How Leaves Enter Furnace Vent Pipes, Removal and Prevention Tips

When it comes to HVAC service calls for furnace lockouts, especially during fall, a common problem encountered are leaves in the PVC intake and/or exhaust vents. However, leaves are not just a problem for furnaces, but also boilers and hot water heaters that utilize unprotected PVC vent terminations. In most cases a qualified HVAC technician can resolve the problem quickly, but not without the expense of the initial service call, hourly on-site fees and material expenses. On this page we are not only going to help you better understand why leaves cause problems in horizontally vented applications, but also how to clear the obstruction yourself and easily prevent the problem from happening in the future.

Leaf Lodged in Furnace Vent PipeIf you look to the image to the right, you will see what we call the $200 leaf. We call it this because the fee for the HVAC service call, time to scope the vents and replace a section of the PVC vent pipe totals about $200 with a coupon. The leaf in this picture came from a furnace’s 2″ fresh air intake vent. Since intake vents draw in air, it’s quite common for leaves blowing around vent terminations to find their way inside an intake vent where they then get lodged at a bend (PVC elbow) in the ventilation pipe run. In the case of the leaf in this picture, it was damp and stuck in a 45° elbow. By using a scope to look inside both the intake and exhaust vents, it was easy to determine why the furnace was in a hard lockout and remedy the problem. This problem was resolved by cutting that section of PVC vent pipe out and replacing it with new. Using the correct 2″ vent screen kit in our store has ensured that this unnecessary expense and inconvenience to the customer would never occur again. You can watch a short video to learn more about vent screen sizing and application use here.

How do leaves get stuck in furnace vents in the first place? This is a common question that many customers ask and knowing the reasons why also aids in both removal and prevention techniques. In most cases, the reason why leaves enter PVC vent pipes is because deciduous trees lose their leaves in the fall, the wind blows those leaves around the intake vent termination where they are drawn deeper into the ventilation system and become lodged in an elbow. In many cases the leaves will lay flat and not cause a large enough restriction to trigger the pressure switch to shut down the furnace in a safety lockout. Those that report strange odors coming from their furnace vents or inside their home in some cases are smelling decaying leaves, other decomposing lawn debris or even dead animals. Dry leaves that enter an intake vent can also find their way directly into the burner compartment where they ignite. Having burning leaves in a furnace’s burner compartment is extremely dangerous as wiring can become damaged and start a dangerous fire that jeopardizes the safety of those occupying the home or business.

Contrary to belief, leaves also get lodged inside exhaust vent pipes too. A leaf or two stuck in a boiler, furnace or hot water heater exhaust vent pipe can occur when wind blows leaves into the exhaust vent while the appliance is idle. Fortunately the force of the exhaust pressure does not pull the leaves deeper into the ventilation system, and the leaves are normally so close to the discharge point that they can be removed without professional assistance. Leaves that are found to be lodged deeper within the ventilation system, or inside the burner compartment, do give rise to additional concerns. Since condensing vents produce condensate, one concern is that condensate will accumulate around the leaves and freeze during winter. When condensate freezes around leaves, this can contribute to furnace icing problems that restrict the exhaust discharge and cause a lockout. However, leaves in vent pipes may also be a clue to an even more serious problem.

While wind is often to blame for leaves and other lawn debris entering PVC intake and exhaust vents, it’s not the only reason. Keep in mind that animals use leaves, grass, straw and other materials for bedding. In cases where animals enter PVC vents, it is quite common for them to bring in the necessary bedding materials and food they need to make your furnace a nice and cozy home. In fact, some service calls for high efficiency furnace noises are the direct result of animals leaving seed and bedding materials inside the furnace’s inducer motor housing or on the draft inducer blower wheel. Should you observe the aforementioned conditions or animal feces inside your ventilation system and/or furnace compartments, please immediately call a qualified HVAC company to inspect the entire system. Once a qualified technician has given your high efficiency appliance a clean bill of health, then immediately install the appropriate vent screen(s) we sell in our store. You never want to install our vent screens when a live animal may be inside your vents or appliance as there is always the opportunity the animal will leave under its own free will, which will save you additional grief and expense if it does leave on its own.

If you suspect that you have a leaf or leaves stuck inside your vent pipes, often resulting in a pressure limit diagnostic fault code, you should perform a good visual inspection of the PVC intake and exhaust termination points to make sure they are clear. Also make sure your furnace filter is clear because clogged furnace filters often throw the same diagnostic error codes as obstructed vent pipes. In some cases a shop-vac may be used to vacuum out the vent pipes and retrieve any lodged leaves. When attempting this, always make sure the power supply to the appliance is off (not just at the thermostat but at the breaker panel itself), the furnace cabinet is open and that the exhaust pipe is detached from the inducer motor. Also make sure the intake vent pipe opening is full exposed in the furnace cabinet. Doing this will not only protect the furnace from damage, but allow you to access the vents inside and outside of your home with a vacuum. Make sure the shop-vac debris container is empty so that you can view its contents when you are completed to see what debris you were able to collect. Because vent pipe runs are often long, having the assistance of another person may be useful as they can block off the open side of the vents you are not vacuuming from. Doing so will create additional vacuum within the vent pipes and increase the likelihood of collecting debris. Ultimately you may need to contact a qualified HVAC company with the necessary equipment to scope the vents to visually confirm that no obstructions exist within the vent pipes. With a professional on-site, he or she will also be able to diagnose additional problems and remedy them in a timely manner.

Preventing leaves from entering PVC intake and exhaust vents starts with the initial installation. In many cases installers have a choice where to terminate vents at, and terminating vents in areas that do not collect leaves, snow, etc. is most desired. However, this is only one factor an installer uses in recommending where vents should be terminated at and is irrelevant when servicing existing systems. Therefore, proper lawn maintenance practices must be followed. If using a side discharge lawn mower, do not discharge cut grass and leaves in the direction of the vents. When deciduous trees shed their leaves, see that they are raked up quickly and never allowed to accumulate near the vent terminations. If using a leaf blower, do not blow leaves towards the vent terminations as this mimics the wind conditions that are largely responsible for leaves getting into intake and exhaust vents. Always keep the area around the vents clear and avoid planting ornamental grasses, plants or shrubs in an effort to hide the vents. Ornamental grasses not only can block the vents themselves, but they aid in allowing other debris and animals to enter the ventilation system as well. Lastly, make sure you are using the appropriate vent screen to protect your vents. Our PVS series vent screens are designed for all-season vent protection, and will prevent animals, insects and leaves from entering your furnace’s intake and exhaust vents.

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