The first real cold snap rolls through the Chippewa Valley, you turn the thermostat up, the furnace starts, and the air coming out of the vents is cool. It is one of the most common calls we get once heating season begins. Sometimes the fix takes a minute and costs nothing. Other times it means a part has worn out, or the furnace has shut its burners off on purpose to protect itself.
Below we walk through the usual reasons a furnace blows cold air. For each one, we will tell you what it is, what you can safely check on your own, and when it is time to call.
Safety first: If you smell gas (a rotten-egg odor) or a carbon monoxide alarm goes off, do not troubleshoot. Get everyone out of the house without switching lights or appliances on or off, then call 911 or your gas utility's emergency number from outside.
Start with the simple stuff
Thermostat settings and batteries
Check that the thermostat is set to HEAT, that the temperature is set above what the room is now, and that the fan is set to AUTO, not ON. With the fan on ON, it runs all the time, including between heating cycles, so some of the air you feel is room air that has not been heated. On AUTO, the fan only runs while the furnace is heating.
If the thermostat runs on batteries and the screen is dim or blank, replace them. Smart thermostats can also be sitting in an away, eco, or schedule setting that holds the temperature lower than you expect. All of this is safe to check yourself and worth ruling out first.
The furnace may just be warming up or winding down
Most furnaces light the burners first and wait for the heat exchanger to warm before the fan starts, then keep the fan running for a short time after the burners shut off to pull out the remaining heat. In those first and last moments, the air can feel cool. If it turns warm within a minute or two, that is normal. If it stays cool through a whole cycle, keep reading.
A dirty air filter
A clogged filter is the most common maintenance problem we see, and it can cause cold air. When the filter is packed with dust, the furnace cannot move enough air across the heat exchanger, so it runs hot. A safety switch then shuts the burners off while the fan keeps running to cool things down, and the air coming out of the vents is cool. The furnace may cycle on and off over and over.
Pull the filter and hold it up to the light. If you cannot see through it, replace it. Our maintenance tips page covers how often to change it.
Quick win: The right thermostat settings and a fresh filter fix a real share of cold-air calls. Start there before anything else.
Wisconsin winter problems
Snow or ice blocking the vent pipes
High-efficiency furnaces usually vent through two white plastic pipes that run out a side wall or the roof of the house. Drifting snow, ice, or heavy frost can block them. When that happens, a safety switch keeps the burners from lighting. You get no heat, and if the fan is set to ON, the air that does come out is cold.
After a storm, take a look at the pipes outside. Gently clear away snow by hand or with a broom, and keep the area around them clear. Do not chip at ice or pull on the pipes. If they keep icing over, give us a call, because that usually points to a venting problem worth fixing before the coldest weeks arrive.
A clogged or frozen drain line
High-efficiency furnaces also make water as they run, and it drains through a small tube and trap near the furnace. If that drain clogs, or freezes where it runs through a cold space, a safety switch shuts the furnace down. If you see water pooling around the furnace or along the drain tube, turn off the furnace at its power switch and call us.
Problems that need a technician
Ignition or pilot problems
Most newer furnaces light with an electronic igniter instead of a pilot light that stays lit. When the igniter wears out, the furnace tries to light, fails, and may lock itself out after a few tries. Some older furnaces still have a standing pilot that can go out. Igniters, pilot assemblies, and gas valves are work for a trained technician. If you are not sure what you have, leave relighting to us, and never try to relight anything if you smell gas.
A dirty flame sensor
If the burners light and then shut off after a few seconds, over and over, a dirty flame sensor is a common cause. The sensor confirms there is a flame. When residue builds up on it, the furnace cannot confirm the flame and shuts the gas off as a safety measure. It is a quick fix for a technician and one of the things checked during yearly maintenance.
Gas supply problems
If the gas valve to the furnace was turned off, for example after other work in the house, or the gas supply is interrupted, the furnace cannot light. If your other gas appliances, like the stove or water heater, are not working either, contact your gas utility. Do not turn gas valves you are unsure about.
Leaky or disconnected ductwork
If some rooms get warm air and others get cool air, the furnace may be fine and the heat is being lost on the way. Ducts that run through an unheated attic, crawlspace, or garage can leak or come apart. Finding and sealing them properly takes a technician with the right tools.
Blower, limit switch, or control board problems
Some cold-air problems come from parts inside the furnace, like a failing blower motor, a worn limit switch, or the control board. Many furnaces have a small window on the front panel where a light blinks a code when something is wrong. If you see one, write down the pattern before you call. It helps our technician know where to start.
Heat pump or hybrid heat? Some cooler air can be normal
If your home has a heat pump, or a hybrid heat system that pairs a heat pump with a gas furnace, the air from the heat pump feels cooler than furnace air even when it is heating correctly. In cold weather, a heat pump also runs short defrost cycles, and the air can feel cooler for a few minutes while it does. If the house stays comfortable, that is normal. Our hybrid heat and heat pumps pages explain how these systems work.
What you can safely check yourself
Before you pick up the phone, run through this list. Any one of these can be the whole answer:
- Thermostat set to HEAT, the temperature set above the room, and the fan set to AUTO.
- Fresh thermostat batteries if the display looks dim or blank.
- A clean air filter you can see light through.
- The furnace power switch, which looks like a light switch on or near the furnace, turned on, and the furnace breaker on in your electrical panel.
- The furnace's front panel closed securely. Many furnaces will not run with the panel loose.
- The outdoor vent pipes clear of snow and ice.
- Supply and return vents open and not blocked by furniture or rugs.
- Any blinking light code on the furnace written down.
When to call a professional
If you have worked through that list and the furnace is still blowing cold air, it is time to bring in a technician. That is especially true for ignition or pilot problems, burners that keep shutting off, water around the furnace, vent pipes that keep icing over, or a blinking error code. Anything involving gas is never a do-it-yourself job.
Kelly Heating & Electric has served the Chippewa Valley since 1997. We are a Bryant Factory Authorized Dealer, and our technicians service and repair all brands, not just the ones we sell. Yearly furnace maintenance catches a lot of these problems, like a dirty flame sensor or a partly blocked drain, before they leave you without heat. Learn more on our heating and gas furnaces pages.
Get your heat back
No heat in a Wisconsin winter is an emergency. Heating emergencies are answered around the clock on our main line, 715-832-6909. For anything that can wait, call during office hours or request an estimate online, and we will get your home warm again.
Written and reviewed by our team in Eau Claire, Wisconsin. Local and serving the Chippewa Valley since 1997.