DEAR TIM: 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 about exhaust gas damage that might be occurring inside my chimney. Do you think it is necessary to have a chimney liner installed? Robert K., Norwich, CT

DEAR ROBERT: 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. This public servant should help you determine if 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, wood burning, 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 contrary, 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 of masonry chimneys. 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 mortars.

Before you installed your new furnace, your old low efficiency furnace cycled on and off frequently. 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 so to speak.

Currently, there are many organizations that produce building codes. For many years, the American Gas Association (www.aga.org) and the National Fire Protection Association, Inc. (www.nfpa.org) have authored the National Fuel Gas Code. This code as well as other uniform building codes contain tables and charts that people use to properly size and install chimneys for fuel burning appliances. Research is currently being conducted by the Gas Research Institute that may create changes used in sizing masonry chimneys. In other words, code refinements are forthcoming with regards to venting modern fuel burning appliances.

You can do a quick check of your chimney to see if it meets an old rule of thumb and some of the current codes. Measure the diameter of the exhaust pipe leaving your hot water heater. If it is a 3 inch diameter pipe, 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 a chimney lining. There are approved corrugated metal pipes that you can snake down the chimney and connect to the exhaust pipe of your 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, by all means make sure you do it under the guidance of your local building department officials.

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