High-efficiency gas furnaces have plastic vent pipes that extend to the outdoors to get rid of exhaust gasses and bring in air for combustion. These PVC or CPVC pipes can be inviting to rodents and birds (especially in the winter) as well as insects (mostly in summer) looking for a home. If the vent pipes are not protected by screens, you can be taking a big risk in having some type of furnace problem due to pipe obstruction or infestations from pests. A simple screen can prevent pest problems, but the wrong type of screen can create other problems.
Is Screening Always a Good Idea?
Screens on furnace vent pipes have their critics. Some argue that a screen will obstruct airflow and cause furnace problems, such as with the pressure switch, or that the vent screen will not withstand corrosive flue gas exhaust. Others point out that screens made to restrict rodents won't keep out insects, so they are ultimately ineffective.
However, most issues are related to improper screens, as homeowners have been known to use any screen they can find, which may not be appropriate. Even some commercial screen products don't perform as they should. Using the right type of screen with the proper specifications takes care of most issues. It can also help to use different types of screening for the summer and winter months.
In addition to using the right screen product, confirm that your furnace vent pipe has been installed with a minimum upward slope from the furnace of 1/4 inch per foot. Also make sure the exhaust pipe is supported at least every 5 feet of along its horizontal run.
Vent Screen Designs
The best vent screen design is based on what you want to keep out. For birds and rodents, you want to use a larger checkered-grid screen installed at a 45-degree angle, creating a diamond pattern. This helps make it more difficult for birds to grab the screen and try to get past it.
Checkered grid screens may have as high as 90 percent airflow rate, the amount of air that is allowed to pass through the screen. Screens made of thin stainless steel wire mesh typically have a much higher airflow rate than many PVC molded screens, which may have flow rates around 70 percent. In general, these large-opening grid screens are most commonly used in fall, winter, and spring months, when protection from insects is not a major concern.
To protect against insects, you will need a much finer screen opening, similar to an insect screen on a screen door. For this level of protection, you look for a stainless steel screen that will reduce the airflow rate from 90 percent in a large-grid screen to about 70 percent overall.
It is not recommended to have a fine screen for winter months, since flying insects are largely dormant. Also, a finer screen mesh promotes snow and ice accumulation, which could significantly restrict the airflow. If that happens, the furnace may shut itself down to prevent further damage.
Installing Vent Screens
Vent screens come in several sizes, including the common 2-inch and 3-inch diameters for PVC hubs and couplings. The screens may be installed by using a dab of silicone caulk applied to the back of the screen frame or by using a special spring clips to retain the screen. Clips are easily removed and are recommended if you plan to add finer screen for the summer months.
Watch Out for Improper Vents
One of the confusing issues associated with PVC vents is that there are several ready-made PVC accessories that people have improperly used as a screen. For example, a PVC termination vent has a horizontal grill and can obstruct over 60 perfect of the airflow. This can lead to furnace problems, such as poor efficiency, pressure switch shut-off, and possibly unsafe operating conditions. PVC floor drains are another improper type of accessory that looks like a grill or vent screen but is not appropriate for furnace vents.
To be safe, make sure any screen you buy is designed specifically for furnace exhaust/intake vents, and follow the manufacturer's instructions for installation. It's also a good idea to check the screens on your vents periodically to make sure there are no obstructions.