Drafts Fine With Window Cracked but Not Closed: Bypass vs Normal Decision Tree

Why this matters

The diagnostic test where the customer cracks a window and the smoke spillage stops is the single most informative test in chimney troubleshooting. It tells you the chimney can draft properly when given enough makeup air, which means the chimney itself is not the fault. The fault is on the house side, and "just leave a window open" is not an acceptable answer. This decision tree gets the sweep from confirming the bypass test to identifying the specific house-side cause and recommending the right fix.

Section 1: Run the test correctly

Before you accept the customer's report, run the test on site with controlled conditions.

Sequence:

1. House in normal operating mode: all interior doors open as customer normally has them, HVAC running as normal, all exhaust appliances customer would normally have running (kitchen hood at usual setting, bathroom fans as usual, dryer if it would be running)
2. Light the appliance and observe spillage at the firebox opening over 5 to 10 minutes
3. Open a window approximately 3 to 4 inches on the windward side of the house
4. Observe whether spillage stops within 1 to 2 minutes
5. Close the window and observe whether spillage returns

If spillage stops with the window open and returns when closed, the bypass test is positive. The chimney drafts when not competing for air. The house is in net negative pressure during normal operation.

If spillage continues even with the window open, the bypass test is negative and you are looking at a chimney-side problem (sizing, height, cold flue, restriction). Do not chase house-side causes for a negative bypass test.

Section 2: Quantify how much air the house needs

The house needs enough makeup air to feed the chimney draft plus any other exhaust appliances running. A masonry fireplace under burning load typically needs 200 to 400 CFM of makeup air; a wood stove needs 50 to 150 CFM depending on size and burn rate; a high-efficiency insert needs less. Add range hood (often 300 to 1200 CFM), dryer (100 to 200 CFM), bathroom fans (50 to 150 CFM each).

The math often comes out to 800 to 1500 CFM of total exhaust demand when everything is running, on a house that may have a measured infiltration rate of only 200 to 400 CFM at normal pressure. The deficit is what the chimney supplies by reversing.

Section 3: Identify the dominant competing exhaust

Cycle through exhaust appliances with the chimney burning and observe.

• Range hood off, then medium, then high: does spillage start at any setting
• Bathroom fans off, then on: does spillage start
• Dryer off, then on (if customer would run it during burning): does spillage start
• HVAC fan on continuous versus auto: any change in spillage pattern
• Whole-house exhaust ventilation (HRV/ERV in exhaust-only mode): cycle and observe

The exhaust appliance that triggers spillage when activated is the dominant competing exhaust. Often it is the range hood; sometimes it is HVAC return-side leakage in an unconditioned space (attic, crawlspace) pulling air out of the home indirectly.

Section 4: Identify the building envelope context

A tight house with low infiltration cannot supply makeup air through cracks and gaps. A leaky house often can. Recent envelope work is a common trigger for chimneys that used to draft fine and no longer do.

Ask about:

• New windows installed in the last 1 to 3 years
• Attic air sealing or insulation work
• Spray foam insulation in walls, rim joist, or attic
• Weatherstripping or door sweep upgrades
• Basement air sealing or encapsulation
• HVAC duct sealing
• Whole-house weatherization program participation

Each of these reduces infiltration. None of them is wrong; the house is tighter and more efficient. But the chimney is now competing with exhaust appliances in a way it did not before.

Section 5: The decision matrix and remediation

Range hood is the dominant competitor:

• Most common case in modern homes with powerful kitchen ventilation
• Solution: dedicated makeup air unit interlocked with the range hood, sized to match the hood CFM
• Code requirement: many jurisdictions require makeup air for hoods over 400 CFM under IRC Section M1503; check local code
• Customer action: turn down the hood or use a lower setting while burning if makeup air install is deferred
• Interim: leave window open during simultaneous range and fireplace use (not a permanent solution but accurate description of physics)

Whole-house exhaust ventilation (HRV/ERV imbalanced or in exhaust-dominant mode):

• Rebalance the ventilation system to neutral or slightly positive
• Adjust supply and exhaust airflows per ASHRAE 62.2 ventilation rates
• HVAC contractor scope, not sweep scope

HVAC system pulling air from unconditioned spaces:

• Duct leakage testing reveals returns drawing from attic, crawlspace, or garage
• Seal duct returns and supplies
• HVAC contractor scope

Building envelope tightened, no specific exhaust dominant:

• House is simply too tight to supply makeup air passively
• Dedicated combustion air supply to the appliance: a duct from outside terminating near the firebox, providing direct combustion air without depressurizing the room
• Code-permitted in most jurisdictions; check appliance manufacturer's specification for whether outside-air kits are listed for the specific appliance
• For masonry fireplaces, exterior combustion air ducts to the firebox are permitted under IRC R1006

Multiple competing exhausts, none dominant:

• Comprehensive building air-pressure consultation
• Building science professional or HVAC contractor with depressurization-testing capability
• The fix may be a combination of makeup air, exhaust reduction, and outside combustion air

Section 6: Communicating the finding

The customer ran the bypass test themselves and may consider it the answer. Reframe.

Cover:

References

• NFPA 211 - Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances
• IRC (International Residential Code) Section M1503 - Range Hoods, Section R1006 - Exterior Air Supply for Masonry Fireplaces
• ASHRAE 62.2 - Ventilation and Acceptable Indoor Air Quality in Residential Buildings
• CSIA (Chimney Safety Institute of America) - Successful Chimney Sweeping Manual