Common furnace issues: What can go wrong with your furnace, and what you can do about it

Few things bring as much comfort as hunkering down in a warm home during the coldest weeks of winter. Your furnace, which spends much of the year in hibernation, suddenly becomes your most important appliance. But like all machines, furnaces can face issues that require maintenance—or even replacement.

Is it time for you to upgrade to a new furnace, or do your furnace problems have easy solutions? Let’s find the answer.

Even with regular maintenance, things can go wrong. Furnace issues may result from faulty components, lack of upkeep, overdue maintenance, improper installation, or even physical or electrical damage to the unit. Below, we’ve outlined four of the most common furnace issues and what you can do to fix them:

If your furnace isn’t getting any power, you’re not getting enough heat. The same goes for your thermostat settings, which tell your furnace when to cycle on and off.

Furnaces are usually hardwired to their electrical source, but some may be connected to an outlet with a cord and plug. Depending on the furnace, this may be a standard 120-volt outlet or a larger 240-volt outlet, like the one you may have for your clothes dryer. Carefully check around your furnace to make sure it hasn’t become disconnected. You should also look for a shut-down switch or button to make sure it wasn’t activated accidentally.

An unresponsive furnace may have tripped its circuit breaker. Access your main electrical panel and locate the breaker switch for the furnace. If it’s tripped, flip the switch back to the “on” position and try your HVAC system again. If the furnace running trips the breaker a second time, you should have a licensed electrician or professional HVAC technician take a look. Repeatedly tripping a circuit breaker is often a sign of a larger problem.

A digital thermostat that isn’t receiving power should be easy to diagnose because the digital display will be blank. Most thermostats are hard-wired as well, but some are battery-operated, so you should check to see if dead batteries are the problem.

If you can’t identify and easily fix the source of your power problems or the solution involves electrical wiring, call an electrician.

Summary:

• Problem: Furnace or thermostat isn’t receiving power.
• Possible causes: Disconnected power cord, tripped breaker, shut-off power switch, dead thermostat batteries.
• How to fix: Check the power connection, reset the circuit breaker, replace thermostat batteries, or call an electrician if the issue isn’t easily resolved.

“Short cycling” is HVAC-speak for a furnace, heat pump, or air conditioner that turns on and off too frequently. This is one of the most common furnace problems, and it can stress important furnace components such as the blower motor, limit switch, or flame sensor.

There are several potential causes of short cycling, including some that are related to your furnace’s built-in safety systems. If your furnace overheats, detects a buildup of gases, has a malfunctioning thermostat or pilot light, or generates some other safety warning, it may shut itself down abruptly to protect you from hazards like a gas leak or exposure to carbon monoxide.

Improper thermostat or furnace installation can sometimes lead to short cycling. If the thermostat is installed too close to an HVAC vent, it could mistake warm room temperature air for the entire home and stop heating too soon. And if the furnace itself is too large for a home, it can heat the home too quickly and unevenly, which isn’t energy efficient and can reduce the furnace’s lifespan.

If you’re experiencing short cycling, check the air filter, vents, and cold air returns in your ductwork to make sure there are no clogs or blockages that could cause system overheating. A clogged air filter, clogged filter, or dirty filter is an especially common culprit.

You may also want to check your furnace’s exhaust outlet if you can do so safely; a bird’s nest or other blockage could be causing a buildup of gas. If you’ve ruled out easy fixes like cleaning a dirty filter, have a professional HVAC technician inspect components like the gas valve, ignition system, and limit switch.

Summary:

• Problem: Furnace turns on and off too frequently ("short cycling").
• Possible causes: Overheating, blocked airflow, poorly placed thermostat, oversized furnace.
• How to fix: Check and clean furnace filters and vents, inspect the exhaust outlet for blockages, and call an HVAC technician if short cycling continues.

If your furnace seems to have power but isn’t producing any warm air, it’s possible that your ignitor has failed or the pilot light has gone out. Many older gas furnaces have a small flame called a pilot light, which is designed to burn continuously and provide a source of ignition for the burner. If the pilot light goes out for any reason – a gust of air near your furnace, dust buildup, or a failed thermocouple, for example – your furnace won’t heat your home.

If you're comfortable with diy appliance care, you can attempt to relight the pilot – but always with caution. The pilot light control knob is usually set to “on” when a pilot light goes out accidentally, which means it will still be releasing a small amount of natural gas. To relight a blown-out pilot light manually, turn the gas control knob to “off” and wait a few minutes to let any remaining gas clear. Then, follow your furnace’s instructions to relight the pilot light.

Today’s gas furnaces use electric ignition systems instead of a standing pilot flame. While they're safer and more efficient, they also come with their own risks — such as a failed ignition system, flame sensor, or gas valve — and are best repaired or replaced by a qualified pro.

Summary:

• Problem: Furnace has power but isn’t producing warm air due to pilot or ignition issues.
• Possible causes: Extinguished pilot light, dust buildup, failed thermocouple, faulty electronic igniter.
• How to fix: Follow relighting instructions with caution for older models, turn knob to “off” to clear gas, and call a technician for igniter replacement if needed.

Your home heating system shouldn’t make much more noise than the gentle whoosh of air passing through your vents. If you hear troubling noises, it’s usually time to call in repair professionals.

Scraping or grinding noises are often associated with fan problems. Fan blades can sometimes bend or break, causing them to rub against other parts in your system. Fans can also run dry of lubrication or experience worn-out ball bearings, resulting in a range of unusual sounds. An HVAC technician can align and lubricate your fan blades or replace them as needed.

Squealing or squeaking noises, on the other hand, are usually related to worn-out drive belts. Just as you need to occasionally replace various belts on a car, you may need to replace the drive belts in your furnace.

Rattling noises are typically signs that something inside your furnace has come loose. This could be a panel, a screw, or even part of the blower assembly. While a minor rattle may not seem urgent, ignoring it can lead to further damage.

If you schedule an annual furnace tune-up, your HVAC technician should notice signs of wear and be able to recommend proactive replacement.

Summary:

• Problem: Furnace is making new or strange noises.
• Possible causes: Bent fan blades, loose parts, worn-out blower belt, failing capacitor.
• How to fix: Schedule an HVAC inspection for lubrication, realignment, or drive belt replacement; regular tune-ups help prevent long-term damage

The good news is that most furnace issues can be avoided with consistent, proactive furnace maintenance.

Most furnaces aren’t high-maintenance appliances, at least until they get old. A furnace typically lasts 15 to 20 years, so plan ahead and be prepared to replace it as it reaches the end of its lifespan. An annual furnace tune-up – which includes lubrication, cleaning, inspection, and scheduled replacement of parts – is an essential service that keeps your equipment running efficiently. If you get that service done at the start of each heating season, you have a greater chance of getting through the winter with no trouble.

Whether you're a homeowner new to heating systems or experienced with diy fixes, keeping an eye out for common furnace problems will save time, energy, and money.

If you could use a little extra peace of mind, check out these protection plans¹.

¹Protection plans offered by NRG Protects Inc. The terms and conditions in the protection plans may limit NRG Protects’ contractual obligations due to coverage caps, coverage exclusions, and any cash-back option available under these plans.