A more efficient furnace may require new flue liner

Inspector's Eye

October 13, 2002|By Dean Uhler

I recently looked at a house that has a brick chimney with mineral deposits on its exterior, apparently the result of moisture condensation inside the chimney. This problem is not uncommon, and may become increasingly common as older furnaces are replaced by newer, more efficient ones.

The chimney on the house runs up the two-story end wall. The mineral deposits, which are white and powdery, are at the bottom third of the chimney exterior.

Mineral deposits like this are typically the result of water inside the chimney leaching minerals out of the mortar used in the construction of the chimney. The dissolved minerals are transported as the water seeps to the exterior of the chimney, and are deposited on the chimney exterior when the water evaporates. Over many years, this process can damage the masonry in a chimney.

The water in the chimney that drives this process can come from two likely sources. One source is rainwater, usually originating at the top of a chimney. This typically occurs when a cracked or crumbling mortar crown on top of the chimney fails to shed water as it should, letting water penetrate the masonry and seep down through it. The chimney I mentioned above had a solid and water-tight crown, making it an unlikely source of water. The other common source of water is condensation inside the chimney, usually from water vapor in the heating system's exhaust. That appeared to be the source in the chimney I looked at.

The house is heated by a mid-efficiency gas furnace in the basement. The furnace exhaust vents into the clay tile-lined flue in the brick chimney. It shares the flue with a conventional, gas-fired water heater exhaust.

Mid-efficiency furnaces operate differently from traditional furnaces in ways that enhance efficiency. They have controlled draft through the burners and heat exchanger and use a draft inducer fan that allows greater heat exchange from the combustion gases. This allows elimination of the draft diverter, which, in conventional furnaces, loses heated dilution air up the flue.

But as a side effect of improved efficiency, exhaust gases are cooler and move up the flue less rapidly. Because of this, water vapor in the exhaust may cool to its dew point, causing it to condense into a liquid state. This is particularly likely in a masonry chimney on the outside wall of a house, or in an oversize flue that was originally designed for a less-efficient furnace.

Condensation problems can be corrected by installing a metal liner in the existing masonry flue, using a double-wall B-vent or a single wall stainless steel liner. This will reduce the size of the flue, allowing exhaust to vent more rapidly with less chance to cool. Also, unlike the huge masonry liner, the metal liner will be warmed quickly by the exhaust, reducing the incidence of condensation. Installation of a metal flue liner will probably cost about $1,000 or less.

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