Q–I recently had my heating system replaced with a high-efficiency system. The new furnace vents directly through the wall to the exterior. My hot water heater is now the only thing venting into the masonry chimney. I am concerned that exhaust gas damage might be occurring inside my chimney. Do you think it is necessary to have a chimney liner installed?
A–Venting of any fuel-burning appliance is serious business. The vent for your recently orphaned water heater may not have been checked by the furnace installer. Many building codes mandate that once modified or changed, all fuel-burning appliance vents must be sized according to code. I suggest that you schedule an appointment as soon as possible with your local building inspector. He or she should help you determine whether the marriage of your hot water heater and your chimney meets code.
The dynamics of drafting are complicated. Traditional masonry chimneys were designed and sized to vent very hot flue gases from old coal, woodburning or low-efficiency gas furnaces. Chimneys are like water pipes, they can only handle so much exhaust at a given pressure.
Imagine if the huge boilers of your local power plant tried to exhaust up your house chimney. There simply wouldn’t be enough room to handle all the smoke and ash. On the other hand, if you tried to exhaust your furnace and hot water heater into a power plant chimney, the exhaust gases very likely wouldn’t rise. The column of cold, dense air inside the huge chimney could cause a portion of the exhaust gas to spill back into your basement.
Herein lies the problem. Your chimney’s existing flue liner may be oversized for the orphaned hot water heater. This can, in some instances, create condensation problems inside a masonry chimney. Water vapor is a byproduct of the combustion of natural gas. If the inside of the chimney cools down too much or there is too much cold air inside the chimney, the water vapor can condense and saturate the masonry inside the chimney before it can escape into the atmosphere. Sometimes the condensed water contains acids that can chemically attack poor-quality mortar.
Before you installed your new furnace, your old low-efficiency furnace frequently cycled on and off. This process kept the inside of the chimney warm. That heat source has been removed. Although the water heater does contribute heat to the chimney during its off cycle, it may not be enough to “prime the vent.”
For many years, the American Gas Association and the National Fire Protection Association Inc. have authored the National Fuel Gas Code. This code, as well as other uniform building codes, contain tables and charts that are commonly used to properly size and install chimneys for fuel-burning appliances.
The Gas Research Institute is currently conducting research that may result in changes in sizing masonry chimneys, so code refinements may be in store for venting modern fuel-burning appliances.
You can do a quick check of your chimney to see whether it meets an old rule of thumb and some of the current codes. Measure the diameter of the exhaust pipe of your hot water heater. If it is a 3 inches, the inside, cross-sectional area of the chimney flue liner should not exceed 49 square inches. If the exhaust pipe is 4 inches in diameter, the flue liner cross-sectional area should not be larger than 88 square inches.
If you find that your chimney does not meet code or it is too large, you can reduce the size of the chimney lining. There are approved corrugated metal pipes that you can snake down the chimney and connect to the exhaust pipe of your water heater.
Some chimney companies can reduce the size of the chimney by pumping a cement slurry around a giant inflatable bladder. Low-moisture-content cement and vermiculite mortars can also be used to create a new inner lining. If you decide to reline your chimney, make sure you do it under the guidance of your local building department officials.
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Have questions about the remodeling process? Write to Tim Carter, c/o The Chicago Tribune, P.O. Box 36352, Cincinnati, Ohio 45236-0352. Questions will be answered only in the column.
For a list of hot water heater venting suggestions, metal chimney relining manufacturers and recommendations and other tips on hot water heaters, send $2 and your name and address to Tim Carter at the above address. Ask for Builder Bulletin No. 200.
