Furnace Blowing Cold Air in Sunnyvale, CA

Furnace Blowing Cold Air in Sunnyvale

When a furnace blows cold air it almost always means the blower is running while the burners are not making heat. Sometimes the burners never light. Sometimes they light and then drop out a few seconds later before the heat exchanger warms up. Both feel the same at the register, and neither one means you need a new furnace. On the postwar ranches that make up a big share of Sunnyvale, the furnace is often the original or a single replacement, and what fails is the cheap stuff: the igniter, the flame sensor, or a thermostat that someone left on the wrong setting.

That last one is worth checking before you call anyone. If your fan is set to ON, the blower runs continuously, including the long stretches between heating cycles when the burners are off. The air coming out during those stretches is room temperature, which reads as cold on a 40-degree morning. Switching back to AUTO fixes it for free. We would rather you try that than pay a diagnostic.

If the thermostat is on AUTO and you are still getting cold air, the burners are not carrying through the cycle. On Sunnyvale homes that have had garage conversions or second-story additions, we also watch for an overheating furnace that trips its high-limit switch, because the original system is often undersized for the bigger conditioned space and the heat exchanger runs hotter than it should. That short-cycles the burners and the blower pushes cold air in between.

Common causes

Cracked or worn hot surface igniter. The most common modern furnace failure. The igniter is a thin element that glows to light the gas. When it cracks or weakens it will not reach ignition temperature, so the burners never light and the blower pushes unheated air. We test it for continuity and resistance, and we can see the crack. Replacement is straightforward and we carry the common igniters.

Dirty flame sensor. The flame sensor confirms the burners actually lit. When it gets coated in carbon it stops sensing the flame, so the control board shuts the gas off as a safety response a few seconds after ignition. You hear the furnace try, then quit, then blow cold. Cleaning the sensor often fixes it. If the rod is pitted we replace it.

Thermostat fan set to ON instead of AUTO. Free fix and worth ruling out first. On ON, the blower runs nonstop and moves room-temperature air during every gap between heat cycles. On AUTO, the blower only runs when the furnace is actually making heat. We check the thermostat setting at the start of every cold-air call.

High-limit switch tripping from overheating. Common on Sunnyvale ranches where an addition or pop-up was added but the furnace was never upsized. Restricted airflow or an undersized system lets the heat exchanger overheat, the limit switch cuts the burners for safety, and the blower keeps running cold. We check the filter, the airflow, and the limit, and we tell you honestly if the real problem is that the system is too small for the square footage.

Gas supply or valve issue. If the gas is not reaching the burners, nothing lights. We confirm the supply, test the gas valve operation, and check that no shutoff was left closed after other work. We carry a CO meter on every gas call and test before we leave.

Control board fault. The board sequences ignition, blower, and safeties. When a relay or trace on the board fails, you can get the blower running without a call for the burners, or an ignition sequence that never completes. We diagnose the board last, after the cheaper wear items are ruled out, because it is the more expensive part and we do not want to swap it on a guess.

How we diagnose it

Thermostat first: confirm fan is on AUTO and the call for heat is actually reaching the furnace.
Watch a full ignition cycle: does the igniter glow, do the burners light, do they stay lit, or do they drop out after a few seconds.
Test the igniter and flame sensor, clean or replace as needed.
Check the filter and airflow, then the high-limit switch, especially on homes with additions where the system may be undersized.
Verify gas supply and valve operation, and run a carbon monoxide test before closing the call.

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

Furnace Blowing Cold Air in Sunnyvale: common questions

Do you actually cover Sunnyvale, or are you a Tri-Valley company?

We are based in San Ramon and we run calls across the Bay Area including the South Bay. Sunnyvale, Mountain View, Santa Clara, and Cupertino are regular territory for us. Same-day is best effort depending on where we are when you call, and we will tell you a real window, not a vague one.

It barely gets cold in Sunnyvale. Is a cold-air furnace problem worth fixing now or should I wait?

Sunnyvale winters are mild, with overnight lows usually in the low-to-mid 40s, so a marginal furnace can limp along longer than it would inland. The catch is that a dying igniter or a dirty flame sensor does not heal. It gets worse and tends to quit on the coldest morning. The diagnostic is $75, credited toward any repair over $200, so it is cheap to find out which part it is before you are without heat.

The furnace turns on but the air stays cold. What does that mean?

It almost always means the blower is running but the burners are not making heat, either because they never lit or they lit and dropped out. The usual culprits are a worn igniter, a dirty flame sensor, or a thermostat fan stuck on ON. None of those is a dead furnace. Check the fan setting yourself first, and if it is already on AUTO, the burners are the next thing we look at.

Nearby and related

Furnace Blowing Cold Air near Sunnyvale: Mountain View · Santa Clara · Cupertino .

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