The thermostat says 70, but the air on your hand feels like a draft from the garage. A furnace blowing cold air tends to show up around the first real cold night of the year in North Jersey, when a system that's been sitting since spring finally has to work.
Sometimes the fix takes ten seconds. Other times the furnace has shut its burners off to protect itself, and the fan is the only part still running. Start at the top and work down.
If you smell gas, stop here
If you smell gas or your carbon monoxide alarm is going off, don't troubleshoot anything. Don't flip any switches or use your phone inside. Leave the house and call the gas company and 911 once you're out. If neither applies, keep reading.
Make sure it's really a problem
Give it a few minutes first. A short burst of cool air when the heat kicks on is normal, because that air has been sitting in the ducts. Many furnaces also wait a bit to start the blower so the air has time to warm up.
It can happen at the other end of the cycle too. Once the burners shut off, the blower keeps going for a little while to pull out the last of the heat, and the air gets cooler toward the end.
If you put your hand over a vent several minutes into a steady run and it's still cold, something's wrong. One exception: heat pumps put out air that feels cooler than a furnace's, so if that's what you have, lukewarm air may be perfectly normal.
Check the fan setting
Look at your thermostat and find the fan setting. If it says On, switch it to Auto. On means the blower runs all the time, including between heating cycles when the burners are off, so you'll feel room-temperature air coming out of the vents even though nothing's broken.
Somebody may have set it to On back in July and forgotten about it. While you're there, make sure the thermostat is on Heat and set a few degrees above what the room is reading. If the screen is blank and it takes batteries, change them.
Pull out the filter
A clogged filter is a common cause. When air can't get through, heat builds up inside the furnace. A safety device called a limit switch shuts the burners off so the furnace doesn't overheat, but the blower keeps running to cool things down.
Set the thermostat to Off before you slide the filter out and hold it up to a lamp. If the light doesn't come through, put in a new one with the arrow pointing toward the furnace and let it cool for 20 to 30 minutes before you set it back to Heat. While it's cooling, walk the house and open any closed vents. Make sure furniture or rugs aren't covering them or the big return grilles.
If the furnace goes right back to blowing cold on a clean filter, set the thermostat to Off and call us. Repeated overheating can wear out that limit switch, and at that point the furnace has a problem a new filter won't solve.
When the burners won't light or won't stay lit
Your furnace has to do a few things in order before you get heat. The igniter lights the burners, and a small part called the flame sensor confirms there's really a flame. If the igniter has failed, the burners never light. If the flame sensor is coated with grime, it can't see the flame, so the furnace cuts the gas within seconds to be safe. Either way, the blower may still push out unheated air.
There's not much for you to do here except gather information. Most newer furnaces have a small light that blinks a pattern when something's wrong, often visible through a little window in the lower panel. If you'd have to take a panel off to see it, check your manual first. On many furnaces, pulling the panel cuts power and can clear the code. If you can see the light, tell us what it's doing when you call. We'd rather you didn't clean the flame sensor or replace the igniter yourself, since both sit right next to the gas burners.
The exception is an older furnace with a standing pilot. If the pilot has gone out, the burners can't light, and if the blower runs at all, the air will be cold. The label on the furnace explains how to relight it. If you smell gas near the furnace, don't try. Leave the house and call from outside. If it won't stay lit, call us.
You can also check the fuel. On a gas furnace, make sure the shutoff valve near the unit is open. The handle should run the same way as the pipe. If it's closed and you don't know why, or the gas company has tagged it, leave it closed and call us. Then see if a gas stove or other gas appliance lights. If none of them will, the problem is likely the gas coming into the house, and that's your utility's job.
On an oil furnace, look at the tank gauge first. It's easy to lose track of the level. If there's oil in the tank and the burner has shut itself off, you can press the reset button one time. Skip it completely if you smell oil or see smoke or soot. If it doesn't start and stay running after that one press, leave it alone and call us. If the tank ran dry, fill it first and have a technician restart the burner, because the line often has to be bled. Pressing the reset button again and again can pump unburned oil into the chamber, and that's dangerous when it finally lights.
High-efficiency furnaces can back up
If your furnace is the high-efficiency kind with plastic vent pipes, it makes water while it runs, and that water leaves through a small drain line. When the line clogs, a safety switch can shut the burners down. Water around the base of the furnace is a good clue. If the plastic pipes end outside, make sure leaves, snow or a nest aren't blocking them. Tell us when you call so the technician knows where to look first.
If only some rooms are cold
When the far bedroom blows cold and the rest of the house is fine, the furnace itself may be okay. Ducts that run through an attic, crawl space or garage can come apart at a joint or leak badly enough that the warm air never makes it to the room. A long run through unheated space can also lose a lot of heat on the way. If you can see the ductwork, look for a section that's fallen away, but leave the rest for a technician. Don't close vents in other rooms to push air toward the cold one. Closing too many can make the furnace overheat.
What's safe for you to do
You can do these yourself:
- Set the fan to Auto and the thermostat to Heat
- Change the filter and give the furnace time to cool
- Clear anything blocking the vents and return grilles
- Make sure the gas shutoff on the pipe is open (leave it alone if it's tagged), or check the oil tank
- Press an oil burner's reset button once, only if the tank didn't run dry and you don't smell oil or see smoke or soot
Apart from relighting a standing pilot by the label's directions, leave the igniter, flame sensor, limit switch, the furnace's own gas valve and anything else inside the burner compartment to a technician. If the furnace still isn't making heat after these checks, set the thermostat to Off and call us on a weekday. If the house is getting cold before then, the last question below has a quick way to protect your pipes.
Frequently Asked Questions
What causes a furnace to blow cold air?
Common causes are a fan set to On instead of Auto, a clogged filter that's made the furnace overheat and shut off its burners, or a problem with the igniter or flame sensor. On high-efficiency furnaces, a clogged drain line can do it too. Oil furnaces may be out of fuel or may have shut themselves off.
Is it normal for a furnace to blow cold air at first?
Yes, briefly. The air in the ducts cooled off while the furnace sat idle. If it's still cold after several minutes of steady running, something's wrong.
Can a dirty filter make a furnace blow cold air?
Yes. A clogged filter restricts airflow and the furnace overheats, so a limit switch shuts off the burners while the blower keeps running. Change the filter and give the furnace 20 to 30 minutes to cool.
Should I turn off a furnace that's blowing cold air?
Yes, if it's still blowing cold after the basic checks. Set the fan to Auto and the thermostat to Off so it stops trying to restart, then call us. If it's freezing outside and the house keeps dropping toward 55, open the cabinet doors under sinks on outside walls and let a faucet drip so the pipes don't freeze.
When to stop and call us
A furnace blowing cold air is often something simple, and the fan setting and filter take about a minute to check. If those are fine and the air's still cold, something inside has probably shut the burners down, and it'll keep doing that until it's fixed. That's the point to bring us in.
Air Temp Service Co has served North Jersey homeowners since 1999, family-owned by Mario and Lori Riccobono. Both NJ licenses are on file (HIC 13VH00127600 and Master HVACR 19HC00254600), and we hold Mitsubishi Diamond Elite and Trane Premier Contractor credentials. We repair and replace furnaces across Passaic, Bergen, Essex, and Morris counties, with upfront pricing and rebate paperwork handled for you.
Call (973) 754-1232 to book a furnace repair, and tell us what the thermostat and that little blinking light are doing.