Furnace Blowing Cold Air in Los Gatos, CA

Furnace Blowing Cold Air in Los Gatos

A furnace that runs but blows cold has a working blower and no heat behind it. The burners didn't light, or they lit and went out, and the fan keeps moving unheated air. This is almost always a single failed part, not a dead system. We find the part, replace it, and confirm the burners hold flame through a full cycle before we leave.

Los Gatos housing varies more than almost anywhere we work, and that shapes the call. Larger hillside customs usually run multi-zone systems, so the complaint here is often 'the upper-level master blows cold while the family room downstairs is fine,' which points the diagnosis straight at one zone's heat source. The microclimate adds to it: a west-facing upper room can read very differently from a shaded lower level on the same afternoon, and the heating loads swing by elevation and orientation. Meanwhile the downtown Victorians and Craftsmans often have no forced air at all, just gravity furnaces or wall heaters, which fail in their own way and call for a different conversation.

The honest version: cold air from a register, even on a complex hillside system, is rarely a dead furnace. Most of the time it's an igniter, a flame sensor, a stuck zone damper, or a thermostat set wrong on one zone. We isolate the affected system and fix the part rather than reaching for a replacement quote.

Common causes

One zone's thermostat set to ON instead of AUTO. On a multi-zone hillside home it's easy to leave one zone's fan on ON, which runs that blower continuously and blows cold air between heat cycles while the other zones behave. We check every thermostat 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 turns brittle until it cracks. When it fails, that zone's burners never light, its blower still runs, and cold air comes out. We test the igniter for continuity, confirm the break, replace it, and watch a full ignition cycle on the affected unit.

Flame sensor fouled by oxide. The flame sensor proves a flame is present so the board keeps the gas on. When it's coated with oxide the board can't confirm flame, so the burners light and shut off within seconds and the blower keeps pushing cold air. Cleaning the sensor usually solves it; we replace it if it's pitted past cleaning.

Stuck zone damper tripping the high-limit. The multi-zone forced-air systems common on Los Gatos hillsides use motorized dampers. A stuck-closed damper starves a furnace of return air, the heat exchanger overheats, the high-limit cuts the burners, and the blower runs on with cold air. We check damper operation and static pressure, then confirm the limit isn't shutting burners down under normal airflow.

Control board not sequencing ignition. The gas furnace behind a zone runs its ignition off a control board, and a drifting or failed board can run 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 feeding sensor is at fault before quoting the board.

Gas supply or valve fault on one unit. No gas to a system means its igniter glows, the burners don't light, and the blower delivers cold air to that zone. We confirm the gas valve opens on the heat call and check supply pressure at the affected unit individually rather than assuming the whole house lost gas.

How we diagnose it

Which zone is affected: we determine whether one zone or the whole house blows cold, which narrows the diagnosis fast on a multi-zone hillside system.
Thermostat fan settings on every stat, set to AUTO, since one zone left on ON is a quick cause to rule out.
A full ignition cycle on the affected unit: igniter glow, gas valve, burner light-off, and flame sensor hold.
Igniter and flame sensor condition, tested and cleaned or replaced as the readings call for.
Zone damper operation and static pressure, to confirm a stuck damper isn't tripping the high-limit and cutting the burners.

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

Furnace Blowing Cold Air in Los Gatos: common questions

Do you cover Los Gatos, including the hillsides, and how fast?

Yes. We service the South Bay including Los Gatos, Saratoga, and the surrounding area from our San Ramon base. The hillside addresses take a little planning, but a furnace blowing cold is a service call we get to quickly. Call (925) 999-4095 for an honest arrival window. Same-day is best effort, not guaranteed.

My downtown Los Gatos house has wall heaters, not a furnace, and it's blowing cold. What now?

Many of the downtown Victorians and Craftsmans have no forced-air system, just gravity furnaces or individual wall and floor heaters, and those fail differently than a central furnace. We service what's there and, if the unit is at the end of its life, we'll lay out options. For these homes that's often a multi-head ductless mini-split, which adds real heat and cooling without tearing into the period architecture.

Only my upper-level master blows cold air in the afternoon. Is the furnace broken?

Probably not. On a multi-zone hillside home the upper level is its own zone, and a west-facing master can swing hard in the afternoon. Cold air from that zone points to its igniter, flame sensor, control board, or a stuck damper, while the lower level stays comfortable. We diagnose that specific zone and put the repair on a written estimate before any work.

Nearby and related

Furnace Blowing Cold Air near Los Gatos: Saratoga · San Jose · Cupertino .

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