Furnace Blowing Cold Air or Not Turning On? 9 Fixes to Try
The first cold mornings in the Willamette Valley are when furnace problems show up. Most of these checks take a few minutes and need no tools.
Smell Gas? Stop Here.
If you smell rotten eggs or your carbon monoxide alarm is going off, leave the house now. Don't flip switches. Call NW Natural's emergency line or 911 from outside. Skip the checklist below.
9 Things to Check Before You Call
Work through these in order. The first five solve a surprising number of no-heat calls.
Thermostat set to HEAT, fan set to AUTO
If the fan is set to ON, it runs between heating cycles and blows room-temperature air that feels cold. Set it to AUTO. Then make sure the mode is HEAT and the set temperature is a few degrees above the room.
Thermostat batteries
A dead battery can leave the screen on but stop the call for heat. Replace them if the display is dim or blank.
The air filter
A clogged filter chokes airflow. The furnace overheats, the high-limit switch shuts the burners off, and the blower keeps pushing cold air. Swap the filter and see if heat returns. Most homes need a new one every 1 to 3 months during heating season.
Breaker and furnace power switch
Look for a tripped breaker labeled furnace or air handler. Then find the light-switch style switch on or near the furnace. It gets turned off by accident more than you'd think.
The furnace door
The front panel presses a safety switch. If it isn't seated all the way, the furnace won't start. Remove it and put it back firmly.
Gas supply
If other gas appliances aren't working either, the issue is the gas supply. Check that the gas valve at the furnace is in line with the pipe (open).
Condensate drain on high-efficiency furnaces
Furnaces with white PVC vent pipes make water. If the drain line or trap clogs, a pressure switch stops the furnace. Look for water in the drain pan or a kinked hose.
Blinking light code
Most modern furnaces have a small window with a blinking LED. Count the blinks and match them to the chart on the inside of the door. The code tells a technician exactly where to look.
Is it actually a heat pump?
Heat pumps deliver air around 90 to 100°F, which feels cooler than a furnace's 120°F+. They also run short defrost cycles in cold, damp weather that blow cooler air for a few minutes. That's normal. Constant cold air is not.
Still Cold? These Are the Usual Culprits
What a Technician Will Look At
- Dirty flame sensor: the burners light, then shut off after a few seconds. A cleaning usually fixes it.
- Failed hot surface igniter: you hear the inducer fan start but the burners never light.
- Pressure switch or inducer motor: often tied to a blocked vent or a clogged condensate trap.
- Bad control board or gas valve: less common, and the blinking code usually points right at it.
These parts carry live gas or high voltage, so leave them to a licensed technician. Valley Flow stocks common furnace parts on every truck, so most of these get fixed on the first visit. See our furnace repair service, or heat pump repair if you have a heat pump.
How to Keep It From Happening Again
- Change the filter on schedule. Every 1 to 3 months in heating season, more often with pets or wildfire smoke.
- Get a fall tune-up. A technician cleans the flame sensor, tests the igniter and checks the heat exchanger before the first cold snap.
- Keep the condensate line clear. On high-efficiency furnaces, a yearly flush prevents the most common mid-winter shutdown.
- Test your CO detectors. Put one on every level of the home and near bedrooms.
Furnace Troubleshooting FAQs
Why is my furnace blowing cold air?
The most common causes are the thermostat fan set to ON instead of AUTO, a clogged air filter tripping the high-limit switch, a failed igniter or dirty flame sensor, or a clogged condensate drain on high-efficiency furnaces.
Why won't my furnace turn on at all?
Start with the thermostat mode and batteries, the breaker, the furnace power switch and the door safety switch. If all of those are fine, a blinking error code on the furnace control board will point to the failed part.
Is it normal for a furnace to smell like burning when first turned on?
A light dusty smell for the first 15 to 30 minutes of the season is normal as dust burns off the heat exchanger. A strong electrical smell or a smell that doesn't go away is not. Turn the furnace off and call for service.
What should I do if I smell gas?
Leave the house right away, don't flip any switches, and call NW Natural's emergency line or 911 from outside. Don't try to diagnose the furnace yourself.
Still No Heat? We'll Come to You.
Valley Flow repairs every major furnace and heat pump brand across Silverton, Salem, Keizer, Woodburn, Stayton and Mt. Angel. Licensed, bonded and insured, CCB# 248792.