Today's high efficiency gas furnaces can be 25% more efficient then the furnaces of just 20 years ago. Much of that added efficiency is achieved by keeping the heat in the furnace and not allowing it to escape up the chimney. This reduces the flue gas temperature, requiring a smaller flue to create proper draft.
Our technicians have encountered failing tiles and misaligned flue tiles in homes as young as 20-years-old.
Existing masonry chimneys are most often too large to create the proper draft required for today's high efficient gas furnaces and water heaters.
This is a picture of a clay chimney flue used to vent a natural gas furnace. Notice the severe deterioration of the clay lining.
This is caused because the flue temperatures leaving the furnace are not hot enough to keep the large masonry flue temperature above its due point. The hot exhaust gases venting into the chimney are cooled by the masonry chimney and then the gas is turned into a liquid when it passes by the surface of the chimney walls.
This is exactly what happens when you take a hot shower and the steam escapes the shower and ends up fogging the mirrors.
The difference is when flue gases condensate into a liquid they do so as sulfuric acid which eats away at the lining of the chimney and will eventually do enough damage to cause the chimney to have to be replaced.
The solution is to line the chimney with a properly sized aluminum or stainless steel liner which will protect the walls of the masonry chimney and provide proper draft.
This is a picture of an outside chimney. The chimney has two separate flues running through it, one for the fireplace and one for the gas furnace and gas water heater. This chimney should have an aluminum or better yet a stainless steel liner in the flue that is used for the furnace and water heater. The liner will also provide a cap for the top of the chimney.
This home has an inside chimney. The majority of the chimney is located within the climate controlled portion of the home.
Note: The outside portion of this chimney does not begin at the roof line. It begins at the floor of the attic where the chimney is first susceptible to the outside cold temperatures.
When upgrading an older gas furnace to a new ultra-high 90% efficiency model that use direct venting (No Chimney), it is important to remember to install a properly sized chimney liner in the existing masonry chimney if a water heater is going to be left venting into the chimney. Orphan chimney's will condensate water and mix with sulfur in the flue gas forming acid that will attach the masonry chimney. A properly sized stainless steel chimney liner will protect your chimney.
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