Furnace Blowing Cold Air in Alamo

Furnace Blowing Cold Air in Alamo

Alamo gets genuinely cold winter nights for the Bay Area, and the larger homes here lean hard on their furnaces. When a furnace runs but blows cold air, the heat side has failed while the blower keeps working. On most homes that is one fixable part. It is worth saying up front that a number of Alamo properties run more than one furnace, so cold air in one part of the house and warm air in another is a single-system problem, not a dead house.

The usual culprits are a worn hot surface igniter or a dirty flame sensor that keeps the burners from staying lit. The board lights the burners, loses flame confirmation, then shuts the gas while the blower keeps pushing unheated air. On the long duct runs common in the bigger Alamo floor plans, that cold air shows up at the far registers first.

Because many Alamo systems are multi-zone, we also look at whether a zone damper is stuck or a zone board has dropped a call for heat. That can leave one zone blowing cold even when the furnace itself is firing fine for the other zone.

Common causes

Worn hot surface igniter. The igniter weakens and no longer lights the burners every cycle, so they drop out and the blower delivers cold air. On a dual-furnace Alamo home we confirm which unit is at fault first, then test and replace the igniter, typically $200 to $350.

Dirty flame sensor. Carbon on the sensor rod stops the board from confirming flame, so it shuts off the gas for safety. Cleaning usually restores it; a degraded sensor gets replaced for $150 to $200. We check this on the specific furnace serving the cold zone.

Stuck zone damper or zone board fault. On Alamo's multi-zone systems, a damper that fails closed or a zone board that drops a heat call leaves one wing blowing room-temperature air while the furnace heats the other zone fine. We run the zone control diagnostics in sequence instead of replacing boards blindly.

Thermostat fan set to ON. With multiple thermostats in a large home, it is common to find one set to fan ON, running the blower constantly and pushing unheated air between cycles. We check every thermostat's setting before condemning a part.

Overheating limit short-cycling the burners. A dirty filter or a partly closed zone restricts airflow, the furnace overheats, and the high-limit shuts the burners while the blower keeps running to cool the heat exchanger. We find the restriction and verify the limit cycles correctly.

Gas supply to one unit. On a two-furnace home, a closed valve or supply issue at one unit leaves that half of the house cold while the other runs. We confirm gas is reaching each furnace's valve before chasing electronics.

How we diagnose it

• Identify which furnace and which zone is producing cold air on a multi-system home.
• Watch a full ignition sequence on the affected unit and test the igniter and flame sensor.
• Run the zone control and damper diagnostics in sequence rather than swapping the board.
• Check filters and return airflow on the affected zone, then verify the high-limit operation.
• Confirm every thermostat is set to AUTO and wired correctly.

$75 diagnostic, credited toward any repair over $200. You get a written quote before any work begins.

Furnace Blowing Cold Air in Alamo: common questions

Nearby and related

Furnace Blowing Cold Air near Alamo: Danville · Blackhawk · Lafayette · Walnut Creek .

This is usually a furnace repair in Alamo job. See our furnace repair overview or the Alamo service area.