Furnace Blowing Cold Air in Portola Valley

Furnace Blowing Cold Air in Portola Valley

Furnace blowing cold air sounds like a dead system and almost never is. The blower keeps running, moving air through the ducts just fine. What's missing is the heat: the burners lit and then dropped out, or never lit in the first place, and the fan keeps pushing room-temperature air at you. In nine out of ten cases it comes down to one part. We replace it, confirm the burners hold flame through a full cycle, and the furnace is back to normal.

In Portola Valley, the larger custom homes on wooded acreage and the ranch properties toward Skyline often run more than one system, sometimes an original unit serving the main living areas and a newer one added for an expanded wing. So the complaint here is frequently 'this side of the house blows cold while the rest is warm,' and that isolates the problem to one system's heat source. The oak-and-redwood hill air also stays cooler and damper than the valley floor, which can mean a furnace on the shaded side of a property gets harder use than one on an open ranch parcel.

Worth saying directly: cold air from a vent doesn't mean the furnace needs to go. Homeowners get told a furnace blowing cold means a full replacement more often than they should. Most of the time it's an igniter, a flame sensor, or a thermostat set wrong.

Common causes

Thermostat fan switch left on ON instead of AUTO. The simplest cause and worth ruling out first. With the fan on ON, the blower runs continuously between heat cycles too, pushing unheated air the whole time. On a property with more than one thermostat covering different additions or wings, it's easy for one to get left on ON. We check every stat on the property and set fan to AUTO.

Cracked hot surface igniter. The most common failure on a modern furnace. The igniter is a brittle ceramic element that cracks with age and repeated heat cycling, and once it fails the burners never light while the blower keeps running on the call for heat. We test it for continuity, confirm the crack, replace it, and watch a complete ignition cycle to verify the burners catch and hold.

Dirty flame sensor shutting burners down. The flame sensor is what proves to the board that a flame is actually burning. A furnace that runs less often, common on a system serving a rarely used wing or an older addition, tends to build up oxide on that sensor faster relative to its run time. The board loses its flame signal and cuts gas within seconds, so burners light, then die, and the blower keeps moving cold air. Cleaning it usually solves it; a pitted sensor gets replaced.

Overheating limit switch short-cycling the burners. A clogged filter or a closed damper starves the furnace of return air, the heat exchanger overheats, and the high-limit switch trips to protect it, cutting the burners while the blower runs on. We check filter condition, static pressure, and damper position, then confirm the limit isn't tripping on otherwise normal airflow.

Control board not sequencing ignition. A drifting or failed control board can call the blower without firing the burners, or fire them out of sequence. We read the board's fault codes, check the sequence against the wiring diagram, and determine whether the board itself is at fault or whether it's getting bad data from a sensor, before quoting a board replacement.

Gas supply interruption to one system. If a furnace loses gas supply, the igniter glows, the burners never catch, and the blower delivers cold air anyway. We confirm the gas valve opens, check supply pressure at the unit, and verify no shutoff was left partially closed after other work, whether that's the main system or one added later for a separate wing.

How we diagnose it

• Which system is affected: we identify whether it's one wing, one addition, or the whole house blowing cold, which narrows the diagnosis right away on a property with more than one furnace.
• Thermostat fan settings on every stat, set to AUTO, since one left on ON is a quick fix.
• A full ignition cycle: watching the igniter glow, the gas valve open, the burners light, and the flame sensor hold, to see exactly where the sequence fails.
• Flame sensor and igniter condition, tested for continuity and oxide buildup, cleaned or replaced based on what we find.
• Airflow and limit behavior: filter, static pressure, and damper position, to confirm the high-limit isn't tripping on restricted return air.

$99 diagnostic, credited toward the repair. You get a written quote before any work begins.

Furnace Blowing Cold Air in Portola Valley: common questions

Nearby and related

Furnace Blowing Cold Air near Portola Valley: Woodside · Menlo Park · Los Altos Hills .

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