The Attic Duct Leak That Made the Fireplace Smoke

Why this matters

A fireplace that smokes into the room is putting combustion products, including carbon monoxide, into the space where people are sitting. The usual suspects are all chimney problems, and a chimney sweep is the right first call. In this case the sweep did everything right, found the flue sound, and noticed the one detail that did not fit a chimney fault: the smoking stopped when the furnace fan stopped. The handoff that followed, from a chimney sweep to an HVAC shop, is the part worth copying.

This case runs in the order it had to: contain the smoke first, then find out why.

First: what the owner was told to do the night it smoked

The owner called the chimney sweep's office on a Saturday evening with a fire burning and smoke rolling out of the top of the firebox opening. The office's script, which is the part of this case any shop that answers the phone should have ready:

• If a carbon monoxide alarm is sounding, or anyone has a headache, dizziness or nausea, everyone leaves the house now and calls 911 or the fire department from outside. Nobody goes back in until responders say so.
• Otherwise, turn the heating system off at the thermostat, including the fan. Anything that pulls air out of the house competes with the chimney; that includes the bath and kitchen fans, so turn those off too.
• Open a window in the room with the fireplace a few inches. That gives the house a source of air other than the chimney.
• Leave the damper open and let the fire burn down. Closing the damper on a burning fire sends all of its smoke into the room. Do not add wood, and do not use water on it.
• If the house has no working CO alarm, open more windows and stay out of the room until the fire is out and the room has aired.

The owner turned the heat off and opened the window, and the smoking stopped within a few minutes. The office booked an inspection for Monday and noted the detail.

The complaint, in the owner's words

A single-story house, about 1,700 square feet, built in the 1980s. A heat pump with its air handler and all of its ductwork in the vented attic. Electric water heater, no other combustion appliance. An open masonry wood-burning fireplace in the living room, used a few times a month in winter.

Owner: "It's smoked a few times this winter. I think it did it in October too, when the air conditioning was still running. It never used to."

The sweep's inspection

The sweep worked the chimney causes in order and ruled each out on evidence:

• Blockage. A camera run of the flue: light creosote, no nest, no fallen liner tile, cap and screen clear.
• Damper. Opens fully, closes fully, linkage sound.
• Chimney height and surroundings. The chimney top clears the roof by the margins in the commonly adopted rule (3 feet above where it passes through the roof, 2 feet above anything within 10 feet, IRC R1003.9 in substance, as adopted); no taller structure or tree nearby to cause downdrafts.
• Opening-to-flue proportion. The firebox opening and the flue size were original and in proportion; nothing had been changed.
• Cold flue at start-up. A cold chimney on an outside wall can resist draft until it warms, which makes a fireplace smoke for the first few minutes of a fire. The owner said this smoked well into a fire, not just at lighting.

Then the sweep ran a simple draft check: damper open, no fire, a smoke source held just inside the top of the firebox opening. With the heat pump off, the smoke went up the chimney. The sweep asked the owner to turn the heat on. Within a minute of the indoor fan starting, the smoke curled out into the room.

The sweep's report said it plainly: "Chimney and flue in serviceable condition. Fireplace draft reverses when the home's heating system runs. This points to the house being depressurized by the HVAC system, not to a chimney fault. Recommend an HVAC contractor check the duct system for leakage, especially in the attic." The sweep's office sent the report and the owner's contact to an HVAC shop it works with, with the owner's permission.

That report is the handoff done right: what was ruled out, the observation that does not fit the trade's own causes, and a named next trade with the reason.

The HVAC visit: reading the house

The HVAC tech brought a digital manometer and took the house's pressure against outdoors, a tube run out under a closed exterior door, interior doors open, fireplace damper open, no fire.

  HOUSE SECTION, blower on, before repair

      ATTIC (outdoors, vented)
   supply trunk off its collar -> conditioned
   air blowing into the attic     \
   +--[air handler]=====  X  ======\=======+
   |    return pulls from house          |
   |    supply delivers less than        |
   |    return removes                   |
   |  LIVING ROOM                         |
   |  house: -5.5 Pa to outdoors          |
   |    ^                                 |
   |    | outdoor air pulled DOWN the     |
   | [FIREPLACE] chimney into the room    |
   +--------------------------------------+
   with blower off: -0.5 Pa

• Blower off: house at about -0.5 pascals relative to outdoors, close to neutral on a calm day.
• Blower on high: about -5.5 pascals.

