Chimney Academy · Water and weather

Chimney Condensation: Signs and Causes

Some chimney water never fell as rain. Burning gas or oil makes water vapor, and when the exhaust cools too much on its way up the flue, that vapor turns back into an acidic liquid inside the chimney.

5 minute readUpdated October 2026

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On this page

Where the water comes from
The dew point is the line that matters
Why efficient appliances changed the picture
Oversized flues: the slow, cool ride
Exterior chimneys run colder
What the acid does to a chimney
Signs you can see
Wood-burning flues condense too
What fixes condensation
Questions homeowners ask
Sources

The short answer

Chimney condensation happens when exhaust from a furnace, boiler, water heater or other appliance cools below its dew point before it leaves the flue. The water vapor in it condenses on the flue walls, and the liquid is acidic. It is most likely when a modern, efficient appliance vents into a masonry flue that is too large for it, or into a chimney on an outside wall that stays cold. Signs include white staining, flaking brick, crumbling mortar and deteriorated tile, often worst near the top, plus moisture or rust near the base or cleanout door, especially in cold weather. In most cases the remedy is a liner matched to the appliance, with insulation where the chimney runs cold.

Where the water comes from

Natural gas, propane and oil all contain hydrogen. When they burn, the hydrogen combines with oxygen to make water vapor, which leaves with the rest of the exhaust. In a 1995 article for ASTM Standardization News, fireplace builder Jim Buckley wrote that more than a gallon of water, laced with acids, is produced for every 100,000 Btu of gas burned.

While the exhaust stays hot, that water remains vapor and leaves the chimney as an invisible gas. The question is whether it stays hot enough all the way to the top.

The dew point is the line that matters

Every mixture of air and water vapor has a dew point, the temperature at which the vapor starts turning to liquid. A cold glass on a humid day shows the same thing. Exhaust from gas appliances carries a lot of water vapor, so its dew point is relatively high. A Journal of Light Construction article on venting gas appliances puts it at about 140°F.

When exhaust touches a flue wall colder than its dew point, water condenses on that wall. If the whole flue stays warm, condensation is brief, for example in the first minutes of a cycle, and evaporates as the flue heats up. If the flue never gets warm enough, condensation continues for most of every run.

Simplified illustration. A small, efficient appliance venting into a large, cold masonry flue gives its exhaust time to cool below the dew point before it leaves the chimney.

Why efficient appliances changed the picture

Older furnaces and boilers sent a lot of heat up the chimney. That waste kept masonry flues warm. As appliances became more efficient, less heat went up the flue. DuraVent's technical literature on gas relining explains that for mid-efficiency appliances, roughly 80 to 83 percent, exhaust leaves just above the dew point, so the dew point is reached in the vent system. Condensing appliances of 90 percent and above condense inside the appliance and use the venting systems their manuals specify.

The furnace's own installation manual is the authority on how it may be vented. One example, a Trane gas furnace installer's guide, prohibits venting into an unlined masonry or concrete chimney, and requires a listed liner when a fan-assisted furnace vents into a chimney with one or more walls exposed to outdoor temperatures.

Oversized flues: the slow, cool ride

A flue sized for an old, large appliance, or for a fireplace, can be far too big for a modern furnace or water heater. Buckley's article gives an example: a new furnace with a 4-inch vent exhausting into an 8 by 12 inch tile flue at about 200°F. In a flue that large, the exhaust slows down and cools, and most of the water condenses before it reaches the top.

DuraVent's literature adds that poor draft results when an efficient gas furnace discharges into a large masonry flue, and that slow, cool exhaust in an oversized flue can allow carbon monoxide to spill into the home. The same mechanism explains the orphaned water heater problem, where a water heater is left alone in a flue once shared with a furnace.

Exterior chimneys run colder

A chimney built on the outside wall of a house, with three sides exposed to winter air, has much colder flue walls than one rising through the middle of the house. DuraVent notes that in cold climates, rapid cooling of flue gases often leads to condensation on cold flue surfaces. A 1983 Canadian government study of oil-fired heating systems (CANMET's Canadian Combustion Research Laboratory) found that in masonry chimneys, particularly outside installations, the range of conditions in which short furnace cycles keep the flue continuously condensing is extended.

Short cycles matter because a flue that only gets a few minutes of exhaust at a time may never warm above the dew point at all.

What the acid does to a chimney

The condensate is not clean water. DuraVent describes acids that break down and erode clay tiles, bricks and mortar from the inside. The CANMET study raised the same concern about corrosion of liners, and about condensed water soaking into mortar where it then freezes and thaws.

That last part ties condensation to weather. Masonry that is wet from the inside goes into a New Jersey winter already saturated, and freeze-thaw cycles do the rest. Damage often concentrates near the top of the chimney, where the exhaust is coolest and the masonry is most exposed.

Signs you can see

These signs overlap with ordinary rain leaks, so they are clues to check rather than proof. The CANMET study and DuraVent's literature list several:

White efflorescence on the outside brick, especially on the upper part of the chimney. See white staining on brick.
Spalling brick and eroded mortar joints. See why chimney brick flakes.
Deteriorated flue tile at the top of the chimney, or chips of tile and mortar collecting at the base.
Yellowish or brown staining on the outside of the chimney.
Moisture or leaks around the cleanout door at the base.
Water collecting at the base of the appliance flue.

A pattern that points toward condensation is trouble that is worse in cold, dry weather and tied to a gas or oil appliance's flue, rather than to rainstorms.

Wood-burning flues condense too

A fireplace or wood stove flue that runs cool also condenses its exhaust. With wood, the condensate includes tars and the result is creosote, a fire hazard as well as a moisture issue. The causes rhyme with the gas case: oversized flues, exterior chimneys and slow, smoldering fires. Our guide to creosote covers that side.

What fixes condensation

Exterior sealers cannot stop water forming inside the flue. The fixes change the flue itself:

A properly sized liner. A liner matched to the appliance's input and the chimney's height keeps exhaust moving fast enough to stay warm. DuraVent's literature describes sizing liners from capacity tables based on the appliance rating, chimney height and lateral runs. Our chimney liner page and the guide to liner options go further.
Insulation where the chimney is cold. The CANMET study suggested a stainless steel liner, possibly insulated, for the chimneys it examined. Insulation approved by the liner maker helps an exterior chimney stay above the dew point.
Correct connectors. The Trane guide recommends double-wall type B vent where vent pipe runs through spaces colder than 60°F, and avoiding extra bends.
Repair of damage already done, such as tile, crown and masonry work, after the cause is fixed.

Any of this starts with an inspection that looks at the flue sizing and condition alongside the appliance it serves.

Questions homeowners ask

Why does my chimney leak only in winter when it is not raining?

Heating appliances run the most in winter, and cold weather keeps the flue walls cold. That combination produces condensation inside the flue, which can look like a leak.

Is condensation in my chimney dangerous?

It damages the chimney over time. It can also be a sign that exhaust is moving too slowly to vent reliably, which raises carbon monoxide concerns. Keep CO alarms working and have the flue checked.

Will waterproofing the outside of the chimney help?

Not with condensation. The water forms inside the flue. A sized liner, and insulation where needed, addresses the cause.

I replaced my furnace. Should I worry?

Changing a furnace, boiler or water heater can change what the chimney needs, especially if a new high-efficiency unit vents elsewhere and leaves an older appliance alone in the flue. That is a good time for a flue inspection.

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Sources

This guide is general information. It can't diagnose a specific chimney; an inspection can.

Guide