Furnace Blowing Cold Air? Causes, Checks and Repair Costs

Check the thermostat first: Heat mode, a setting above room temperature, and the fan on Auto, not On. Then replace a dirty filter, because an overheated furnace shuts off its burners and keeps the fan going. A short burst of cool air when the furnace starts is normal.

How warm should it be? Carrier puts gas furnace supply air at roughly 120 to 140°F. On startup, Carrier says some furnaces hold the blower for about 30 to 60 seconds after the burners light. If a part has failed, Fixr reports $66 to $122 for a limit switch and $390 to $725 for a control board, service call included.

Why is my furnace blowing cold air?

Carrier, Trane, Lennox and Bryant all publish cause lists for this symptom. They overlap. What you notice tells you which cause is likely and whether it is yours to fix.

Is it normal for a furnace to blow cold air at first?

Briefly, yes. Lennox says a short burst of cool air at startup is normal, and that continuous cold means a problem. The furnace lights its burners first and starts the main blower later, so the heat exchanger can warm up.

The delay varies by model. Carrier's owner's manual for one condensing furnace says the blower starts about 60 seconds after ignition. Carrier's 58-1T troubleshooting guide for an older two-stage furnace gives 45 seconds after the gas valve opens. Your owner's manual lists your furnace's delay.

The end of a cycle works the other way. When the thermostat is satisfied, the burners stop but the blower keeps going. The 58-1T guide lists blower off delays of 90, 135, 180 or 225 seconds, set at the factory to 135.

How to fix a furnace blowing cold air: checks you can do

These steps follow the "before you request a service call" list in Carrier's owner's manual, plus the manufacturer pages cited below. Stop at the first problem you find, fix it, and wait through one heating cycle.

1. Thermostat. Set it to Heat, a few degrees above room temperature, with the fan on Auto. Carrier says to check whether a programmed schedule shifted after daylight saving time or a power outage. Lennox says weak batteries can cause malfunctions even before they are fully dead.
2. Filter. Pull it and look. Carrier recommends replacing 1-inch filters every 1 to 3 months. Trane says every 30 to 90 days. Carrier's manual says never to run the furnace without a filter, since that can damage the blower motor.
3. Vents and returns. Open every supply register. Move furniture and rugs off the return grilles. Bryant says both should be open and unobstructed.
4. Power. Make sure the furnace power switch is on and the breaker has not tripped. If the breaker keeps tripping, Bryant says to stop resetting it and call a qualified professional.
5. Fuel. On natural gas, the shut-off valve on the pipe to the furnace should be open. If your stove or water heater is also out, Carrier says to contact the gas utility. On propane or oil, read the tank gauge.
6. Drain. High-efficiency furnaces drain water through a plastic line. Trane says a clog stops the burners from igniting. It has a technician clear the drain line and pan and check the condensate pump, if you have one. Lennox treats clearing the line as a homeowner job. Our pressure switch guide covers drain problems in more detail.
7. Blinking light. Count the flashes through the window in the lower door. Carrier's manual says to record the code before you open the blower door or turn off power, because either one erases it. A phone video works.
8. Do not keep resetting it. Bryant recommends professional service rather than repeated attempts to restart the furnace. Carrier's manual says its control locks out after four failed ignition attempts.

That is where the owner's part ends. Carrier's manual says untrained people can do basic maintenance such as cleaning and replacing filters. All other work must be done by trained service personnel.

Furnace blowing air but no heat: the parts behind it

If the checks above did not fix it, one of these parts is the usual suspect. Knowing which one helps you read a repair quote.

High-limit switch

Carrier says a high-limit switch can shut down the burners while the blower keeps running to cool the equipment. Carrier lists a dirty filter, closed or blocked vents, blocked return grilles, a failing blower motor and a dirty evaporator coil as the airflow problems behind it. Trane warns that repeated overheating can make the switch fail altogether.

On the board covered by Carrier's 58-1T guide, code 33 means the limit or flame rollout switch is open. Its checklist starts with a dirty filter or restricted duct system. The rollout switch on that furnace needs a manual reset, which is a technician's job.

Flame sensor

Bryant names a dirty flame sensor as one cause when a furnace starts and then shuts down shortly afterward. Carrier explains why: a dirty sensor may not detect the flame, so the furnace shuts the burners down as a safety precaution. Trane lists cleaning the sensor as the fix and a professional for replacement. Carrier recommends a professional. Our short cycling guide covers this on-off pattern.

