Furnace Blowing Cold Air [Causes and Fix]

Gas Furnace Overview

A gas furnace is a type of heating system. The heat exchanger uses natural gas instead of oil as its fuel source. It generates heat through ductwork in the home by using a pilot light connected to a thermostat that homeowners can set to regulate the temperature in their homes. When considering whether to purchase a gas furnace or oil furnace, homeowners should consider where their home’s location and their heating needs.

Furnace blowing Cold Air:

Your furnace may be blowing cold air because the filter is too dirty. A dirty air filter blocks airflow over the furnace’s heat exchanger, causing it to overheat. When overheating, your furnace can trip a high limit switch, causing the furnace burners to shut off so that the heat exchanger does not crack.

Some reasons your furnace is blowing cold air:

1. INCORRECT THERMOSTAT SETTING.

You might have walked by a vent and noticed the air seemed cool. But is your home staying warm, at the temperature you set it at? If it is, the problem might be a simple thermostat setting.

So if you feel the air blowing out of the vents while the furnace is off, the air will seem cool since it’ll be at about room temperature.

Troubleshooting:

Check your thermostat and make sure that it is set to “auto” rather than “on.” When set to “on”, the fan on your furnace will blow continually, even when the furnace isn’t actually heating the air.

2. OTHER THERMOSTAT ISSUES.

 Have you installed a new thermostat lately? If you did it yourself instead of calling a professional, you may have chosen a thermostat that’s not compatible with your heating system.

Troubleshooting:

Consult an HVAC expert for advice. If you’ve had the thermostat for a while, the battery might be low. Try changing the batteries.

3. EMPTY OIL TANK.

If you have an oil-fired furnace, you may be out of fuel. If so, your blower may keep circulating cold air even after the burners fail to light and produce heat.

Troubleshooting:

Fill up the empty oil tank and try restarting your furnace.

4. OVERHEATED FURNACE.

Have you changed your air filter lately? If not, the compromised air flow can cause your furnace to overheat, shutting down the burner. That’s why the furnace is not blowing hot air.

Troubleshooting:

Try changing the air filter and restarting the furnace.

5. EXTINGUISHED PILOT LIGHT.

If you have an older furnace with a continuous pilot light, the pilot may have gone out. This could happen from a gust of air caused by someone working near the furnace.

Troubleshooting:

Solution 1: Relight the furnace pilot light. If the flame stays on, great!

Solution 2: If the pilot refuses to light, ensure that gas is flowing into the furnace. Make sure the furnace’s gas valve switch is in the “on” position (parallel/in line with the gas supply pipe. if the switch is on, then check to see if the pilot light is dirty. if it is, clean it to allow gas to come through.

Solution 3: If the pilot lights, but won’t stay lit, then there’s a problem with the thermocouple (the copper rod you see the pilot flame hitting). It needs to be adjusted or replaced. If you don’t feel comfortable doing this, then contact a professional.

6. VALVE PROBLEM.

If the pilot won’t light at all, you may have a faulty gas valve. Or, if your system has not been cleaned in a while, a valve could be stuck due to dirt and debris. 

Troubleshooting:

Have your system cleaned and checked by an HVAC professional.

7. FAULTY THERMOCOUPLE. 

If you do manage to relight the pilot but it keeps going out, a faulty thermocouple is likely causing your furnace to blow cold air. The thermocouple is a sensor that controls the gas valve and ignition. 

Troubleshooting:

This is usually a simple fix, but you’ll need to call a technician to adjust or replace it.

8. CLOGGED OIL FILTER.

If you have an oil-fired furnace, its filter can become clogged with dirt and debris, just like an air filter on a gas-fired system. When the filter is clogged, ignition can be affected and the result is your heating not blowing hot air. 

Troubleshooting:

If you know how, you can clean or replace the filter yourself, but it’s messy and not as easy as an air filter so you may want to leave this job to the pros.

9. IGNITION PROBLEM.

If your gas furnace seems incapable of blowing hot air, the problem could be with the ignition system. In such a case, your heater is not lighting the fuel, as a result of which it cannot heat your home. Confirm whether the pilot light is out.

Troubleshooting:

• If it’s out, try the following:

Check for instructions on how to ignite your model’s pilot light. You’ll find the instructions on the unit itself or in the user’s manual provided. Relight it and wait for a few minutes. If the flame goes out, then all is not well. You may have a thermocouple-related problem, meaning the component needs adjusting or replacing. Because the work involved is technical, trying a do-it-yourself replacement could do you more harm than good.

• If you cannot relight the pilot, check your gas supply. Make sure the gas valve is switched on. If it is, you probably have a dirty pilot light. Clean it to ensure proper gas flow.

10. LEAKING DUCTS.

Your furnace blowing cold air could be caused by hot air escaping through holes in the duct work that carries the heat throughout your building.

Troubleshooting:

Have an expert inspect your ducts along with the furnace.

11. COMPROMISED FLAME SENSOR.

If your furnace starts blowing warm but then quickly turns cold, the flame sensor may need attention. A flame sensor that’s covered with grime or worn out can cause the burner to keep shutting off.

Troubleshooting:

You’ll need a visit from an HVAC professional to make this furnace repair.

12. DIRTY OIL OR GAS BURNER.

If the furnace burner itself is neglected, accumulated dust and grime can cause trouble igniting.

Troubleshooting:

Have your system professionally cleaned to take care of this problem.

13. The Pilot Light is Out

If your gas furnace is older and it’s blowing cold air, chances are its pilot light has gone out. 

Troubleshooting:

If you’re comfortable doing so, just follow the manufacturer’s instructions for how to re-light it safely.

Alternatively, you can call your local utility company or an HVAC technician to make sure it is lit safely.

Once its lit, if it doesn’t stay lit, it may need some routine maintenance from an HVAC professional.

14. Inadequate Supply of Gas

Your furnace might lock down for safety reasons if it is not getting the amount of gas it requires to provide warm air. If your gas supply is not turned off, then you probably have a severed line.

Troubleshooting:

In case of severed line case you’ll need some professional help.

15. Condensate line

If you have a high-efficiency furnace and you see water pooling around the furnace. Then the furnace’s condensate line (usually a PVC pipe) may be blocked, which causes the furnace to shut down. 

When high-efficiency furnaces run, they create water (condensate), which is emptied out a drain line. However, if that line gets blocked, water backs up into the furnace, causing an overflow kill switch to shut down the furnace to prevent water damage.

Common causes of condensate blockage are:

Besides blockages, the condensate overflow may also be caused by a broken condensate pump. 

Troubleshooting:

If you suspect that ice is the problem, then wrap the condensate line with heat tape and pipe insulation. While in some cases, you’ll need a professional the fix the pump.

Maintenance

Regular maintenance can prevent HVAC problems like your furnace blowing cold air

Many of the causes of furnace problems can be prevented with regularly-scheduled cleaning and maintenance. If you haven’t done that yet this winter, call in an HVAC service expert to keep your system in top shape and prevent unexpected failures.