High-efficiency condensing furnaces have plastic vents for combustion air intake and exhaust gas discharge. With the special heat exchanger design used in high-efficiency gas furnaces, exhaust gases are depleted of heat until the water condensate drips out of the furnace's heat exchanger and the low-temperature flue gases escape from a plastic PVC or CPVC vent pipe instead of a chimney. Combustion air is brought to the furnace from a PVC pipe also commonly vented to the exterior.
These vent pipes typically horizontally penetrate the exterior wall and are inviting to rodents, birds, and insects looking for a home, especially in the winter. With the vent pipes unprotected and no screens installed, you are taking a big risk in having some type of furnace problem due to pipe obstruction or infestations from pests.
However, there are two schools of thought on putting a screen on your condensing furnace’s exhaust flue and air intake vent, “Do It” or “Don’t”.
The “Don’t” folks are concerned that a cover screen will obstruct airflow and cause furnace problems (the pressure switch can also be tripped by an obstructed exhaust flue gas vent pipe) or that the vent screen will not withstand corrosive flue gas exhaust. However, those problems are due to an improper specification of the vent screen.
As the “Do It” folks understand, both of the above problems are easily resolved with a properly specified and installed stainless steel screen mesh vent screen. So installation of a proper vent screen is recommended. One other point, make sure to check that your furnace vent pipe has been installed (minimum upward slope from a furnace of ¼” per foot) and make sure the exhaust pipe is supported every 5 feet of its horizontal run.
Vent Screen Designs
The vent screen design you use will be affected by what you want to keep out. So for birds and rodents (e.g., squirrels), you want to use a larger checkered grid screen installed at a 45-degree angle (diamond pattern). This helps make it more difficult for birds to grab the screen and try to get past.
These types of checkered grid screens may have as high as 90% air flow rate (if a screen is used that uses a thin stainless steel wire mesh). If a PVC molded screen is used the air flow rate will be reduced to around 70%. These large opening grid screens are most commonly used in fall, winter and spring months and where 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 will be purchasing a stainless steel screen but it will reduce the air flow rate from 90% in a large grid screen to about 70%. There is no real reason 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 block the furnace from breathing. If that happens the furnace could essentially shut down to prevent further damage.
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Installation of Vent Screens
Vent screens come in several sizes, including the common 2” and 3” diameters for PVC hubs and couplings. The screens may be installed by using a dab of silicone caulk to the back of the screen frame, or by using a special installation spring clip to retain the screen.
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. These vents have a horizontal grill but obstruct over 60% of the air flow. This will lead to furnace problems such as poor efficiency, pressure switch shut off, unsafe operating conditions, etc. PVC floor drains are another improper type of accessory that looks like a grill or vent screen but is not correct and will cause problems if used.
In the end, use of a vent screen for the air intake and flue exhaust is a decision you will have to make based on the merits as you see them. But the use of a properly specified and installed screen will do much to prevent insects and rodents or birds from nesting in your open vent pipes and will inexpensively and easily prevent damage to your high-efficiency furnace heating system.