Why Is My Furnace Blowing Cold Air/Not Heating?

Is your furnace giving you the cold shoulder by blowing cold air?

Dang, that’s no good. Especially during Minnesota’s Russian-esque cold seasons.

So why is it happening? 

Could be lots of things. But you can fix these 4 common issues before calling a professional for help.

Thermostat’s fan is set to ON

Does your furnace blow cold air only sometimes?

Your thermostat fan setting may be switched to ON, meaning the blower runs 24/7, even when the furnace isn’t heating the air

How to fix it: Set the fan setting to AUTO. Now the blower will run only when the furnace heats the air. No more cold shoulder.

Furnace air filter is dirty

Your dirty air filter may be overheating the furnace, causing it to shut down.

How can the filter do all that?

Well, a dirty filter blocks airflow over the furnace’s heat exchanger (the part that heats the air). Without enough airflow, the heat exchanger overheats, tripping a high limit switch, which shuts down the furnace burners.

How to fix it: Turn off the furnace and change the furnace filter if it’s dirty,

Check the filter at the blower.

Pilot light is out

If you have an older furnace— one installed installed before 1990— you probably have a standing pilot flame which may have gone out. No pilot light means the burners won’t ignite.

Most of standing pilot furnaces can be re-lit with these steps: 

  1. Turn the power switch off. (The switch looks like a light switch and is usually found near the furnace wall or ceiling.) 
  2. Carefully remove the furnace door. 
  3. Do you see the pilot light? No? Drat. It’s out
  4. Locate the control valve. You should see a turnable knob with these options “Off”, “On” and “Pilot”
  5. If there’s no pilot flame, turn the knob to the “Off” position. 
  6. Wait five minutes. 
  7. Go get a match or BBQ lighter and keep it on hand. 
  8. After five minutes, turn the knob to “Pilot.” 
  9. Press the pilot button and hold it while you use a match or BBQ lighter to relight the pilot. You can locate the pilot by following the tubing from the control panel.
  10. Keep holding the pilot button for 1 minute. 
  11. Release the pilot button. The pilot flame should stay lit. If it goes out, start over.
  12. Once the flame stays lit, stand back at arm’s length and turn the control valve’s knob from “Pilot” to “On.”
  13. Flip the furnace power switch on. 
  14. The furnace burners should now light. 
  15. Carefully place the door back on the furnace.
  16. Celebrate, cause you should have heat again!

What to do if the pilot won’t light or stay lit

Sadly, we’ve done all we can to help you here. You’ll need to call a Minnesota furnace technician for further help

Condensate line is clogged or frozen

For those of you who have a high-efficiency furnace, the furnace’s condensate line may be clogged or frozen, which can cause the furnace to shut down. You’ll know this is the problem if you see water pooling around your furnace.

You see, when high-efficiency furnaces run, they create water (condensate), which is usually,  drained out a condensate line. But if the line gets blocked or frozen, water backs up into the furnace, causing an overflow kill switch to shut down the furnace to prevent water damage.

Condensate lines can become blocked due to:

  • Dust
  • Mold
  • Dirt
  • Ice (only if the line is long and runs through an unconditioned space)

To unclear most condensate clogs: Follow this tutorial on Energy Vanguard.

If the condensate line is frozen: Wrap the condensate line with heat tape and pipe insulation.

Need a furnace repair in Minnesota?

Why not give On Time Service Pros a call? We’ve been serving Minnesota for 70 years. That’s like 497 dog years.

We promise to show up on time, or you get $5 for every minute we’re late (up to $300).

Deal? Contact us online.


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