You’ve seen it often–an older masonry chimney damaged by moisture, with loose mortar joints, missing mortar, spalling brick and missing brick. When this masonry damage above the roof line is combined with signs of backdrafting, metal flue condensation and damage at a gas water heater, you may be looking at an “orphaned” gas water heating crying for adoption. (This condition can also be caused by 85% furnaces and boilers.)
From about 1900 through 1980, many homes were built with masonry chimneys and clay tile liners. A skilled mason built the chimney to carry products of gas or oil combustion out of the living space to a point above the roof. These masonry chimneys depended on the fact that heated air rises, taking with it the products of combustion, which were pushed out into the atmosphere.
This type of chimney worked well back then, when oil and natural gas furnaces and boilers operated at about 60% efficiency; that is, 60% of the fuel energy went into heating the home and the remaining 40% literally went up the chimney. That was a good arrangement when fuel was cheap and chimneys needed to be kept warm for proper draft. Masonry chimneys were warm and happy, and the temperature inside the flue and vent connectors stayed well above the dew point of the products of combustion.
Efficient Furnaces Changed
As fuel prices increased, we made our homes more efficient. We vented furnaces and boilers directly to the outside with no masonry chimney. Many 90%-efficient condensing gas furnaces vented with PVC pipe. Boilers became about 80% efficient and now modern condensing boilers vent with a PVC pipe, too.
Deprived of the heat produced by a 60% furnace, the clay-tile chimney turns cool. The “orphaned” water heater just doesn’t send enough heat up the chimney. When the temperature of the liner tile dips below the dew point of the water heater’s products of combustion, moisture condenses on the tile, masonry and mortar. Leaks can occur. Wet masonry chimneys alternately freeze and thaw, and eventually they fall apart.
Fixing the Chimney
We can rescue the chimney by inserting a thin metal liner inside the tile. Often, this liner is made of expandable aluminum, with a special connector to the water heater flue. At the top of the masonry chimney, the clay tile is capped with metal, and a metal cap tops the metal liner as well.
When the water heater operates in the winter, the small metal liner heats quickly and there is no condensation. Similar liners are also needed to prevent condensation on chimneys serving 85% furnaces and boilers.
If you see a newly rebuilt chimney or a new metal flue pipe with an orphaned water heater, you should suspect that the orphaned water heater caused damage, requiring parts to be replaced and the chimney rebuilt. Consider including this in your inspection report. Remember: outside chimneys and cold climates compound the problem.
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors boost their business. Copyright ©2015 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.