Furnace Blowing Cold Air in Los Altos, CA

Furnace Blowing Cold Air in Los Altos

A furnace that runs but blows cold has a healthy blower and no heat behind it. The burners didn't light, or they lit and dropped out, so the fan circulates unheated air. This is almost always a single failed part rather than a dead system. We isolate it, replace it, and confirm the burners hold flame through a complete cycle before we finish.

Los Altos housing shapes the call. The town is large-lot 1950s-through-80s ranches, and the bigger footprints push a lot of these homes onto dual-zone systems by default. So when cold air shows up, it's often on one zone while the other heats fine, which tells us the heat source on that one system is the problem and narrows things quickly. The marine-influenced climate zone 4 here is mild, with a roughly 36-degree heating design, so furnaces don't log heavy hours. A unit that sits idle through a mild winter is the one most likely to greet the first cold morning with an oxidized flame sensor or a cracked igniter.

This town also has a lot of additions and second-story pop-ups, and homeowners who've grown their homes in place over decades. That sometimes leaves one room or wing always running cold, which can look like a furnace blowing cold but is really an airflow or sizing issue we'll diagnose separately. Either way, true cold-air-from-the-vents is usually one part, not a reason to replace the system.

Common causes

Thermostat fan on ON, not AUTO. With the fan on ON the blower runs continuously, including between heat cycles, so it pushes room-temperature air. On a dual-zone Los Altos home it's easy to leave one zone's fan on ON by mistake. We check both thermostats and set fan to AUTO so each blower runs only when its burners are firing.

Cracked hot surface igniter. The 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 that zone blows cold. We test the igniter for continuity, confirm the crack, replace it, and run a full ignition cycle to verify the burners catch and hold.

Flame sensor oxidized on a mild-climate furnace. In the mild climate here the furnace runs few hours, and the flame sensor builds an oxide film while idle. The board then can't confirm flame, the burners light and shut off within seconds, and the blower keeps moving cold air. Cleaning the sensor fixes most of these; we replace it if it's pitted.

High-limit short-cycling on restricted airflow. A clogged filter or a closed register starves the furnace of return air, the heat exchanger overheats, and the high-limit cuts the burners while the blower runs on. The dual-zone systems common here are especially sensitive to a closed-down zone restricting airflow. We check filters, measure static pressure, and confirm the limit isn't tripping the burners under normal airflow.

Control board not sequencing ignition. The variable-speed dual-zone equipment common in Los Altos runs off a control board that can drift or fail, calling the blower without firing the burners. We read the fault codes, check the ignition sequence against the wiring diagram, and isolate whether the board or a sensor is at fault before quoting the board.

Gas valve or supply 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 heat call and check supply pressure at the furnace, including making sure a shutoff wasn't left partly closed after other work.

How we diagnose it

Thermostat fan setting on both zones, set to AUTO, as the fastest cause to rule out.
Whether one zone or both blow cold, which immediately narrows the diagnosis on a dual-zone system.
A full ignition cycle: igniter glow, gas valve, burner light-off, and flame sensor hold, to see where the heat drops out.
Flame sensor and igniter condition, tested and cleaned or replaced, since both fail more readily on lightly-used mild-climate furnaces.
Filter, static pressure, and zone damper position, to confirm restricted airflow isn't tripping the high-limit.

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

Furnace Blowing Cold Air in Los Altos: common questions

How fast can you get to Los Altos for a furnace blowing cold air?

We run the South Bay including Los Altos, Mountain View, and Cupertino daily from our San Ramon base. A furnace blowing cold is a service call we prioritize on cold mornings. Call (925) 999-4095 and we'll give you an honest arrival window. Same-day is best effort, not guaranteed.

One room has always run cold. Is that the same as my furnace blowing cold air?

Usually not, and it's a common Los Altos situation because so many homes here grew through additions and pop-ups without the HVAC being resized. A single room that's always cold points to airflow or undersizing on that part of the house, which we diagnose by re-running the load on your current floor plan. A furnace blowing cold from every vent is a different problem, normally an igniter or flame sensor.

My furnace blew cold on the first cold morning of the year. What failed?

Most likely the flame sensor or the igniter. In this mild climate the furnace sits idle for long stretches, the sensor oxidizes, and the igniter weakens, so the first real cold-start is when one of them gives out. The repair is usually a sensor cleaning or an igniter, and we put it on a written estimate before any work.

Nearby and related

Furnace Blowing Cold Air near Los Altos: Palo Alto · Mountain View · Cupertino .

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