Furnace Blowing Cold Air? Causes and Fixes

You turn up the thermostat, hear the furnace start, put your hand over a register and feel cool air. In a Denver cold snap that is more than an annoyance. The good news is that many causes are simple, and a few you can check yourself in a few minutes without tools. Others need a technician, and one or two are safety problems you shouldn't ignore.

This guide goes through the causes in the order a technician would check them, from free fixes to parts that need replacing. Repair prices quoted are published national ranges from HomeGuide, not MrFixer Heating & Cooling prices.

First: is it actually a problem?

Some cool air is normal:

• At start-up. On most gas furnaces the burners light and warm the heat exchanger before the blower starts. If the ducts run through a cold crawlspace, garage or attic, the first air out of the register can feel cool until the ducts warm up.
• At the end of a cycle. After the burners shut off, the blower keeps running briefly to pull the remaining heat out of the furnace. That air gets cooler as it goes.
• With a heat pump. If you have a heat pump or dual-fuel system, its air is warm but not as hot as a gas flame's, and during defrost it can feel cool for a few minutes. That is normal; our heat pumps in Denver winter guide explains defrost.

If the air stays cool for more than a few minutes after start-up, or the furnace cycles on and off without warming the house, keep going.

What you can safely check yourself

1. Thermostat settings

Look at the fan setting. If it says ON, the blower runs constantly, including when the burners are off, so you feel room-temperature air between heating cycles. Switch it to AUTO. Also confirm the system is set to Heat, the setpoint is a few degrees above room temperature, and the display isn't blank. Many thermostats run on AA or AAA batteries; weak batteries cause strange behavior. Smart thermostats can also be in an away or schedule mode you didn't expect. If the thermostat itself seems to be the problem, see our thermostat troubleshooting service.

2. The air filter

A clogged filter starves the furnace of airflow, and the heat exchanger overheats. The limit switch senses this and shuts the burners off to protect the furnace, but the blower keeps running to cool things down. The result: cold air from the vents, often after a few minutes of warm air. Pull the filter and hold it up to a light. If you can't see light through it, replace it. In Denver's dry, dusty air and during wildfire season, filters can clog faster than the package suggests. Our filter replacement page has more on choosing one.

3. Power and the furnace door

The furnace has a power switch that looks like a light switch on or near the unit, plus a breaker in your panel. Make sure both are on. The blower door has a safety switch that cuts power if the panel isn't seated properly, so check that the door is closed tightly.

4. The error code

Most furnaces made in the last few decades have a small LED on the control board, visible through a sight glass in the lower door. It blinks a code, and the legend is usually printed on a label inside the door. Write down the pattern before you turn anything off, since cycling power often clears it. A code such as "pressure switch open" or "flame sense failure" points straight to the cause and speeds up a service call.

5. Outdoor PVC vents (high-efficiency furnaces)

If your furnace vents through two white plastic pipes out a side wall or the roof, it is a condensing furnace. One pipe brings in combustion air and the other carries exhaust out. After heavy snow, drifting or a deep freeze, either pipe can get blocked. Consumers Energy describes snow packing into the intake and keeping the furnace from starting. A pressure switch senses the blockage and won't let the burners light, but on some furnaces the blower may still run.

Check from outside. If snow or ice is covering or packed into the pipe openings, gently clear it by hand or with a soft brush. Don't hack at ice with tools, which can crack the pipe, and don't block the pipes with covers. Clear the area around the vents after every big storm. If the pipes ice up repeatedly, the vent may need to be raised above the snow line or rerouted, which is a job for a technician.

6. Condensate drain

Condensing furnaces also make water, which drains through a plastic trap and hose to a floor drain or a small pump. If the trap clogs, the drain line freezes in a cold garage or crawlspace, or the pump fails, water backs up and a safety switch shuts the furnace down. Look for water around the furnace, a full condensate pump reservoir or a hose running through a cold space. A frozen line outside or in an unheated area may need to be thawed and insulated or rerouted.

7. Gas supply

Check whether your other gas appliances, such as the water heater or stove, are working. If none are, the problem may be with the gas service. Check that the gas valve handle on the furnace supply line is parallel to the pipe (open). If you find it closed and don't know why, don't just open it. Call a technician or your gas utility, because someone may have closed it for a reason.

What needs a technician

Cost ranges: HomeGuide national furnace repair data, parts and labor. Not MrFixer prices.

