You just switched on the heat, so you should be nice and toasty in no time… yet it seems your furnace is blowing cold air and isn’t helping you stay warm at all. In most modern HVAC systems, the blower fan shouldn’t turn on until the heat exchanger reaches the proper temperature, so if you notice your furnace is blowing cold air when the system is set to heat, it could indicate an issue. But don’t worry, this is a common question we receive at North East Air Conditioning, Heating & Plumbing, and often the fix is simpler than it seems. Here’s why your furnace might be blowing cold air and when to call in the experts.
1. The Thermostat Setting
Check your thermostat before you dive deeper. Though perhaps the most obvious, an incorrectly set thermostat is one of the most likely reasons cold air is blowing from your furnace.
If the thermostat is set to “ON”, the blower fan will blow continuously, even when the furnace isn’t heating the air. To the untrained eye, this may look like the furnace is blowing cold air, when it’s actually working as it should. Switch the setting to “AUTO” so the fan only kicks on when the furnace is heating.
Is your thermostat set to the right mode?
Double-check what mode your thermostat is set to. Increasing the temperature on “cool” won’t make your HVAC unit blow warm air. To activate your furnace, you’ll need to set your thermostat to “heat”.
2. Furnace Overheating or Safety Shutoffs
If a furnace overheats, the safety sensors (limit switches) will automatically shut off the burners to protect internal components from sustaining damage. Depending on the cause, the blower may still run to cool the system down, but since the burners are off, all it can push out is unconditioned air which may seem cooler in comparison.
Common causes of furnace overheating include:
- Dirty filter: A clogged air filter restricts airflow, limiting the amount of air that can make it to the heat exchanger and causing the system to overheat. Replace your filter every 1–3 months, depending on the type of filter you use and the air quality in your home, to ensure your system runs efficiently and without any hiccups.
- Blocked return grilles: Obstructed return inlets (where the furnace draws in air) also restrict airflow and can cause your furnace to overheat. Make sure all return grilles are unobstructed, with at least half a foot of clearance around each one.
- Mechanical failure: Weak capacitors, a faulty relay, or a blower motor failure can prevent the furnace from moving enough air to transfer heat from the exchanger. Note: in cases where the blower fan stops working, you may not feel any air come from the registers/vents.
If your furnace shortcycles or shuts off after running for only a few minutes, check your air filter and return grilles. If your returns are clear and your air filter is clean, turn off the system and contact a technician.
3. Leaky or Damaged Ductwork
The issue may not always stem from your furnace, but your ductwork. If you notice inconsistent temperatures across rooms in the same zone, despite your system being on and your registers being open, your ductwork may be the problem. Supply ductwork is the network of tubes that carry treated air to different parts of your home. If your ductwork has leaks or holes, or is improperly sealed or connected, it may allow treated air to leak out before it reaches its intended destination.
A professional duct inspection will help you locate leaks and other issues, such as damaged or missing insulation. Repairing leaky ducts can help you stay more comfortable and lower your energy bills.
4. Fuel or Power Supply Problems
For your furnace to heat your home, it needs a ready fuel or power supply.
If something has interrupted your gas supply, whether a kink in your gas line or an outage, then your furnace burners will not ignite and the system will shut off.
If your furnace runs on electricity, check the circuit breaker. A tripped breaker means no power, which also means no heat.
Note: Don’t try to fix a gas problem on your own. If you suspect a gas-related problem, turn off your system and contact a professional immediately.
When to Call a Professional
Still stumped? When in doubt, call an HVAC professional. At North East Air Conditioning, Heating & Plumbing, we’ve helped San Antonio homeowners stay warm and cozy for over 40 years. Our technicians can quickly diagnose and fix heating problems without making unnecessary repairs.
