Should I close the damper on my Ottawa wood fireplace during March freeze-thaw cycles or leave it cracked to prevent moisture buildup?

This is one of those questions where Ottawa's specific climate creates a genuine tradeoff, and the right answer depends on whether you are still burning fires or your fireplace is done for the season.

If you are still burning fires in March, the damper question is straightforward — close it fully between fires, full stop. Leaving it cracked does not meaningfully reduce moisture buildup, and a partially open damper during Ottawa's March cold snaps is an open invitation for cold air to pour down the flue, chill your living room, and drive up your heating costs. More critically, a damper left cracked after a fire can trap residual carbon monoxide in your home if any embers are still smouldering. Always confirm the fire is completely cold — not just visually dark, but cold to the touch — before closing the damper.

If your burning season is finished, the conventional wisdom of "close the damper tight" still applies, but Ottawa's March freeze-thaw cycle does complicate things slightly. Here is the honest reality: a closed damper does trap some humidity inside the flue, and during March when temperatures swing from -15°C overnight to +5°C in the afternoon, that moisture can condense on the interior flue walls and mix with creosote deposits to form an acidic sludge that accelerates deterioration of clay tile liners. However, leaving the damper cracked to address this introduces a different problem — cold outside air rushing down the flue creates a persistent cold draft that can actually draw more moisture-laden interior air into the chimney system, not less.

The better solution is not about damper position at all. A properly fitted chimney cap is what actually protects your flue from March moisture, ice, and freeze-thaw infiltration from above. If your chimney does not have a cap, or if the cap is damaged, Ottawa's March weather will work water into the flue no matter what you do with the damper. A chimney cap installation in Ottawa runs $200 to $600 and is one of the highest-return maintenance investments you can make given our climate.

If moisture and condensation inside the flue are a recurring concern, that is often a signal worth noting for your spring chimney inspection. A WETT-certified chimney sweep can assess whether your clay tile liner has developed cracks from freeze-thaw cycling — a very common finding in Ottawa homes with chimneys more than 20 to 30 years old — and whether the chimney crown shows deterioration that is allowing water infiltration from the top. Schedule that inspection for April or May before the summer rush, and you will have a clear picture of what your chimney needs before next burning season.

Close the damper fully when not in use, invest in a quality chimney cap if you do not already have one, and let a professional assess the liner condition in spring. That combination will protect your chimney through Ottawa's punishing March weather far more effectively than any damper position adjustment.