Flame sensor

The flame sensor is a thin metal rod in the burner flame. If it can't detect flame within seconds after the gas valve opens, the control board shuts off the gas. A coated or corroded sensor causes the classic pattern: burners light, run briefly, shut off, try again, and after several tries the furnace locks out while the blower runs. Cleaning a flame sensor is a common tune-up task, but it involves removing a part from the burner assembly, so we recommend leaving it to a technician.

Limit switch

If you replaced the filter and the furnace still overheats and trips the limit switch, the cause may be closed or blocked registers, undersized return ducts, a weak blower or a failing switch. Repeated overheating stresses the heat exchanger, so don't keep resetting it.

Igniter, inducer and pressure switch

The start-up sequence is: thermostat calls for heat, inducer fan starts, pressure switch proves the vent is clear, igniter heats up, gas valve opens, flame sensor proves flame, then the blower starts. A failure at any step stops the burners. In Denver, a furnace that wasn't set up properly for altitude can have trouble at the pressure-switch and flame steps; see our furnace repair cost guide for more on high-altitude conversion and what these parts cost.

Ducts

If the furnace is producing heat but some rooms get cool air, look at the ducts. A disconnected or crushed duct in a crawlspace, garage or attic dumps heat into an unheated space and can pull cold air into the system. Our duct repair service handles this.

When cold air is a safety issue

Most cold-air problems are about comfort, not danger. A few are serious.

Why does this belong in a cold-air article? Because some causes of a furnace shutting down are also causes of CO problems: a blocked or disconnected vent, a failing inducer, or a cracked heat exchanger. The CPSC has warned that vents blocked by debris or nests, and loose or cracked vent pipes, can let exhaust into the home, and estimates that CO from heating systems, water heaters and ranges kills about 80 people a year in the U.S. Its advice:

• Have fuel-burning appliances, including the furnace, flue and vents, inspected every year by a professional.
• Install a CO alarm in the hallway near each sleeping area, check it monthly and replace batteries yearly.
• Never run a generator or charcoal grill indoors or in an attached garage.

If a technician tells you the heat exchanger is cracked and shuts off the furnace, that is a safety call, not a sales tactic. You can still get a second opinion before deciding on a replacement. Our carbon monoxide safety check covers combustion and vent testing.

Things not to do

• Don't keep resetting the furnace. Repeated restarts after a lockout can flood the burners with gas or hide an overheating problem.
• Don't bypass safety switches. Jumping a limit or pressure switch to get heat is dangerous.
• Don't use the oven or a grill for heat. Use a space heater with tip-over and overheat protection if you need temporary heat, and keep it clear of anything that can burn.
• Don't cover the outdoor vents to keep snow out.

Preventing a repeat

• Get a fall tune-up before the first cold snap. Cleaning the flame sensor and checking the igniter and safeties at regular rates beats an after-hours call.
• Change the filter on schedule, more often in smoky or dusty months.
• Clear snow from PVC vents after every storm, and insulate condensate lines that pass through cold spaces.
• Keep supply and return registers open and unblocked.
• If your furnace is over 15 years old and needing repeat repairs, read our repair-or-replace guide and compare options in Denver HVAC system costs.

Frequently asked questions

Why is my furnace running but blowing cold air?

The most common causes are the thermostat fan set to ON, a clogged filter tripping the limit switch, a dirty flame sensor, or a blocked PVC vent on a high-efficiency furnace. Check the thermostat, filter and outdoor vents first.

Can a dirty filter cause a furnace to blow cold air?

Yes. A clogged filter restricts airflow, the furnace overheats and the limit switch shuts off the burners while the blower keeps running, so cool air comes out. Replacing the filter often fixes it.

How do I know if my flame sensor is bad?

The burners light, run a few seconds, shut off and try again, and the furnace may eventually lock out. HomeGuide lists flame sensor cleaning or replacement at about $80–$250.

Can snow make my furnace stop working?

Yes, if you have a high-efficiency furnace with PVC vents. Snow or ice blocking the intake or exhaust trips the pressure switch, and the furnace won't light. Clear the vents gently by hand after storms.

Is it dangerous if my furnace blows cold air?

Usually it is a comfort problem, but some causes, such as blocked vents or a cracked heat exchanger, can create carbon monoxide risk. If a CO alarm sounds or you smell gas, leave the house and call 911 or your gas utility.

Should I reset my furnace?

Turning power off and on once can clear a lockout, but write down the error code first. If it locks out again, call a technician instead of resetting it repeatedly.