Some noise from your furnace during its operation is normal. The development of a loud humming or buzzing noise, however, can be concerning. The fact is, a loud hum could indicate either a basic maintenance issue or it could also, potentially, mean there is a more serious problem.
The Likely Culprits
The blower motor, a key component of your HVAC system, is responsible for directing the heat generated by the furnace into the home. It is also the most common source of humming and buzzing noises. Often, noise from the blower motor is a result of insufficient lubrication. The hum may also result from dirt and debris getting inside the unit and causing damage.
Noise from loose equipment is typically the result of age or poor maintenance. Belts, nuts and bolts can loosen over time. When left unaddressed, this can lead to a serious equipment failure.
The joints and seems in your home’s ductwork can wear over time and become loose causing the ductwork to vibrate and hum. This can also indicate the presence of air leaks, which mean your system is not operating as efficiently as it should be.
Furnaces have two types of capacitors. The start capacitor helps the motor start up when the heater is first turned on. The run capacitor supplies electricity to the fan motors to help the blowers move the hot air throughout the home. When the unit is on and making noise but the motor is not turning, the issue could be that the capacitor has gone out and needs to be repaired or replaced.
This is the least likely cause of humming in an HVAC system, nevertheless, a failing transformer can hum when electrical current passes through it. If this is the case, the transformer should be replaced before the furnace stops altogether.
Troubleshooting the blower motor
Not all “hums” mean the same thing. Here are some ways to identify the specific issue with your blower motor.
Humming at slow speeds
Running at a slower speed once the home has reached the set temperature is an energy-saving measure of the furnace. If a noticeable hum accompanies the slowdown, it could mean that the motor is working harder than it should be for normal operation. If that is the case, the motor and the capacitor will need to be replaced. This problem can be avoided with proper maintenance and regularly replacing the air filter to prevent dust and debris from getting inside.
Humming without turning
If the hum occurs when the motor is on but not turning, the capacitor may be damaged and, if so, should be replaced. This could result from a poor quality capacitor or a blocked filter.
Humming when off
Humming when the blower is off usually indicates that the pilot light needs to be readjusted to the appropriate level.
Humming when on
Humming when the blower is on may mean the gas burners are dirty and need to be cleaned, adjusted or replaced.
The two best ways to avoid all of these problems are changing your furnace filter regularly and scheduling annual maintenance.