The short answer
A Lennox furnace in Northern Virginia blowing cold air usually has the thermostat fan set to ON instead of AUTO, burners that won’t light, or a limit switch that shut the burners off while the blower keeps running. On a heat pump, cool air for a few minutes during defrost is normal. Check the fan setting and the filter first, then look for a code on the furnace or the thermostat.
If you smell gas, skip all of it. Leave the house and call the gas company and 911 from outside. If a carbon monoxide alarm is going off, get everyone out first (what to do when the CO alarm sounds).
1. The fan is set to ON
ON runs the blower all the time. Between heat cycles it’s moving room-temperature air, and room-temperature air on your hand feels cold. Set the fan to AUTO and the blower only runs when there’s heat behind it.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
2. The burners aren’t lighting
With the fan on ON and a furnace that’s locked out on ignition, you get air and no heat. Look for E270 on the display, or a no-ignition count on a Merit’s red light. Lennox furnace not igniting walks through what each failure looks like.
3. The limit tripped
When the furnace gets too hot inside, the limit switch shuts the burners off and the blower keeps running to cool it down. You feel lukewarm air, then cool air. A clogged filter is the most common cause, then closed or blocked vents. The code is E250, and Lennox turns it into a one-hour lockout if the limit stays open 3 minutes.
4. The flame keeps dropping out
Burners light, run a few seconds, go out, and try again. You get short puffs of warm air between cool ones. That’s almost always a coated flame sensor: E240 or E273.
5. It’s a heat pump, and it’s defrosting
Lennox’s heat pump manual says frost builds on the outdoor coil when it’s under 45°F outside, and the unit runs a defrost cycle of 5 to 15 minutes to melt it. Steam coming off the outdoor unit during defrost is normal. The thermostat may bring on backup heat while it does. Heat pump air is warm rather than hot, so it can feel cool against your hand even when it’s working. More on aux heat vs emergency heat and a heat pump covered in ice.
6. The fan runs on its own (E150)
Newer Lennox systems on R-454B refrigerant have a leak sensor indoors. If it detects refrigerant, the indoor fan comes on to clear it, and the compressor and any electric heat shut off. So the fan blows and nothing heats. The thermostat shows E150. Call; it can’t clear while the sensor still sees a leak.
What to check before you call
- Thermostat on Heat, fan on AUTO, set a few degrees warmer than the room.
- The filter. If you can’t see light through it, change it (which filter to buy).
- Supply vents open and the big return grille not blocked by furniture.
- Furnace switch on, breaker on, blower door on and seated.
- Any code on the furnace or the thermostat: photo it, then reset once (how to reset a Lennox furnace).
- On a heat pump: watch the outdoor unit. Steam and a quiet fan means defrost. Give it 15 minutes.
What it usually turns out to be
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| The cause | What you’d notice | Our price |
|---|---|---|
| Clogged filter tripping the limit | Lukewarm, then cool air, E250 | Often just a filter |
| Dirty flame sensor | Burners light and drop out | $95 to $180 |
| Cracked igniter | No flame at all, E270 | $240 to $410 |
| Blower wheel packed with dust | Weak airflow, limit trips | $300 to $440 |
| Heat pump defrost board | Outdoor coil iced solid | $450 to $1,150 |
| Heat pump contactor | Outdoor unit silent, backup heat doing all the work | $290 to $425 |
| Thermostat set up wrong for the heat pump | Backup heat at odd times, cold air at others | $300 to $640 |
The diagnostic is a flat $99, and the price comes in writing before we start. HVAC blowing cold air in winter covers every brand.
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade. A packed blower wheel is one of the quiet reasons a furnace trips its limit and blows cool air.
If the repair is a big one on an older furnace, get a second look before you spend it. Our free second opinion costs you nothing, and the estimator shows what a new system costs with $1,500 off already in the price. Thinking about brands? Lennox vs Trane lays out the difference.
Every Lennox code in one place: Lennox error codes. Other brands too: furnace and AC error codes.
The system we put in first
Built to take a beating. Put in by our crew.
Heat lamps, snow and rain on one outdoor unit: that’s Trane’s own film. We’re a Trane Comfort Specialist, on Trane’s own dealer locator, and when you pick a Trane, our crew puts it in.
Films: Trane’s own.
Call or book a Lennox repair
Call (703) 997-9757. We answer day or night. Or book online and pick a time. The diagnostic is a flat $99, and you see the repair price in writing before we touch anything. Furnace running fine and you want to keep it that way? Book the $67 heating check from goswiftpro.com/cookiesoffer (book by November 30) and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276. More on Lennox repair in Northern Virginia.
Questions people ask
Why is my Lennox furnace blowing cold air?
Check the thermostat fan setting first: ON runs the blower between heat cycles. If it’s on AUTO, the burners aren’t lighting or a limit switch shut them off, often from a clogged filter.
Why does my Lennox heat pump blow cold air in winter?
It’s probably defrosting. Under 45°F, frost builds on the outdoor coil, and Lennox runs a defrost cycle of 5 to 15 minutes. Steam from the outdoor unit is normal. If it lasts longer, call.
Why is my Lennox furnace fan running but there’s no heat?
A limit trip (E250), an ignition lockout (E270), or a refrigerant leak alert on newer R-454B systems (E150), which runs the fan on purpose. Photo the code.
Can a dirty filter make my furnace blow cold air?
Yes. A choked filter overheats the furnace, the limit switch shuts the burners off, and the blower keeps running. Change the filter, and if the code was E250, give it an hour or reset it once.
