Installing a gas insert into a masonry fireplace without addressing the flue creates a specific problem: gas combustion produces water vapour that condenses into a mildly acidic liquid inside an oversized old flue, attacking mortar and clay tile from within.
Why gas can damage a chimney with no soot
People reasonably assume that because gas burns clean, it cannot harm a chimney. The mechanism is different from wood, but it is real.
Burning natural gas produces carbon dioxide and a substantial amount of water vapour. In a correctly sized flue that vapour stays hot enough to exit as vapour. In an oversized masonry flue, originally built for a wood fire producing far more heat, the exhaust cools on the way up and the vapour condenses on the flue walls.
That condensate is mildly acidic. Over years it dissolves mortar joints between clay tiles and degrades the tiles themselves, and none of it is visible from the room.
Why Romeoville sees this often
Romeoville has a high rate of gas insert conversions, homes where a gas unit was installed into an original wood-burning masonry fireplace. Done properly, with a correctly sized stainless liner, that is an excellent upgrade: more efficient, more convenient, and easier to maintain.
Done without addressing the flue, it starts a slow degradation the homeowner has no way of observing. The insert works fine, the room is warm, and the chimney quietly deteriorates.
How to check where you stand
If your gas insert was installed as part of a conversion and you do not know whether a liner was fitted, that is worth resolving. A camera inspection shows the flue condition and whether a correctly sized liner is present.
The remedy where one is missing is a stainless liner sized to the appliance’s specification, which restores a correctly proportioned path so exhaust exits before condensing. It also improves the appliance’s draft and efficiency.
While we are there, annual gas fireplace service covers the appliance side: burner and pilot cleaning, log repositioning to manufacturer specification, millivolt testing on the thermopile, and gas leak testing at accessible connections.
Talk to a local chimney pro
If your pilot keeps going out, that is usually a weakening thermopile rather than the valve, and we measure before replacing anything. We cover Romeoville on our regular Will County route.
Written by Josh Amber, co-owner of Ember Chimney Sweepers, Oswego IL. Published October 2, 2026.