Furnace Troubleshooting
Why is my furnace blowing cold air?
The quick checks that fix most Des Moines furnaces blowing cool air, what a technician looks at next — and when no heat in an Iowa cold snap is an emergency.
In a Des Moines winter, the most common reason a furnace blows cool air is simple: the thermostat fan is set to ON instead of AUTO, so the blower keeps running and pushes room-temperature air between heating cycles. Other frequent causes are a dirty filter choking airflow, or a safety control shutting the burners down. Many are quick DIY checks — but no heat during a deep Iowa freeze is an emergency, so don’t wait it out.
Start with these quick checks
- 1 Set the fan to AUTO, not ON. On ON, the blower runs even between heating cycles and pushes room-temperature air that feels cold. AUTO runs the fan only when the furnace is actually making heat. This is the single most common cause.
- 2 Confirm the thermostat is on HEAT and set above the room temperature. If the setpoint is at or below the current temperature, the furnace never calls for heat.
- 3 Replace a dirty filter. A clogged filter chokes airflow, which can overheat the furnace and trip the high-limit safety, shutting the burners off while the fan keeps blowing.
- 4 Open and unblock vents and returns. Closed supply vents or furniture over return registers starve the system of airflow.
- 5 Check the power switch and breaker. Make sure the furnace switch is on and its breaker has not tripped.
- 6 Confirm the gas is on. For a gas furnace, verify the gas supply valve is open before assuming the worst.
What a technician checks next
If the basics look right and you still have cold air, the cause is usually inside the furnace. These checks need tools and training:
- Flame sensor. A classic culprit: a dirty flame sensor lets the burners light, then shuts them off seconds later, so the furnace blows cool air. Cleaning or replacing it is a common fix.
- Igniter and ignition system. A failed igniter means the burners never light, even though the blower runs.
- Limit and pressure switches. Safety controls that shut the burners down when airflow, venting, or temperatures are out of range.
- Condensate drain. High-efficiency furnaces lock out if the drain backs up, which is easy to miss without a trained eye.
- Heat exchanger. Inspected for cracks, which is a safety issue tied to carbon monoxide and a reason never to ignore a furnace that keeps failing.
Safety first: if you smell gas or a burning odor, or a carbon-monoxide alarm sounds, leave the house and call from outside — don’t try to troubleshoot it yourself. Staying on top of a yearly heating tune-up is the best way to catch these issues early; see what’s in an HVAC tune-up .
Iowa myth vs. truth
Myth: “Cold air means my furnace is shot and I need a new one.”
In Iowa the usual culprits — the fan set to ON, a clogged filter, or a dirty flame sensor — are minor and fixable. A furnace rarely needs replacing just because it blew cool air once. If it’s genuinely aging and unreliable, that’s a different conversation; our repair-or-replace guide helps you tell the difference.
Emergency service
Still no heat in your Des Moines home?
If the quick checks don’t restore heat — or you’re in the middle of a deep freeze — don’t wait it out. All Seasons HVAC offers emergency HVAC service across the Des Moines, IA metro — with no extra charge for emergencies.
Common questions
Why does my furnace blow cold air sometimes?
The most common reason is the thermostat fan being set to ON instead of AUTO, so the blower runs and pushes room-temperature air between heating cycles. A brief cool puff at the very start of a cycle is also normal while the heat exchanger warms up.
Can a dirty filter make my furnace blow cold air?
Yes. A clogged filter restricts airflow, which can overheat the furnace and trip its high-limit safety. The burners shut off for protection while the blower keeps running, so you feel cold air. Replacing the filter is the first thing to check.
Why does my furnace start warm then blow cold?
That pattern often points to a dirty flame sensor or another safety shutting the burners down shortly after they light. The fan keeps running, so warm air turns cool. It usually needs a technician to clean the sensor or diagnose the safety.
Is a furnace blowing cold air an emergency?
In IA's deep cold, treat any extended no-heat situation as urgent, especially with infants, seniors, or anyone medically vulnerable at home. All Seasons HVAC offers emergency service at (515) 417-0296 when heat fails at the worst time.