Furnace Blowing Cold Air in Lafayette, CA

Furnace Blowing Cold Air in Lafayette

When a furnace runs but blows cold, the blower is fine. It's the heat that dropped out. The burners either failed to light or lit and went out, and now the fan is moving unheated air through the house. The good news is that this almost always comes down to one replaceable part. We find which part, replace it, and watch the furnace hold flame through a full cycle before we leave.

Lafayette's housing pushes the odds toward certain causes. The town runs heavily to 1950s through 80s hillside customs, and a lot of those gas furnaces are past the 18 to 20 year mark. Older systems mean older igniters and flame sensors, the two parts most likely to leave you with cold air. The upper hillsides also drop below freezing on the coldest nights, which is exactly when an aging furnace gets pushed hard and a marginal igniter finally cracks. Tight crawl spaces here, the same ones that make condensate clogs common, also collect dust that fouls a flame sensor faster than you'd expect.

It's worth being direct: cold air from the vents on a 20-year-old furnace makes people assume the system is finished. Usually it isn't. A flame sensor cleaning or an igniter gets it running. The replace-versus-repair conversation only comes up if we find a cracked heat exchanger, and we show you that on camera before we say a word about a new system.

Common causes

Cracked hot surface igniter on an aging furnace. The most common failure on the older furnaces common in Lafayette. The ceramic igniter glows to light the burners and grows brittle with age until it cracks. When it fails the burners never light, the blower runs anyway, and cold air comes out. We test it for continuity, confirm the break, replace it, and run a full ignition cycle to be sure the burners catch.

Flame sensor fouled in a tight crawl space. The flame sensor confirms a flame is burning so the board keeps the gas on. Dust and oxide, both plentiful in the cramped hillside crawl spaces these systems sit in, coat the sensor and the board can no longer read flame. Burners light for a few seconds, then shut off, and the blower keeps pushing cold air. Cleaning the sensor usually solves it; we replace it if it's pitted.

Thermostat fan set to ON, not AUTO. If the fan switch is on ON, the blower runs around the clock, including the long gaps between heat cycles, so it blows room-temperature air. People often mistake this for a broken furnace. We confirm the fan is set to AUTO so the blower only runs when the burners are actually firing.

High-limit tripping on restricted airflow. A dirty filter or a closed register starves the furnace of return air, the heat exchanger overheats, and the high-limit switch cuts the burners while the blower runs on. The result is intermittent cold air. We check the filter, measure static pressure, and confirm the limit isn't shutting the burners down under normal airflow.

Heat exchanger crack on a 20-year furnace. On furnaces approaching the 18-to-20-year window common here, a cracked heat exchanger can cause the burners to behave erratically and is a carbon monoxide safety issue. We inspect it on camera. If it's cracked we show you the footage, shut the system down, and document it. That's the one case where cold air points toward replacement, and we run the numbers honestly.

Gas supply or valve fault. No gas to the burners means the igniter glows, nothing lights, and the blower delivers cold air. We confirm the gas valve opens on the call for heat and check supply pressure at the unit, including making sure a shutoff wasn't left partly closed after unrelated work on the hillside.

How we diagnose it

Thermostat fan setting first, set to AUTO, since a fan stuck on ON is the quickest cause to rule out.
A full ignition cycle: igniter glow, gas valve, burner light-off, and flame sensor hold, to see exactly where the heat is dropping out.
Igniter and flame sensor condition, tested and cleaned or replaced, since these are the top failures on the older furnaces here.
Filter and static pressure in the crawl space, to confirm restricted airflow isn't tripping the high-limit and cutting the burners.
Heat exchanger inspection on furnaces past 18 years, on camera, so a crack is documented before any replacement conversation.

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

Furnace Blowing Cold Air in Lafayette: common questions

Do you cover the Lafayette hillsides, and how fast?

Yes. We're a San Ramon company and run the Diablo Valley, including Lafayette, every day. Hillside access takes us a little longer to scope on installs, but a furnace blowing cold air is a service call we get to quickly. Call (925) 999-4095 and we'll give you an honest window. Same-day is best effort.

My furnace is about 20 years old and blowing cold. Is it cheaper to just replace it?

Not usually, at least not for this symptom. A cold-air complaint on a 20-year furnace is most often an igniter or a flame sensor, a few hundred dollars, not a new system. The exception is a cracked heat exchanger, which we inspect on camera. If we find one we show you and explain why replacement makes sense. Otherwise we fix the part and you keep the furnace.

It's below freezing tonight and my furnace blows cold. Should I keep it running?

If the burners aren't lighting, running the blower just circulates cold air and wastes energy, so set the fan to AUTO and use space heaters safely until we get there. If you smell gas, shut the system off and call us. On the cold upper-hillside nights we prioritize no-heat calls.

Nearby and related

Furnace Blowing Cold Air near Lafayette: Orinda · Moraga · Walnut Creek · Alamo .

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