A grinding noise inside a furnace often indicates an animal has found its way into an unprotected exhaust vent and introduced nuts, seeds and bedding debris into the draft inducer motor housing. While this type of grinding sound is more commonly encountered at the beginning of fall, when the furnace is first turned on after a long period of no use during summer, it can occur throughout the year as animals seek shelter from the elements. For this reason, many HVAC companies, plumbers and home owners utilize preventative measures to guard vent terminations. Unfortunately for many consumers, they first learn about protecting vents after a costly infiltration event has cost them hundreds of dollars in diagnosis and service related repairs.

High efficiency furnaces utilize a draft inducer motor to mechanically supply fresh air for combustion and to discharge the exhaust. A blower wheel, often referred to as a squirrel cage, is connected to the draft inducer motor shaft and contained within a housing. The exhaust vent connects to this housing and often traps animals, insects and debris on furnace vent terminations that have not been properly protected. As you will see in the picture of a draft inducer motor and housing above, an animal entered the exterior exhaust vent termination, traveled deep within the vent pipe where it packed the housing full of nuts and seeds. As the blower wheel rotates, the blower wheel comes into contact with this debris and creates a grinding sound. In extreme cases, the debris left behind from animals may also seize the blower wheel and result in a system lockout after failing a pre-ignition safety check. While in some cases the draft inducer motor housing can be disassembled and cleaned by a technician, often damage to the blower’s fins necessitates replacement. Should the entire draft inducer motor need to be replaced, consumers can expect to pay between $500-$800 with pricing dependent on the make, model and BTU rating of the furnace.

Prevention

Preventing animals from entering vents is easily accomplished with the use of vent screens, which are available in our store. For example, our 90% airflow rated PVS-R (pictured right) and CVC-R vent screens prevent birds, mice, chipmunks, squirrels and other rodents from entering intake and exhaust vents. PVS-R and CVC-R series vent screens exceed NFPA (National Fire Protection Association) minimum airflow standards for use on high efficiency furnace vent terminations during winter. By guarding intake and exhaust vents at the termination, rodents and debris are unable to find their way into the furnace. When insects are active, we recommend the use of our PVS-I and CVC-I series insect screens to prevent bees and wasps from building nests within vent pipes and inside furnaces.

Diagnosis and Service

In many cases a home owner can determine if an animal entered a furnace vent by simply looking at the exterior terminations. When animals enter vents, they often leave clues behind such as food, feces, bedding, etc. which are visible at the termination without the need for specialized skills or equipment. In some cases an odor may be present at the termination or by the furnace, which may indicate an animal has died within the ventilation system.

Since grinding sounds heard in a furnace can be the result of a number of mechanical failures, it’s best to turn off the furnace and immediately contact a locally licensed HVAC technician to inspect the unit. The technician will perform a variety of tests to determine the cause and is better prepared to address infiltration events involving animals. For your safety, do not continue operating a furnace making grinding sounds or detach the exhaust pipe from the draft inducer motor as the pipe may contain a live animal.