One of the most important safety features on a furnace is the metal pipe known as the flue, whose job is to carry carbon monoxide and other dangerous gases outside of your home. A flue that isn't working properly can put you and your family at serious risk. Here are two common causes of a restricted flue.
In order to work properly, a flue must have what is known as a negative pressure differential. In other words, the flue pressure must be less than that of the unwanted outlet gases. This creates a vacuum that draws the gases upward and out through the flue. When the pressure inside of the flue becomes too great--a condition known as a restricted flue--the vacuum necessary to evacuate carbon monoxide gas ceases to exist, creating a potentially hazardous situation.
Flue Pipe Is Too Small
The most common cause of a restricted flue is improper sizing. This often occurs when a new furnace is installed while still utilizing a pre-existing flue. If the old flue is too small to handle the volume of combustion gases being generated by the new furnace, restriction will occur. The solution here is to contact an HVAC installation expert to help determine the necessary size of the flue. Such a calculation must account for such factors like the following:
- the furnace's BTU capacity
- vertical and horizontal length of the flue
- number of elbows present
- width of the flue pipe
- type of connector attaching flue pipe to the furnace
Flue Has An Excessive Number Of Elbows
Elbows, as you can probably deduce, are any right angles present along the course of the flue. Such angles have the effect of reducing the flow rate of gases through the flue. Too many elbows, in other words, means that there will be a higher pressure inside of the flue.
Most flue designs account for two elbows. Yet the physical idiosyncrasies of a house sometimes requires a few additional twists and turns. If this is the case, such extra elbows must be accounted for by installing a flue pipe with a greater width. Where this is not possible, the flue may need to be redesigned so as to minimize the number of elbows. If neither of these options are possible, it may be necessary to install an induced draft fan, or a similar type of powered vent.
For more information, talk to an HVAC professional.