A duct system that is tight, or whose supply and return leak outside the house by the same amount, barely moves the house's pressure. A system that leaks more on the supply side than the return side outside the envelope pushes house air into the attic and pulls the same amount back into the house through every other opening. The largest opening in this house is the open fireplace damper. See related: Duct Leakage Outside the Envelope, which owns that mechanism.

A 5-pascal swing that appears and disappears with the blower told the tech where to look before he climbed the ladder. A swing toward positive would have pointed at return leaks in the attic instead.

What was in the attic

The flexible supply trunk feeding the three bedrooms had pulled off its collar on the supply plenum. The strap had failed, and the trunk sat a few inches from the collar, blowing straight into the attic insulation, which showed a scoured patch. The tech asked the owner whether the bedrooms had been weak this winter; they had, and the owner had assumed it was the cold.

Two smaller leaks: an unsealed seam at the plenum takeoff and a boot in the hall ceiling with a gap at the drywall.

The working conditions. The attic was entered on a cold morning, but attics can be extreme in either season; the tech walked only on framing, wore a respirator for the fiberglass under the shop's written program per 29 CFR 1910.134 (29 CFR 1926.103 applies the same rule where the work counts as construction), and turned the system off at its disconnect before working at the plenum, so the blower could not start with hands in the duct.

The repair and the proof

The tech reconnected the trunk to the collar with a new draw band, sealed the inner liner and the outer jacket, and mastic-sealed the plenum seam and the hall boot. Then, out of the attic with power restored at the disconnect, the same two readings:

• Blower off: about -0.5 pascals, unchanged.
• Blower on high: about -0.8 pascals.

The swing the blower caused fell from about 5 pascals (from -0.5 to -5.5) to about 0.3 (from -0.5 to -0.8). The same smoke check at the firebox opening, no fire, blower on: smoke went up the chimney. The bedroom registers, which the owner had called weak, now felt strong at the hand, and a flow hood reading at the largest bedroom showed close to design.

Return to service. The owner lit a fire that evening with the heat running, a CO alarm in the living room, and the shop's instruction to repeat the Saturday steps if it smoked. It did not. The shop recommended, in writing, a CO alarm on every level and near the bedrooms if the house did not already have them.

Why it only happened with the system running

The fireplace drew fine on its own. A fire's draft is real but small at the firebox, and a few pascals of house depressurization working the other way is enough to stall or reverse it. The October smoke matched too: the air conditioning used the same blower and the same leaky trunk, so the house went just as negative with the AC running as with the heat. The cause was never heating or cooling; it was the fan. See related: How Draft Actually Works.

What would have changed the read. If the house had kept a draft-vented gas water heater or furnace, the same 5.5 pascals would have threatened that appliance whenever the blower ran, fire or no fire, and the HVAC tech would have stopped and brought in a gas or combustion-safety tech for a spillage test before leaving. If the blower-on reading had been near neutral, the HVAC system would have been cleared and the next suspects would be the house's other exhaust fans and a tighter house since a recent air-sealing job.

What each office added to its intake

Both shops changed one thing about how they take this call, and between them it would have saved the Saturday.

The chimney office now asks, on any "fireplace smokes" call: "Does it happen when the heat or air conditioning is running, or when a kitchen or bath fan is on?" A yes does not skip the chimney inspection, since a chimney can have a fault and the house can be negative at the same time, but it puts a pressure check on the referral list from the start and the manometer on the truck.

The HVAC shop added the house-to-outdoors reading, blower off and blower on, to every attic-duct service call in a house with an open fireplace or any draft-vented appliance. It takes about five minutes, and a blower-on swing of more than a couple of pascals sends the tech into the attic before the customer ever reports a symptom.

References

• See related: Duct Leakage Outside the Envelope; How to Find Duct Leaks Outside the Envelope; How Draft Actually Works
• See related: The House as One Pressure System; How a House Pulls Flue Gas Back Down a Vent
• IRC R1003.9, chimney termination, as adopted by your jurisdiction
• OSHA 29 CFR 1910.134, respiratory protection
• OSHA 29 CFR 1926.103, which applies 1910.134 to construction work