drawing is the cause, not the test. The house is negative to outdoors, so outdoor air takes the easiest path in, which is down the vent. The flue gas rising off the burner meets that downflow at the draft hood and is pushed out into the room, where the burner draws it back in.

What pulls the room negative

The pressure at the appliance is the sum of everything acting on that room. See related: The House as One Pressure System, for how the drivers add.

Stack effect. In winter the bottom of a heated house is negative to outdoors, and the basement, where most natural-draft appliances sit, is the bottom. A leaky top of the house pushes the neutral plane up and makes the basement more negative still. See Stack Effect and the Neutral Pressure Plane.

Exhaust fans and the dryer. Every exhaust device pulls the house down, and several together pull harder than their separate effects added up. See Counting Every Exhaust Device in the House, and The Clothes Dryer as the Fan Nobody Counts.

Duct leakage at the appliance. A return duct leaking in the same basement room as the water heater, an open filter slot, or a missing return panel on the furnace cabinet pulls directly on that room whenever the blower runs. This one is an HVAC-caused backdraft that starts when the blower comes on, often in the same minute the water heater fires. See Duct Leakage Outside the Envelope.

A closed door. A door that separates the appliance room from the rest of the house cuts it off from the house's leaks, so whatever is pulling on that room has to be fed through the vent. See The Combustion Appliance Zone and the Door That Closes It.

Wind. An appliance room on the leeward side of the house, or a chimney top in a positive-pressure zone of a taller roof, adds an intermittent push. See Wind Pressure on a House and What It Moves.

A fireplace. A wood fire draws a large volume of house air up its own flue, enough to depressurize a house on its own, and a cold chimney with its damper open can act as a second, competing vent.

Worked example: why it happens on some mornings

A house with a natural-draft water heater in a basement utility room, a door to that room, and a 1980s furnace whose return duct has an open panel in the same room. These are illustrative contributions, chosen to show how the drivers stack, not measurements.

Mild Tuesday, nothing running. Outdoors near indoor temperature, no wind, no fans, utility door open. The basement is within a pascal of outdoors. The water heater lights, its vent warms, it drafts. Any tech testing today passes it.

January Saturday. Outdoors cold: stack puts the basement, say, 3 Pa negative. The dryer and a bath fan running add about 2 Pa. The furnace blower kicks on, and the open return panel pulls on the utility room, which has its door closed, say 2 Pa more. The room sits around 7 Pa negative.

What the water heater experiences. A shower ends and the water heater calls. Its vent is cold. With the room 7 Pa negative, cold outdoor air is already coming down the vent and out of the draft hood. The burner lights, the flue gas meets the downflow at the hood and spills. A few pascals of hot-vent draft might eventually establish against a mild pull; against this one, the vent may never warm enough to turn around, and the spillage continues for the whole burn.

Take away one driver at a time. Open the utility door and the room shares the house's leaks; the 2 Pa from the return panel spreads across the house and mostly disappears from that room. Close the return panel and that term is gone whether the door is open or not. Stop the dryer and the fan term drops. Warm the weather and the stack term shrinks. Any one of these might bring the room back under what the vent can handle on this morning. None of them fixes the house, because the next combination of drivers will find the same weak point. The durable fixes are on the appliance side (sealed combustion, a power-vented or electric water heater) or the house side (makeup air sized to the exhaust), decided after a real worst-case test.

What changes the answer. A sealed-combustion appliance, sometimes called direct vent, takes its combustion air from outdoors through a pipe, has a sealed combustion chamber and usually pushes its exhaust out with a fan. It has no draft hood open to the room, so house depressurization of the size discussed here does not reverse it. A power-vented appliance also pushes its exhaust with a fan but still takes combustion air from the room; it resists depressurization far better than a draft-hood appliance. A fan-assisted, induced-draft furnace venting into a natural-draft vent is more resistant than a draft-hood appliance but not immune. The appliance type, read off the rating plate and the venting, is the first fact a tech should know. See Combustion Safety After a Partial Conversion for the orphaned water heater left behind when the furnace goes sealed.

Signs that a house has been pulling on a vent

Things any trade can notice without testing anything:

• Soot, scorching or melted plastic on or around the draft hood or diverter, or discolored paint above the water heater.
• Rust and corrosion on the top of the water heater or the vent connector, and condensation dripping from the hood area. Flue gas carries water vapor, and spilled gas condenses on cool surfaces.
• A cold draft coming out of the draft hood with the burner off, felt on the back of the hand.
• A CO alarm that sounds intermittently, or none installed at all near a natural-draft appliance.
• A recent job that tightened the house or added exhaust: windows, attic air sealing, a new hood, a moved dryer, a basement finish with a door to the utility area.

Any of these goes to a qualified gas tech, with the observation and the date written on the work order, and the customer told why.

References

• See related: How Draft Actually Works; Combustion Safety After a Partial Conversion
• See related: Kitchen Hood Makeup Air (combustion safety depressurization limits, BPI)
• See related: The Combustion Appliance Zone and the Door That Closes It; How to Screen for Spillage After You Add an Exhaust Fan
• See related: Counting Every Exhaust Device in the House; Stack Effect and the Neutral Pressure Plane