Why Is My Furnace Blowing Cold Air?

By James K. Kim

Compared to some Northern regions, New York City and the surrounding areas including Westchester County and southwestern Connecticut enjoy relatively milder winters with average temperatures typically staying above sub-freezing most days.

However, studies have shown that most people feel most comfortable at temperatures around 70 degrees F, and as a result, most homeowners rely on their home’s central heating system to maintain a cozy indoor climate throughout the colder months.

That’s why the last thing anyone wants to feel when they fire up their forced air heating system, such as a furnace, is cold air blowing from the supply air register vents. If you are experiencing issues with your furnace, give Cottam Heating and Air Conditioning a call now to have our trained HVAC service technicians diagnose and troubleshoot your home heating problems.

If you are curious to learn the possible reasons why your furnace might be blowing cold instead of hot air, here are the top 11 most common reasons why.

1. Check for possible issues at the thermostat

Sometimes it can be a simple user error on the thermostat, such as a mistake on the programming, that can set the thermostat incorrectly, such as the fan mode set to ON, and result in cool air blowing without the furnace firing its heating element.

Look at the thermostat and double check that it is set to HEAT instead of COOL or FAN.

However, if you find the thermostat is indeed set to heating and the furnace is still blowing cold, it may indicate a different issue other than the thermostat.

2. The furnace is overheated

Today’s modern natural gas-fired furnaces are engineered with safety systems and checks in place to help prevent explosive fire disasters from occurring that involve gas.

One such safety system of a furnace is a high limit switch that can detect when the furnace is dangerously close to overheating and shuts the entire heating system down.

When this occurs, it is sometimes possible that the blower fan keeps running, resulting in cool air still circulating through the ductwork.

3. The furnace is in Safe Mode

Another modern function of today’s furnaces is the Safe Mode function that prevents heat from accelerating to dangerously high levels.

When in Safe Mode, the furnace ignition is killed so the burners (the round metal tubes that alight when the furnace fires up) do not ignite.

Your furnace could be in Safe Mode for a number of reasons, including:

• A corroded thermocouple or flame sensor
• A faulty high limit switch
• The furnace overheating
• A crack in the furnace heat exchanger
• If it’s an older furnace, the Safe Mode could be malfunctioning

Your trusted local HVAC service technician should be able to diagnose the Safe Mode issue and recommend a solution to get your furnace back to blowing warm instead of cold air.

4. The furnace filter needs to be replaced

Furnace filters are usually located in the return air portion of the system, secured within an access plate, These replaceable or reuseable filters are responsible for trapping particles, dust, and debris from collecting and potentially damaging the machine.

Depending on indoor air quality factors including pets, smoking, allergies, and other factors, most HVAC experts would agree that furnace filters be replaced several times a year, preferably at the beginning of each season and as needed in between.

A dirty clogged air filter can restrict airflow into the system and trap heat in the furnace, resulting in cold air blowing.

Filters should be replaced with fresh, clean ones and are typically part of a seasonal maintenance service appointment performed by an HVAC service technician.

5. The circuit control board is malfunctioning

A furnace blower motor (the component that blows heated air through your ductwork) and the thermostat can be considered the heart and soul of a central HVAC system, while the circuit control board can be considered the brain.

That’s because the circuit control board tells the furnace what it should do and when to do it, while also coordinating communications between various components of the heating system.

The circuit control board also relays the thermostat’s commands from to the heating burner and blower motor.

When the circuit control board malfunctions, the entire central heating system goes down, or it can activate the blower without igniting the burners, resulting in the furnace blowing cold air.

6. There is no more fuel in the furnace

If a furnace does not run on natural gas, other fuel sources include oil, propane, or other combustible material. Without fuel, a furnace may still run, but just blow cold air. A call to your furnace fuel provider should be able to refill your fuel.

If the furnace is fueled by natural gas, it’s possible that the gas supply line could be shut off. If the gas line has a gas valve near the furnace, it’s possible it may not be open or have a malfunction.

If you’re still unsure if natural gas is flowing, it’s best to contact either your trusted local HVAC company or your gas utility company for further assistance.

7. The furnace pilot light is out

Older model furnaces may still use a pilot light that is constantly burning in order to fire up the burners.

However, due to age and use, the pilot light can extinguish for a variety of reasons. Homeowners could re-light the pilot light if they feel it is safe to do so.

However, if excessive natural gas is detected or the homeowner is not comfortable relighting the pilot, it may be best to instead contact a qualified HVAC technician rather than risking a potential accident from natural gas leaks.

8. Your ductwork is in need of repair

Your home’s ductwork transports the conditioned air from your central HVAC system to the various rooms of the house.

Over time, it’s possible for dust, debris, and possible organic growth to clog and create blockages in the ductwork. Older ductwork can sometimes also break or crack.

Damaged or clogged ductwork may lead to furnaces blowing cold air if heat cools down before it reaches the vent. A ductwork inspection by an HVAC specialist will help with a solution to repairing or unclogging problems in the ducts.

9. A clog in the vents or the evaporator coil

The vents in the floor, ceiling, or wall that emit air from the furnace could be blocked and prevent heat from entering the space. It’s possible you may be able to unclog the vent yourself, or you may be considering contacting a reputable duct cleaning company to help clear and clean ducts and vents.

If you have central air conditioning as part of your HVAC system, then you may have an evaporator coil that is typically connected to your furnace. Even when in heating operation, air will still pass through the evaporator coil.

However, a clogged evaporator coil could also result in cool air emitting from your vents. A qualified HVAC service technician should be able to diagnose an issue within the evaporator coil and remedy it accordingly.

10. The condensate drain line is clogged

If you see pooled water collected around the base of the furnace, it may indicate a clog in the condensate drain line. Clogged drain lines can lead to water backing up back into the furnace, causing both cold air blowing as well as potential damage that might require entire system replacement.

While some homeowners may be able to successfully clear out a drain line, it is also a task that can be expertly performed by an HVAC service technician as part of a regular furnace maintenance appointment.

11. Dirty flame sensors

Modern furnaces that do not have a pilot light will have different types of flame sensors that detect when the burners are alight.

However, if the flame sensor is covered in carbon, dirt, and other buildup from combustion, then normal heating operation may not happen when the thermostat calls for heat, leading to cold air blowing from the vents.

A qualified HVAC service technician will be able to expertly analyze an issue with the flame sensor and either be able to clean and re-use it or replace it entirely with a new flame sensor to get your heating back online.

Conclusion

The best way to prevent your furnace from blowing cold air instead of heat is by getting regular HVAC maintenance visits from qualified service technicians.

The HVAC tech can run through their seasonal maintenance checklist of heating items to be inspected which will help prevent minor issues into developing into future HVAC disasters.

A well-maintained heating and cooling system should have a reliable operational lifespan between 10-15 years. If your HVAC system is nearing its end of life, your HVAC technician can help you understand your options for replacement and keep you and your family cozy and warm all winter long.

Want to Stay Warm This Winter? Call Our HVAC Heating Service Experts

Our Westchester expert HVAC technicians can troubleshoot all your furnace issues and let you know if a new furnace can help you save money in the long run. Call us today or schedule an HVAC service appointment online.