Vent, Valve & Volt
Furnace blowing cold air instead of heat

You can fix this

Likely cause: the fan is set to On

With the fan on On, the blower runs even when the furnace isn't heating, so you feel cool air between cycles. Set it to Auto.

Why this happens

Carrier lists the thermostat and fan mode first among the causes of a furnace blowing cold air. On the On setting, the blower keeps moving air through the house whether or not the burners are lit. Lennox and Trane also list the thermostat as a cause.

How to fix it

  1. Set the mode to Heat and the fan to Auto.
  2. Set the temperature a few degrees above the current room temperature.
  3. If the thermostat runs on batteries and the screen looks dim, replace them.
  4. Wait for the furnace to start, give it a minute, then feel the air at a vent.

If this does not fix it, tell the HVAC technician

  • What the thermostat was set to.
  • Whether the air warmed up once the fan was on Auto.
  • The thermostat brand and model.

Build the full list: what to tell the HVAC technician.

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Carrier: Furnace blowing cold air
  2. Lennox: How to fix a furnace blowing cold air
  3. Trane: Furnace not blowing hot air

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