Igniter or pilot

Newer furnaces light with an electronic igniter. Trane says electronic ignition has to be evaluated and fixed by an HVAC professional. Lennox says most furnaces made since 2010 have no pilot light. On older ones, Carrier says a pilot that lights but will not stay lit may mean a dirty orifice or a faulty thermocouple. Lennox says a flickering flame or a yellow base instead of blue suggests a blockage.

Gas valve or supply

Carrier says a gas furnace blowing cold air might have a stuck valve or a supply line problem. If you have fuel but no heat, Carrier says a professional should inspect the gas valve or burners. Lennox notes that damage to the gas line would also affect your water heater and stove.

Control board

The board runs the startup sequence. Lennox says a malfunctioning board could turn on the blower and not the burner. Carrier says not to repair or replace it yourself, and to open the cabinet only if your owner's manual permits homeowner access.

Warm at the furnace but cold at the vents?

Then look at the ducts. ENERGY STAR says in a typical house about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts. It lists rooms that are hard to heat and ducts in an attic, crawlspace or garage as signs to check. Trane adds that leaky ducts pull cold air in during winter.

The sources differ on how much ducts explain. ENERGY STAR treats hard-to-heat rooms as a sign of duct problems. Bryant says duct leaks generally do not explain genuinely cold air at the supply register.

For ducts you can reach, ENERGY STAR says to seal leaks with mastic sealant or metal tape. It says never to use duct tape, because it does not last. Carrier also names metallic foil tape or mastic. For ducts inside walls, Carrier points you to an HVAC professional.

Why is my emergency heat blowing cold air?

Emergency heat is a heat pump setting. Lennox says it shuts the heat pump off completely and runs only the backup source. For most homes that is electric resistance heat. Some dual-fuel systems use a gas or oil furnace instead, so every furnace cause above applies. Lennox also says you will still feel warm air on emergency heat.

If you do not, Carrier's page on a heat pump that is not heating gives one check you can do. The outdoor and indoor units have separate breakers. Reset a tripped one, and call a professional if they keep tripping. Carrier warns that heating with electric resistance alone can significantly raise your electric bill. Our dual-fuel guide explains how the two heat sources share the load.

Normal heat pump air also feels cooler than furnace air. Carrier says heat pump supply air is typically around 85 to 100°F, against roughly 120 to 140°F from a gas furnace. A defrost cycle of about 5 to 10 minutes can also blow cool air. Refrigerant work is not a homeowner job. The EPA requires technicians who service equipment that could release ozone-depleting refrigerants to be certified under Section 608. Our Houston page covers backup strips in a city with a light heating load.

How much does it cost to fix a furnace blowing cold air?

A thermostat setting, a filter or an open vent costs little or nothing. Past that, it depends on the part. Fixr reports that furnace repairs cost $349 to $650, with a national average of $485. Trane's cost guide gives a range of $150 to $3,000 and a service call fee of $50 to $200.

The two sources disagree, sometimes by a lot, so treat both as rough. Fixr says technicians charge $87 to $162 an hour depending on location, and that weekend, holiday and after-hours calls always cost more. Trane says emergency rates can be as much as double to triple the regular hourly rate. Our guide to comparing HVAC quotes covers what to ask.

Is it safe to run a furnace that blows cold air?

Usually, for a short time. Carrier says cold air is usually not an immediate danger, but it indicates a malfunction. If you smell gas or see soot around the furnace, Carrier says to turn the system off immediately and contact a professional. Carrier also says not to diagnose or repair a gas leak yourself.

The NFPA says fuel-burning heating equipment can be a source of carbon monoxide. The CPSC says to install battery-operated CO alarms, or alarms with battery backup, on every level of the home and outside sleeping areas. It lists headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion as symptoms. The NFPA says to have fuel-burning heating equipment inspected by a professional every year before cold weather. See our guides to furnace carbon monoxide and cracked heat exchangers.

Call a pro now if

• You smell gas or see soot around the furnace. Leave first if you smell gas, then call.
• A CO alarm sounds or anyone has symptoms. The CPSC says to move outside to fresh air immediately and call 911.
• The burners light and go out again and again, or the furnace locks out.
• The filter and thermostat are fine but the air stays cold. Bryant lists this as a reason to call.
• The condensate drain is clogged, or a blinking code keeps coming back.
• You hear banging, rattling or squealing, which Bryant also lists. See furnace noises.
• The house has no heat at all. Start with our no heat checklist.

Lennox says annual maintenance by a certified professional can prevent many of these problems. Our furnace tune-up guide covers what that visit should include.

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