why is my furnace blowing cold airThere are few systems more important than your furnace. It doesn’t just keep you warm. It also keeps you healthy. Cold homes can increase your risk of depression, respiratory infections, even heart attack and stroke! In short, a furnace blowing cold air is a serious problem. Most of the causes are systematic, requiring the help of a professional contractor, while a few are simple enough to tackle on your own. Some of the most common reasons furnaces blow cold air:

Your Furnace Hasn’t Warmed Up Yet

Even the best furnaces require a few minutes to heat up. If you notice your furnace blowing cold air after being switched on, wait a few moments to see if the problem clears up on its own. An older furnace might need as much as 10-15 minutes before it starts blowing hot air. Anything longer indicates a genuine problem.

Thermostat Set Incorrectly

Perhaps not surprisingly, many problems with furnaces can be traced to thermostats. In many cases, they’re set to the wrong temperature or operating mode, FAN instead of HEAT, for example. Before calling a technician, check to see whether the settings on your thermostat are correct. It’s the most likely reason your furnace is blowing cold air.

Pilot Light is Out

Older furnaces require a pilot light to activate the burners. Without it, the gas runs, but doesn’t ignite the burners. Pilot flames are not particularly strong, so there’s a chance it was blown out by a draft. In that case, homeowners can relight it themselves. If it won’t light, contact a professional. The valve may be clogged with dirt, dust, or debris, or the thermocouple may need repair. (The thermocouple deactivates the furnace when the pilot is out. It’s a safety device designed to prevent gas from leaking into your home.)


To protect against fires, modern furnaces come with built-in safeguards that shut down the burners automatically whenever they begin to overheat. However, the safeguards aren’t connected to the fan, which will continue to run and circulate cold air. It’s uncommon for furnaces to overheat, but when they do, it’s normally due to:

  • Clogged Filters. Steady airflow keeps the fan motor operating at a safe temperature. However, as the filter absorbs more particles, the fan compensates by spinning faster. Eventually, if the filter isn’t replaced, the fan motor can overheat and shut down.
  • Dirty Components. Dust, dirt, and grime are common issues in most furnaces, especially if they’ve been operating for several years. Regular maintenance clears them away, but if they’re allowed to build up, they can hinder the movement of internal components. Same as with clogged filters, the motor responds by working harder until it overheats.

Replacing filters is one of the few ways homeowners can care for furnaces on their own. Cleaning your motor, on the other hand, is a delicate operation best left to a professional.

Condensate Line is Blocked

In the old days, a significant amount of heat generated by a furnace was absorbed by the gas flue. Modern furnaces are more efficient. Most of the heat they generate is distributed to your home. As a result, excess moisture, which, in the past, would have evaporated, condenses inside the gas flue. This condensation is collected and drained through the condensate line. If the line is blocked, water backs up into the furnace and triggers a shutdown. Cleaning condensate lines isn’t hard, but if you’re not familiar with the technique, it’s best to entrust it to a professional.

Lack of Fuel

Blocked fuel lines prevent your burners from igniting. Constricted lines prevent them from generating significant heat. Unless you’ve been certified to work on HVAC systems, never try to repair blocked or restricted lines on your own. However, before calling for help, it’s always a good idea to check your gas valve first.

The valve is controlled by a bright red handle sitting on top of your gas line. When the handle’s parallel to the line, the valve is open. When it’s perpendicular, it’s closed. If the valve is closed, your problem is solved. If it’s open, contact an HVAC contractor. They’ll determine whether you’re dealing with a simple blockage, an empty gas tank, or a busted valve.

Leaky Ducts

Bad ducts allow cold air to sneak into your ventilation system and warm air to sneak out. Besides driving up heating costs, they also reduce the air temperature in your home. While homeowners don’t need help sealing ducts (caulk is the most effective remedy), they may need help finding them. If you’re worried your ducts are leaking air, an HVAC technician can pinpoint the source of the problem.

Broken Flame Detector

Modern furnaces use an ignition switch instead of a pilot light. When gas is released into the burners, it activates the switch, which sparks to ignite the flames. This is supposed to trigger the heat exchanger, but if the exchanger can’t detect the lit flame, it won’t run and the blower fan will start circulating cold air instead.

Mechanical Failure

Like cars, furnaces gradually wear out. And the longer it’s been operating, the more problems it will likely encounter. Most times, when a furnace is blowing cold air, worn parts are the reason. Though the flame detector is normally the culprit behind this type of malfunction, several other parts could be at fault as well, such as:

  • Main Limit Switch. The device that cuts power to the furnace when it overheats. If it’s near the end of its lifespan, the switch may cut power when the furnace is operating at normal temperatures instead.
  • Circuit Control Board. Acts as your furnace’s central nervous system, conveying messages from one component to another. Each part of your furnace is supposed to activate in sequence to generate heat. But when the circuit board burns out, the signal might not reach one or more parts of your furnace (e.g. gas valve, ignition switch, burners), stopping the process in its tracks.
  • Fan Limit Switch. Tells the blower fan to start circulating air once the heat exchanger has warmed up. It also deactivates the fan once the house has reached the right temperature. However, a defective switch might tell the fan to run continuously, even after the heat exchanger has cooled down.

Protect Your Heating System

Agway doesn’t just power your home. It helps save money on repairs. Our EnergyGuardTM Program covers your heating, cooling, and electrical systems* against damage caused by wear and tear. Home insurance doesn’t cover worn parts, but we do.

Rather than waste time searching for a qualified technician, customers call us when the need for repair occurs. We send a contractor to your home as quickly as possible. There are no service fees or deductibles. We pay for the visit and all covered parts. Contact us today to sign up and enjoy the benefits of EnergyGuardTM!

*Coverage depends on which commodity you purchase.