Furnace Blowing Cold Air in San Leandro

Furnace Blowing Cold Air in San Leandro

San Leandro is largely post-war single-family across Marina Faire, Estudillo Estates, and the central neighborhoods. The climate is mild, summer highs in the 75 to 85 range near the bay, so both heating and cooling matter but neither is extreme. The furnaces here are usually a home's second or third system, and they sit on top of ductwork that is frequently original and well past its useful life. That last detail changes the cold-air diagnosis.

When the furnace runs and the air feels cold, the cause is usually one part. A flame sensor that has fouled, an igniter that has failed, or burners short-cycling on a limit. We diagnose those the same way everywhere. But in San Leandro we also check whether the furnace is heating fine and the heat is leaking out of old ducts in the crawl space before it reaches the rooms. Air that is warm at the furnace and cool at the far register is a duct story, not a furnace story.

Either way, 'runs but blows cold' is a fixable condition, not a dead system. We figure out which of those two things is actually happening before we quote anything.

Common causes

Fouled flame sensor. The most common cause across all the post-war stock here. Carbon film on the sensor makes the board read no flame and cut gas right after light-off, so the blower runs cold. We clean it, check the signal, and only replace if it will not hold. Quick fix on most older units.

Failed hot surface igniter. On the newer furnaces installed in these older homes, the igniter element cracks and stops glowing. No light, blower cycles cold. We ohm-test it and replace, roughly $200 to $350, with the price on the estimate first.

Leaky original ductwork bleeding off the heat. Specific to San Leandro's aging duct stock. The furnace heats fine, but old, unsealed ducts dump warm air into the crawl space and deliver cool air to the rooms. We measure supply-air temperature at the furnace versus the far registers. If the gap is large, sealing or replacement is the real fix, not a furnace part.

Thermostat fan set to ON. Fan ON runs the blower nonstop, so you feel cool air between heat cycles. We confirm the setting before anything else. It is free to fix and we would rather catch it than charge you for a repair you do not need.

Limit short-cycling on a clogged filter. Restricted airflow overheats the furnace, the high-limit trips the burners, and the blower keeps running cold to cool the heat exchanger. We check filter, blower wheel, and airflow before blaming the switch. On old systems this is a frequent culprit.

How we diagnose it

• Confirm thermostat call and fan setting before touching hardware.
• Run a full ignition cycle and note where it drops: igniter, light-off, flame sense, or hold.
• Clean and test the flame sensor, ohm-check the igniter, verify gas valve operation.
• Measure supply-air temperature at the furnace versus the far registers to separate a furnace fault from duct loss.
• On older units, test the high-limit and check airflow restriction from filter or duct condition.

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

Furnace Blowing Cold Air in San Leandro: common questions

Nearby and related

Furnace Blowing Cold Air near San Leandro: Oakland · Hayward · Castro Valley .

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