Short answer
A furnace blowing cold air usually has a thermostat fan set to On, a dirty filter tripping the high-limit switch, a dirty flame sensor, a failed igniter or, on high-efficiency units, a blocked condensate drain or vent pipe. Check the thermostat, filter and vents; call for service if the burners won't stay lit.
Rather have a licensed tech look at it?
Call (702) 602-9362Quick checks
Start with the simple causes.
- Thermostat set to Heat and fan set to Auto
- New batteries in the thermostat
- Clean filter
- Furnace door fully closed (it presses a safety switch)
- Outdoor PVC vent pipes clear of snow and ice
Watch the startup
A normal cycle goes: inducer fan starts, igniter glows or sparks, burners light, then the blower starts. If the burners light and go out after a few seconds, the flame sensor is often dirty. If nothing lights, the igniter, gas supply or pressure switch may be the problem.
Frozen condensate lines in Northern Nevada
High-efficiency furnaces in Reno and Carson City garages and attics can lock out when their condensate line freezes. The fix is thawing and protecting the line with insulation or heat tape.
Safety
If a carbon monoxide alarm sounds or you smell gas, leave the home and call 911 or Southwest Gas from outside. Don't reset the furnace repeatedly after a lockout.
Questions
Why does my furnace blow cold air at the start of a cycle?
Some furnaces run the blower briefly before the heat exchanger warms up. Cold air that lasts more than a minute or two points to an ignition or flame problem.
Can I clean a flame sensor myself?
It's a common fix, but it involves working inside the furnace near gas components. If you're not comfortable turning off power and gas safely, have a technician do it.
This guide is general information for Nevada homeowners. For anything involving gas, refrigerant or electrical parts, use a licensed HVAC contractor.