Furnace Blowing Cold Air in Cupertino

Furnace Blowing Cold Air in Cupertino

Cold air from a furnace that still hums along means the blower is fine and the heat side is not. We see three causes more than any other. A hot-surface igniter that glows but no longer lights the gas. A flame sensor with enough carbon on the rod that the burners light and quit a few seconds later. Or a thermostat fan left on ON, so the blower keeps circulating room-air between burner cycles.

Cupertino sits in a mild winter zone, so furnaces here do not run hard. They run in short, frequent cycles on cool mornings, and that start-stop pattern is what wears igniters and lets flame sensors build up the carbon film that causes nuisance shutdowns. A lot of the housing stock is mid-century single-story ranches now on their second furnace, and the equipment installed in the 2000s is reaching the age where these small parts start failing.

Cold air from a running furnace is usually a $150 to $350 part, not a reason to replace the system. We diagnose it on a live ignition cycle and put the finding in writing before any work.

Common causes

Cracked hot-surface igniter. The silicon-carbide element glows but no longer lights the gas, so the blower runs and delivers cold air. We check continuity and resistance and look for a crack. Replacement is roughly $200 to $350 depending on the furnace.

Carbon-fouled flame sensor. Burners light, then shut off within seconds because the sensor cannot confirm flame. A thin oxide film is the common cause. We clean the rod and read the microamp signal; if it stays weak we replace the sensor, $150 to $200.

Thermostat fan on ON, not AUTO. On Nest and ecobee thermostats common in updated Cupertino homes, the fan can get left on ON, which runs the blower nonstop and circulates cool air between burner cycles. We confirm it at the thermostat and switch to AUTO.

High-limit trip from restricted airflow. Old return ducts or a clogged filter let the heat exchanger overheat, so the limit switch trips the burners while the blower keeps running. We check filter, static pressure, and the limit, then fix the airflow restriction, which in older ranches is sometimes the undersized return itself.

Weak or low gas supply. If burners light inconsistently, we put a manometer on the gas valve and check inlet and manifold pressure. Older homes occasionally have a partially closed valve or a regulator issue we trace back from the furnace.

Ignition control board fault. On furnaces past 15 years the control board can fail to call for gas or drop the burner mid-cycle. We read the diagnostic flash code and verify voltages before quoting a board, because it is the wrong part to replace on a guess.

How we diagnose it

• Run a complete ignition cycle and watch whether the burner lights and stays lit or drops out after a few seconds.
• Pull the board flash code to separate igniter, flame sensor, limit, and board faults.
• Test igniter continuity and read the flame sensor microamp signal under flame.
• Inspect the filter and measure static pressure, which matters on older ranch ductwork prone to restriction.
• Verify thermostat fan is on AUTO and check gas pressure at the valve.

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

Furnace Blowing Cold Air in Cupertino: common questions

Nearby and related

Furnace Blowing Cold Air near Cupertino: Sunnyvale · Saratoga · Los Altos .

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