Why is my furnace blowing cold air?  Pippin Brothers, Lawton, OK
December 17, 2014

Just great. It's cold as penguin's beak outside and now your furnace is blowing cold air (sometimes intermittently) out the air vents.

Why is this happening?

There are a few causes of this issue.

1) The thermostat fan is set to "ON"

"ON" means the furnace's blower is running constantly--whether the furnace is heating it or not.

Simple fix: Turn the fan setting to AUTO. Now only heated air will come out the vents.

2) Dirty air filter is causing furnace to overheat

A dirty air filter decreases airflow to the heat exchanger (the part that heats the air), causing it to overheat. If it overheats, a limit switch shuts down the furnace burner (the part heating the heat exchanger).

Basically, your furnace is trying not to kill itself.

Simple fix: Check the air filter. If it's dirty, change it. Here's how to change the filter.

3) Pilot light's flame is out

Note: Only read this section if you have an older furnace with a pilot light.

When the pilot light goes out, the gas valve shuts, preventing the burners from igniting.

Simple fix: Try relighting the pilot light (instructions should be on your furnace door.)

If the pilot light goes out after a minute, something is wrong. Issues could include:

  • Dirty pilot light
  • Dirty/damaged thermocouple (the copper rod that senses if the pilot flame is lit)
  • Flame isn't reaching thermocouple properly

Call an HVAC contractor to find the problem if the pilot light does not stay lit.

4) Issues with draining condensate

Note: Only read this section if you have a high efficiency furnace (90%+ AFUE)

To work properly, a high efficiency furnace has to drain condensate out of the home through a drain.

If there's an issue draining this condensate, a float limit switch will prevent the furnace burners from igniting to prevent any more condensate from forming.

A few issues with draining the condensate include:

  • Clogged condensate piping
  • Clogged condensate drain
  • Issues with the condensate pump

Simple fix: Call an HVAC contractor to find the issue and fix it.

Pippin Brothers is the Lawton-area's most trusted heating, air conditioning and plumbing company. We've been serving Oklahoma since 1978. Contact us for more information.