Many people don’t think much about the furnace until it stops working. But the last thing you want is a furnace failure in the middle of winter.

Besides uncomfortable temperatures, a broken furnace can also cause serious safety hazard. Picking up early signs of furnace trouble can help you call for repairs before a full breakdown happens, which can be frustrating when temperatures are at their worst.

In this article, we’ll cover the common signs that your furnace needs repairs. If you need furnace repair in Centennial, CO, be sure to choose experienced and reputable technicians.

Signs Your Furnace Needs Repairs 

1. Yellow Flame 

If you have a natural gas furnace, the flame color can indicate if something is wrong. On modern furnaces, you can usually check the flame color through the service door or peepholes.

A bright blue flame indicates complete combustion, meaning your furnace is heating properly. This happens when there is a correct mix of oxygen and fuel.

However, a yellow flame suggests something is wrong with the combustion process. It could mean the fuel isn’t burning completely, which can be caused by low oxygen or other issues. Incomplete combustion not only reduces your furnace’s efficiency but also risks carbon monoxide buildup.

2. Strange Noises

A running furnace will make some noise. This comes from the blower motor, which pushes air through the house. After using the furnace for some time, you should be familiar with the sound it makes. But if you start hearing weird sounds, it could signal a serious problem.

Abnormal sounds include screeching, popping, shaking, and banging. A screeching sound might mean there is an issue with the blower fan. Meanwhile, a popping sound during startup can indicate ignition or combustion problems. 

If your furnace starts making strange noises or its usual sound stops suddenly, call a technician to inspect it.

3. Unusual Smells

A properly functioning furnace shouldn’t produce strange smells. If you notice odors like melting plastic, burning rubber, fuel, or rotten eggs,  this is a strong sign that something is wrong.

A melting plastic or burning rubber smell could come from an overheated blower motor. If you use natural gas, a gas leak may smell like rotten eggs. If your furnace gives off unusual odors, get it checked immediately.

4. Rising Heating Expenses

While a malfunctioning furnace may still heat your space, it will operate inefficiently. For instance, it might need to work extra hard or run longer to maintain the desired temperatures, consuming more energy.

If your heating bills are rising without an increase in energy rates in your area, your furnace could be wasting energy. 

5. Short Start-Stop Sequence

Your furnace should start and run until the thermostat detects that the right temperature has been reached. At that point, the furnace will shut off and remain in standby mode until the thermostat calls for heat again.

But if your furnace repeatedly turns on and off in short cycles, it indicates a problem. Short cycling reduces efficiency and can shorten your furnace’s lifespan if left unaddressed.

This issue could be caused by a malfunctioning fan motor, a broken ignition system, or dirty filters, flame sensors, and burners.

6. Furnace Blowing Cold Air

When the furnace burner ignites, the blower fan waits a moment for the heat exchanger to warm before circulating air. This ensures the furnace doesn’t blow cold air into your home in winter.

If your furnace blows out cold air, it points to a serious issue. Common causes include a faulty thermostat, burners, or the ignition system. Address this problem quickly and have an expert check your furnace.

7. Weak Airflow Pressure 

To properly heat your home, your furnace needs to blow the hot air with sufficient force. Weak airflow can cause the furnace to run longer. This can lead to excessive fuel consumption and a shorter lifespan for your furnace.

Weak airflow could be due to a malfunctioning blower motor or clogged filters. Filters collect dust, dirt, and other particles to keep the air in your home clean. But when they get clogged, airflow becomes restricted.

Restricted airflow can cause the heat exchanger to overheat. If left unchecked, your furnace may stop producing hot air completely.

Don’t Ignore Furnace Warning Signs

Many furnace problems start small but get worse over time. If you ignore minor furnace problems, you could face expensive repairs down the road or have to replace the furnace when you are not financially prepared.

Fixing problems early not only improves your safety but also saves you money in the long run. As with most things, regular maintenance is key to keeping your furnace in good condition. Therefore, be sure to schedule routine checkups and tune-ups.

Jenny Kakoudakis likes to blog about interiors. She launched award-winning Seasons in Colour in 2014 and the luxury interior design blog All The Pretty Homes in 2024.

When she is not chasing criminals out of the financial system (her day job), she gets creative by redecorating her own home.