By Derim “Mike” Vinca, Owner of A-1 Milwaukee Heating & Cooling · 17+ years in the trade · Updated October 2026 · More HVAC guides
Cold air from the vents usually means the blower is running but the burners are not, or not for long. A few of the causes you can check yourself in minutes. Go through these in order. Still cold at the end? Call or text 414-595-5175.
If you smell gas, leave the house and call We Energies at 800-261-5325 or 911 from outside before you call anyone else. If a carbon monoxide alarm is sounding, get everyone outside and call 911.
1. The thermostat fan setting
If the fan is set to On instead of Auto, the blower runs all the time, and between heating cycles it moves room-temperature air that feels cold on your hand. Set the mode to Heat, the fan to Auto, and the temperature a few degrees above the room.
2. Give it a few minutes
A furnace waits for its heat exchanger to warm up before the blower starts, and keeps the blower running for a minute or two after the burners shut off to pull out the last of the heat. That air feels cool. Cool air for the first or last minute of a cycle is normal; cold air for the whole cycle is not.
3. The filter
A clogged filter makes the furnace overheat. A safety switch called the high limit then shuts the burners off while the blower keeps running, so you feel cool air. If you cannot see light through the filter, replace it, then turn the furnace switch off for 30 seconds and back on. Our furnace filter guide covers sizes and ratings.
4. The status light
Most furnaces built in the last 25 years have a small window on the lower panel with a blinking light. Count the blinks and match them to the code chart on the inside of the furnace door. Write the code down; it saves time on the call. Leave the panels on.
5. The condensate drain on high-efficiency furnaces
If your furnace vents through white plastic pipes, it makes water, and that water has to drain away. When the drain line, trap or condensate pump clogs or freezes, a safety switch stops the burners. Look for a full pump tank, a kinked hose or water around the furnace. More in Furnace leaking water?
6. The gas supply
Make sure the gas valve on the pipe to the furnace is open, with the handle in line with the pipe. If your stove and water heater are out too, the problem is the gas supply, so call We Energies.
Still blowing cold? Here is what it probably is
If the checks above did not fix it, the usual causes are a dirty flame sensor (the burners light, then shut off after a few seconds), a failed hot-surface ignitor, a pressure switch or inducer motor problem, a failed gas valve or control board, or a furnace overheating because of closed registers or undersized ductwork. Ducts that leak in an attic, crawl space or garage can also lose so much heat that the air reaches the room lukewarm. If you have a heat pump, a defrost cycle can blow cool air for a few minutes on cold days; that is normal.
What to expect when you call A-1
Calls ring Mike’s cell at any hour. We read the error code, test the ignition, the flame sensor, the pressure switches and the gas pressure, show you what failed, and quote a flat-rate price before any work starts. Repairs are guaranteed for one year, parts and labor. More on our furnace repair and 24/7 emergency service. Typical prices are in our Milwaukee HVAC price guide.
Frequently asked questions
Why does my furnace blow cold air, then warm, then cold again?
That pattern usually means the burners light and then shut off: a dirty flame sensor, a furnace overheating behind a clogged filter, or a drain or venting problem tripping a safety switch. After a few failed tries, many furnaces lock themselves out for an hour.
Is it safe to keep running a furnace that blows cold air?
Usually, yes. The burners are off because a safety switch is doing its job. If you smell gas, leave and call We Energies or 911 from outside. If you hear a loud bang when it lights, turn it off and call.
Can you fix it today?
In most cases, yes. Calls ring Mike’s cell 24/7, no-heat calls get same-day service in most cases, and you get a flat-rate price before any work starts.