Why Is My Furnace Blowing Cold Air? A Portland Tech Answers

We know how infuriating it is to feel cold air coming from your furnace on a freezing day.

Don’t worry—we’ll help you troubleshoot this frustrating problem.

First, let’s try to pinpoint your particular situation:

  • Does your furnace seem to always blow cold air? If so, read the “Problem #1” section below.
  • Does your furnace blow cold air when your furnace first turns on? Jump to the “Problem #2” section below.
  • Does your furnace blow warm air for a little bit before the air turns cold? If this describes your problem, skip to the “Problem #3” section of this article.

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Problem #1: Furnace always blows cold air

The following issues will cause your furnace to blow cold air continuously:

  • The thermostat is set to COOL instead of HEAT
  • The ductwork has major leaks

Let’s look at each of these causes in more detail below:

Cause #1: The thermostat is set to COOL instead of HEAT

We know it may sound silly, but the reason you’re getting cold air could be because your thermostat is set incorrectly.

Solution:

Go to your thermostat and double-check it’s set to HEAT instead of COOL.

If your thermostat is set to HEAT but you’re still getting cold air, read on…

Cause #2: The ductwork has major leaks

If your home’s ductwork has major leaks, it could explain why you’re feeling cold air coming from your vents—even though your furnace is working perfectly fine.

Here’s why: If your ductwork has major leaks, then heated air from your furnace could be escaping into unconditioned spaces such as the attic. While this doesn’t necessarily lower the temperature of the air coming from your vents, it weakens the airflow, which can make the air feel colder than it really is.

Solution:

If you think you have leaky ductwork, you should have a professional inspect your ducts. They can find out if you have disconnected or leaky ducts and recommend the needed repairs.

Problem #2: Furnace blows cold air when it first turns on

Your furnace could be blowing cold air when it first turns on because of a problem with the blower motor.

Normally, the blower turns on 1.5 to 3 minutes after the furnace has ignited and started to heat your home’s air. But if the blower is turning on prematurely, then it may be pushing cold air into your home before it’s had a chance to be heated.

Common problems that cause a blower to turn on too soon include:

  • Thermostat is wired incorrectly
  • The control board is malfunctioning
  • The fan control switch is faulty or set incorrectly

Solution:

Because each of these issues requires special expertise to fix, you’ll need to contact a professional to repair your furnace.

Problem #3: Furnace blows warm air for a little bit before it turns cold

The following issues will cause your furnace to only blow warm air for a little bit:

  • The thermostat is set to ON instead of AUTO
  • The furnace is overheating

Let’s look at each of these causes in more detail below:

Cause #1: The thermostat is set to ON instead of AUTO

When your thermostat is set to ON, it signals the blower in your furnace to run non-stop—even when your furnace isn’t heating any air.

This could explain why you’re only feeling warm air part of the time your furnace is running.

Solution:

Go to your thermostat and make sure your thermostat is set to AUTO instead of ON. When your thermostat is set to AUTO, the blower will only run while your furnace is heating air, which should eliminate the cold-air problem.

Make sure your thermostat fan setting is set to AUTO instead of ON

If your thermostat is set to AUTO but you’re still feeling cold air sometimes, read on…

Cause #2: The furnace is overheating

If your furnace starts to overheat, it will trigger a switch that will shut down the system to prevent damage.

During this shutdown mode, your furnace will stop heating air but the blower will continue to run to cool off overheating components such as the heat exchanger. This could explain why you feel warm air at first, then cold air later.

Your furnace could be overheating for several reasons:

  • Dirty air filter: A dirty air filter restricts airflow to your furnace, which can lead to overheating.
  • Dirty components: A clogged blower can overwork the motor, which can also lead to overheating.
  • Blocked return register: If something is obstructing the return vent, your system could eventually overheat.

Solution:

First, check your air filter. If it is dirty like the one in the image below, replace it with a new one.

Clean air filter versus clogged air filter

Clean air filter (left) vs dirty air filter (right)

Next, make sure nothing is blocking or covering the return vent that goes into your furnace.

A return vent

If you still have cold-air issues after doing both of those things, call a professional to inspect your system and find the cause of the overheating issue

Need a furnace repair from a Portland tech?

Schedule appointment today

We offer fast, same-day service and 100% satisfaction guarantees for all furnace repairs. Visit our furnace repair page for more information about what to expect when you hire us.