High-efficiency gas furnaces have plastic vent pipes that extend to the outdoors to get rid of exhaust gases and bring in fresh 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 take a big risk for furnace problems 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.
The Debate About Vent Screens
The practice of placing screens on furnace vent pipes has critics. Some argue that a screen will obstruct airflow and cause furnace problems, such as affecting the operation of the pressure switch. Another argument is that the vent screen will not withstand the corrosive influence of flue gas exhaust. Others point out that screens that are constructed to restrict rodents are insufficient to keep out insects and are ultimately ineffective.
However, most problems occur because homeowners use whatever screening material they can find, and these choices are sometimes not appropriate to the need. Even commercial screen products sometimes don't perform as they should. However, using the right type of screen with the proper specifications prevents most problems. It can also help to use different types of screening for the summer and winter months, swapping them out with the changes in the seasons.
In addition to using the right screen product, make sure that your furnace vent pipe has been installed with a minimum upward slope from the furnace; 1/4 inch per foot is recommended. 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 one that targets the pest you are trying to keep out. For birds and rodents, it's best 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 pull it open.
Checkered grid screens may have an airflow rate as high as 90 percent (airflow rate is a measurement of how much air passes through the screen vs. a fully open pipe). 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 of only about 70 percent. In general, these large-opening grid screens are best used in fall, winter, and spring months, when protection from insects is not a major concern. The best choice is a stainless steel wire mesh with an airflow rating of roughly 90 percent.
The situation changes during the summer months when insects are the main problem. To protect against insects, you will need a much finer screen opening, similar to the insect screening found on a screen door or storm window. For this level of protection, look for a stainless steel screen that provides an airflow of about 70 percent. Stainless steel is largely unaffected by the corrosive influence of furnace exhaust gases. Although these summer screens do impede the airflow more, this is not a big problem in the summer, when the furnace is not heating as often.
Change back to the larger grid screen for winter months. Flying insects are not a problem during this time, and the furnace will need better vent circulation since it will be running much of the time. Also, if the finer screen is left in place, it may cause snow and ice to accumulate, which can 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 special spring clips to hold it in place around the end of the vent pipe. Clips are easily removed and are recommended if you plan to routinely change screens for the summer months.
Avoid Improper Vent Covers
A common mistake when adding screening to PVC furnace vents is to use ready-made PVC accessories, such as PVC termination vents, which can obstruct as much as 60 percent of the airflow. While these are fine for their intended use for blocking the end of a plumbing system vent pipe, they are not acceptable to shield the ends of furnace vents. Using them in this way can lead to a variety of furnace problems, including poor efficiency, pressure switch shut-off, and possibly unsafe operating conditions. PVC floor drain covers are another improper accessory; these also are inappropriate for covering a furnace vent because they greatly reduce airflow.
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 or damage to the screens.