Furnace Blowing Cold Air in Los Altos Hills

Furnace Blowing Cold Air in Los Altos Hills

When a furnace runs but blows cold, the blower is working and the heat is missing. The burners failed to light, or lit and went out, and now the fan pushes unheated air through the ducts. This is almost always a single failed part. We find it, replace it, and confirm the burners hold flame through a full cycle before we leave.

Los Altos Hills changes how this complaint reads. These are one-acre-minimum estates with large floor plans, and a single system rarely covers the house evenly, so most homes run multiple air handlers and zoning. That means the call here is often 'the east wing blows cold while the rest of the house is warm,' which immediately tells us the problem is isolated to one system's heat source. The long duct and line-set runs and the way the equipment is tucked around these steep, wooded lots also mean we plan our access before we start. The foothill climate runs warmer than the bayside towns, so these furnaces aren't worked as hard as an inland system, and a unit that sits idle is the one most likely to have an oxidized flame sensor or a cracked igniter when you finally call for heat.

Worth saying plainly: cold air from a register, even in a big estate, is rarely a dead system. Most of the time it's an igniter, a flame sensor, or a thermostat set wrong on one zone. We isolate the affected system and fix the part.

Common causes

One zone's thermostat set to ON instead of AUTO. On a multi-zone estate it's easy to leave one zone's fan switched to ON, which runs that blower continuously and pushes cold air between heat cycles while the other zones behave normally. We check every thermostat in the house and set fan to AUTO so each blower only runs when its burners are firing.

Cracked hot surface igniter on one system. The igniter glows to light the burners and grows brittle until it cracks. When it fails on one of the home's systems, that zone's burners never light, its blower still runs, and that wing blows cold. We test the igniter, confirm the break, replace it, and watch a full ignition cycle on the affected unit.

Flame sensor oxidized on a lightly-used furnace. In the mild foothill climate these furnaces run fewer hours, and the flame sensor builds oxide while sitting idle. The board then can't confirm flame, so the burners light and shut off within seconds and the blower keeps moving cold air. Cleaning the sensor usually fixes it; we replace it if it's pitted past cleaning.

Stuck zone damper tripping the high-limit. Zoned forced-air systems use motorized dampers, and 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 position and operation, confirm static pressure, and verify the limit isn't shutting the burners down under normal airflow.

Control board not sequencing one zone's ignition. The gas furnace serving a zone runs its ignition off a control board. When that board drifts or fails it can run the blower without firing the burners, or fire them out of order. We read the fault codes, check the ignition sequence against the wiring diagram, and confirm whether the fault is the board or a sensor feeding it bad data before quoting the board.

Gas supply interruption to one unit. If one system loses gas, its igniter glows, the burners don't light, and the blower delivers cold air to that wing. We confirm the gas valve opens and check supply pressure at the affected unit individually rather than assuming the whole estate lost supply.

How we diagnose it

• Which system is affected: we identify whether one zone or the whole house blows cold, which narrows the diagnosis fast on a multi-air-handler estate.
• Thermostat fan settings on every stat, set to AUTO, since one zone left on ON is a quick fix.
• 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, since both fail more readily on lightly-used foothill furnaces.
• Zone damper position 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 Altos Hills: common questions

Nearby and related

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

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