Furnace Blowing Cold Air in Atherton

Furnace Blowing Cold Air in Atherton

Atherton's Peninsula climate is mild, with cool winters that rarely freeze, so heat demand here is moderate rather than punishing. The large homes still run their furnaces enough that ignition parts wear out, and when a furnace blows cold air the heat side has failed while the blower keeps pushing. In nearly every case the fix is one part on one system.

Many of these properties carry more than one air handler, with separate equipment serving different parts of the house. So the first thing we settle is which unit is actually producing the cold air. A weak igniter or a flame sensor coated in carbon on one air handler will leave its area cold while the rest of the house stays warm. That is one furnace asking for one part, and we treat it that way.

Where a home is set up for zoning, we also check whether a damper has stuck shut or a control has dropped a heat call. A zoning fault can make a single area blow cold even when the equipment is firing correctly for the rest of the house, so we rule it in or out before touching the furnace itself.

Common causes

Weak hot surface igniter. The igniter no longer reaches full temperature, the burners fail to stay lit, and the blower delivers cold air. We confirm which air handler is affected, watch the ignition cycle, and replace the igniter, generally $200 to $350.

Carboned flame sensor. When the sensor cannot confirm flame, the board shuts the gas as a safety step. We clean the sensor or replace it if it is degraded, $150 to $200, on the specific unit serving the cold area.

Stuck zoning damper. On a zoned home, a damper that fails closed leaves one area blowing room-temperature air while the furnace heats the open zones. We test the dampers and zone controls in order instead of replacing parts on assumption.

Control board dropping the burner circuit. A furnace control board can drift over time and de-energize the burner side while the blower stays on. We confirm the board as the fault through the controls before quoting a replacement, since most board complaints turn out to be wiring or sensors.

Thermostat fan set to ON. A home with several thermostats often has one set to fan ON, so the blower runs constantly and pushes unheated air between cycles. We check every thermostat before touching a component.

High-limit short-cycling on a restricted system. A dirty filter or a closed-down zone restricts airflow, the furnace overheats, and the high-limit shuts the burners while the blower runs to cool the heat exchanger. We find the restriction and verify the limit resets normally.

How we diagnose it

• Determine which air handler and area is producing cold air across the home's heating systems.
• Watch a full ignition sequence on the affected unit and test the igniter and flame sensor.
• Where the home is zoned, test the dampers and zone controls to isolate a stuck zone from an ignition fault.
• Confirm thermostat fan settings on every thermostat.
• Check filters and return airflow, then verify high-limit operation on the affected system.

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

Furnace Blowing Cold Air in Atherton: common questions

Nearby and related

Furnace Blowing Cold Air near Atherton: Menlo Park · Palo Alto .

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