Why is my furnace blowing cold air?

Carrier's Comfort 59SC5B gas furnace is certified for a temperature rise of 40 to 70°F. That means the air leaving the furnace should be 40 to 70 degrees warmer than the air going in. Carrier describes typical gas-furnace supply air as roughly 120 to 140°F. If your registers feel cool, an ordinary probe thermometer will narrow down the problem before anyone bills you a service fee.

First, make sure it's actually a furnace problem

Four normal behaviors account for a lot of "cold air" service calls, and none of them needs a part.

The fan is set to On. Carrier calls this the most common cause. With the fan on, the blower runs all the time and pushes room-temperature air between burner cycles. Trane gives the same fix: switch it to Auto so the furnace controls the fan. Also check that the mode says Heat, not Cool.

It's the first minute of the cycle. Carrier says some furnaces delay the blower about 30 to 60 seconds after the burners light, so the heat exchanger can warm up first. Lennox's SL280DFV runs on a fixed 30-second blower-on delay. Air that blows cool for a moment at startup is just the air that was sitting in the ducts.

It's the end of the cycle. After the thermostat is satisfied, the blower keeps running to pull leftover heat out of the heat exchanger. Lennox ships that furnace with a 90-second blower-off delay and tells installers to tune it so the supply air is 90 to 110°F when the blower stops. Lukewarm air right before the fan shuts off is the system working as designed.

You have a heat pump, not a furnace. Carrier puts heat-pump supply air at about 85 to 100°F, which feels cool on your hand compared with furnace air. If the outdoor unit is also coated in ice, our frozen heat pump guide covers the defrost cycle.

Measure the rise

Set the thermostat to Heat, a few degrees above room temperature, with the fan on Auto. Wait until the burners have been lit for about five minutes. Then hold a probe thermometer in the supply register closest to the furnace and write down the reading. Next, take a reading at the return grille, where air is pulled back to the furnace. Subtract the return reading from the supply reading.

It's rougher than a technician's plenum reading but close enough to sort the problem. Compare it with the certified range on your furnace's own rating plate, usually inside the blower door.

Trane also puts a typical service fee at $50 to $200 and labor at $75 to $150 an hour. Both come on top of the part costs in the table.

Zero rise: the burners never lit

Listen: inducer hum, ignitor glow or click, then the burners' soft whoosh. No whoosh means the sequence is breaking somewhere.

Start with the fuel, since that's the one cause you can check without opening anything. Trane says to make sure the furnace gas valve is open on a natural gas furnace, and that the tank isn't empty on a propane or oil furnace.

On a high-efficiency condensing furnace, also look at the condensate drain line. Trane notes that a clogged drain triggers a safety switch that keeps the burners from lighting, while the blower still pushes cool air through the registers. ENERGY STAR's maintenance checklist lists the condensate drain as something a tune-up should check. A clog that's slow to drain or backing up is a normal service-call fix. If water is pooling around the furnace, shut it off at the thermostat before you call.

This is where you stop. The ignitor, gas valve, and control board all sit in the burner compartment. If the gas valve is open and the burners still won't light, that's a technician's job. If the burners light for a few seconds and then go out, the flame sensor is the usual suspect. That's a different symptom, and our short-cycling guide explains how to time it.

Warm, then cold: the limit switch tripped

Carrier explains the high-limit switch this way: when the heat exchanger gets too hot, the switch cuts power to the gas valve and the burners go out. The blower keeps running to cool things down, so from your couch it feels like the furnace is blowing cold air. Trane adds that repeated overheating from a dirty filter can eventually wear out the switch, leaving the furnace blowing cool air without ever burning fuel.

So start with the filter. Put in a fresh filter matching the equipment's specified size and rating, open any closed supply registers, and clear furniture away from the return grille. Then measure again over a full cycle.

Flat rule: if the rise drops off mid-cycle on a clean filter with open registers, book the service call. Don't try a third filter brand. At that point the switch, the blower, or the duct system is the problem, and Carrier puts a professional switch replacement at $150 to $400. Our filter habit guide explains how a clogged filter turns into a much bigger repair bill.

Good rise, cold room: look at the ducts

If the register nearest the furnace reads well above the return but a bedroom at the far end of the house blows cool air, the furnace is fine. The heat is getting lost on the way. ENERGY STAR estimates that 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. Ducts running through an unheated attic or crawlspace lose the most heat.

A disconnected flex duct in the attic can make one room feel like the furnace failed. Getting into an attic or crawlspace is a fall and tight-space hazard, so if the run isn't visible from a safe floor, have a contractor inspect it. Ask them to seal the joints with mastic or foil tape. ENERGY STAR says never to use ordinary duct tape for this, because it does not last.

Flat rule: if the temperature rise at the furnace is inside the nameplate range, you probably don't need a new furnace. That reading shows the furnace is producing heat. Fix where the heat is going before you pay for more heating capacity. If you're comparing bids anyway, our HVAC quote guide shows what a real one should list.

When cold air is a safety call

Cold air on its own isn't an emergency. Cold air along with a rotten-egg smell is. Leave the house and call your gas utility from outside. Do the same if your carbon monoxide alarm sounds. EPA recommends a CO alarm certified to UL 2034 in the hallway near every separate sleeping area, and treats it as a backup to proper maintenance of fuel-burning appliances, not a replacement for it. ENERGY STAR's checklist has a tune-up check gas pressure, burner combustion, and the heat exchanger.

When you book the call, tell the dispatcher your fan setting, your supply and return readings, and whether the rise ever showed up at all. "Zero rise, no burner sound, gas valve open" sends a technician to the ignition sequence first.

Research sources

• Carrier: Why Is My Furnace Blowing Cold Air?
• Carrier: What Is a Furnace Limit Switch?
• Carrier: Heat Pump Not Heating? Troubleshooting Cold Air Issues
• Carrier: 59SC5B Comfort Gas Furnace Product Data
• Lennox: SL280DFV Installation Instructions (508246-01)
• Trane: Furnace Not Blowing Hot Air
• Trane: Average Furnace Repair Cost
• ENERGY STAR: Duct Sealing
• ENERGY STAR: Heating and Cooling Maintenance Checklist
• EPA: What About Carbon Monoxide Detectors?