Four Furnace Noises You Shouldn’t Ignore, and Their Remedies

By  //  November 30, 2021

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The house is home to various noises: creaky doors and floors, children throwing tantrums, anger from losing an online match, etc. They’re troublesome in their own right but probably nowhere at the level of banging and booming from the furnace. Noises like these can make one lose sleep, not just because of the noises themselves but the worry that the furnace isn’t working properly.

Today’s furnaces are designed to minimize noise while working, so a noisy one points to all sorts of issues. If left unattended for too long, it can compromise the entire HVAC system and leave the house without a way to keep warm, especially with winter around the corner. 

To help you out, keep in mind the following unusual sounds and ways to fix a noisy furnace

1. Banging when firing up

Without a doubt, a loud bang when firing up the furnace would startle anyone at home. In most cases, the root of the problem can be found along the ducts, typically a sign of flexing ductwork or, as experts call it, ‘oil canning.’

Commonly occurring in cold-rolled metal products, oil canning refers to the wavy twisting of the metal surface. While an aesthetic issue in roofing applications, oil canning in the ductwork can signify undersized or improper duct fittings.

However, banging can also come from the furnace itself, a result of delayed ignition. In this case, dirt or some form of blockage prevents the furnace from igniting immediately. The loud bang is from the gas buildup caused by the blockage resulting in a small explosion inside the unit. These explosions can damage the heat exchanger over time and prompt a costly replacement. 

Fortunately, dealing with this noise is as easy as keeping the furnace clean before firing it up. So, it’s vital to have the furnace cleaned out at least every year, preferably before fall, to have a working unit come winter. It also pays to check the air filter if it has accumulated too much dirt and dust, preferably every 90 days (30 to 60 days if living further north). 

2. Grinding while in operation

Grinding sounds are the reason metal parts shouldn’t come in direct contact with one another. A modern furnace has many moving components, any of which may be scraping metal off of itself or another part. 

HVAC contractors often encounter grinding sounds on three furnace parts:

Blower wheel – This component is responsible for pushing air in and out of the furnace. A grinding blower wheel may point to it being loose and rubbing against its casing, or it may already need a replacement.

Blower bearing – The bearing holds the shaft that spins the entire blower wheel when in operation. The rubber that encapsulates it keeps it from rubbing against the surrounding components but is prone to long-term wear and tear.

Motor mount bracket – This component keeps the entire blower fan assembly upright, including the parts above. A broken bracket can knock the system out of alignment and grind against the housing.

Unless you have experience fixing furnaces, doing the repairs yourself isn’t ideal. It would be wiser to stop working the furnace for a while and contact an HVAC repair service, especially if the unit’s still under warranty.

3. Humming but no warm air

Modern furnaces activate when the thermostat setting exceeds the indoor temperature. It isn’t unusual to hear it start, but the lack or absence of warm air blowing out signifies an issue with one of two components. The first is with the blower assembly, explained in detail earlier.

However, the majority of these cases stem from a faulty capacitor. Similar to those installed on other circuits, a furnace’s capacitor stores enough electrical charge to activate the furnace when prompted. In this case, the capacitor is more concerned with starting the blower motor to begin blowing warm air.

The nature of the problem with the HVAC system can point to the faulty capacitor in question.

Here’s a rundown on the three typical kinds of capacitors used:

Run capacitor – Derives electrical charge as the motor runs, common among furnaces, air conditioners, and heat pumps.

Start capacitor – Stores enough charge to turn the compressor on and disconnects once it has done its job, used on some air conditioners and heat pumps.

Dual run capacitor – Allows jumpstarting both the motor and compressor, found in most modern HVAC systems.

When suspecting a faulty capacitor, it’s better to conduct some basic troubleshooting before coming to that conclusion. The following steps will only require a good pair of ears and a stick:

Crank up the thermostat above indoor temperature to fire up the furnace.

Listen for the hum of the blower motor while continuing to crank up the setting

If no warm air’s coming in, it may be that the blower’s not turning at all.

Turn off the furnace and shut off power to it.

Use a stick to test if the blower wheel spins freely.

If the wheel does spin, the capacitor may be to blame. 

Replacing a capacitor can be complicated, starting with getting the right one. Run capacitors have a rating of 3 to 70 microfarads (µF) and voltages of either 370 V or 440 V. On the other hand, start capacitors have a rating of over 70 µF and voltages of 125 V, 250 V, and 350 V. If these numbers alone confuse you, leave the task to a professional instead.

4. Rattling furnace

The last of these typical furnace sounds is an incessant shaking, signifying an issue with either the furnace or ductwork. It may be some loose screws with the furnace or ductwork, a problem that’s easily fixable with the right screwdriver.

Otherwise, the rattling may be coming from something worse, like a damaged heat exchanger. A replacement runs around USD$ 1,500, which is a few hundred dollars shy of getting an entirely new furnace.

Final Remarks

Whatever noise your furnace at home makes, it points to a problem that requires your undivided attention. Dealing with the problem right away will ensure a less stressful winter for the entire family.

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