Few things are more frustrating in winter than turning on your heat and getting a blast of cold air instead. Before you panic, know that some causes are simple and easy to fix. Here’s what might be going on and what you can do.

1. The Furnace Is Still Warming Up

When your furnace first turns on, it may blow cool or lukewarm air for a few minutes while the burners heat up.

What to do:
Give it a few minutes to run. If the air never gets warm, move on to the next checks.

2. Thermostat Fan Is Set to “On.”

If the fan setting is on “On”, the blower runs constantly, even when the furnace burners are off. That means it may blow room-temperature or cool air between heating cycles.

Fix:

Set the fan to “Auto,” so it only runs when the furnace is actively heating.

3. Dirty or Clogged Air Filter

A clogged filter can cause the furnace to overheat. When that happens, a safety switch may shut off the burners while the blower continues to run, leaving you with cool air.

Fix:

  • Turn off the system.
  • Check the filter and replace it if it’s dirty.
  • Make this a habit every 30–60 days during the heating season.

If the furnace repeatedly overheats, call Penn Mechanical to check for deeper airflow or mechanical issues.

4. Pilot Light or Ignition Problems (Gas Furnaces)

If your burners aren’t lighting, the furnace can’t produce heat, but the blower may still run.

Possible causes:

  • Dirty flame sensor
  • Bad ignitor
  • Pilot light issues
  • Gas supply problems

Fix:

  • Do not try to disassemble gas components yourself.
  • If you smell gas, leave the home and call your gas company.
  • Call a professional technician to safely inspect and repair the ignition system.

5. Ductwork Leaks or Issues

Sometimes the furnace is heating properly, but by the time the air reaches certain vents, it feels cool.

Possible causes:

  • Leaky ducts pulling in cold air from attics, crawlspaces, or basements
  • Disconnected duct sections
  • Poorly insulated ductwork

Fix:

  • Check accessible ducts for visible gaps, holes, or disconnections.
  • Seal minor leaks with foil tape.
  • For thorough testing and sealing, schedule a duct inspection with Penn Mechanical.

6. Incorrect System Size or Design

If your furnace is undersized or your ductwork is poorly designed, the system may struggle in cold weather, sending out lukewarm air and running constantly.

Fix:

A professional can perform a load calculation and airflow test to determine if your system size and layout are correct. If not, they can recommend the right fixes.

If basic thermostat and filter checks don’t solve the problem, it’s time to call Penn Mechanical. A furnace blowing cold air is usually fixable, but it’s important to get to the root cause before it turns into a bigger issue.