Furnace Blowing Cold Air in Danville

Furnace Blowing Cold Air in Danville

Cold air from a furnace that still runs tells us the blower works and the heat side does not. Most of the time the cause is a cracked hot-surface igniter, a flame sensor fouled with carbon so the burners light and then quit, or a thermostat fan set to ON instead of AUTO, which runs the blower between cycles and moves unheated air. None of those is a furnace replacement.

Danville's housing splits two ways, and so does the diagnosis. The 1960s and 70s ranches off Diablo Road and the older Danville Boulevard tracts often still run original or near-original gas furnaces past 25 years, where igniters, flame sensors, and limit switches are all near the end of their service life. The Blackhawk and East Danville side runs newer multi-zone systems on Carrier Infinity and similar zoned controls, and on those a cold-air complaint can come from a zone damper or a control board rather than the burners.

Danville winters are cool but mild, with January overnight lows commonly in the high 30s, so furnaces cycle short and often. That is hard on igniters and flame sensors. We watch a full ignition cycle, read the control, and tell you which part it is before we quote.

Common causes

Cracked hot-surface igniter. Common on the older Diablo Road furnaces. The element glows but will not light the gas, so the blower delivers cold air. We test continuity and resistance and inspect for a crack, then replace it, about $200 to $350.

Carbon-coated flame sensor. Burners light and shut off within seconds because the board cannot prove flame. We clean the rod and read the microamp signal; if it is degraded we replace the sensor, $150 to $200. This is the single most common cold-air cause on Danville's aging furnaces.

Thermostat fan on ON instead of AUTO. The blower runs continuously and pushes room-temperature air between heating cycles. We confirm it at the thermostat and switch to AUTO. On the multi-zone Blackhawk systems we also verify the zone panel is not holding the fan on.

Zone damper or control-board fault on multi-zone systems. On the zoned Carrier Infinity and similar systems common in Blackhawk and East Danville, a stuck zone damper or a drifting control board can blow conditioned air to the wrong zone or call the blower without heat. We read the board diagnostics and test damper operation before quoting.

High-limit trip from restricted airflow. A clogged filter or a restricted return makes the heat exchanger overheat and trips the limit, killing the burners while the blower runs. We check filter, static pressure, and the limit switch, then clear the restriction.

Gas valve or supply problem. If burners light inconsistently we put a manometer on the valve and check inlet pressure. Older Danville furnaces occasionally need a gas valve, which we confirm by reading before replacing.

How we diagnose it

• Run a full ignition sequence and watch whether the burner stays lit or drops after proving flame fails.
• On multi-zone Blackhawk and East Danville systems, pull the zone-panel and furnace board diagnostics and test damper operation.
• Test igniter continuity and read the flame sensor microamp signal under flame.
• Inspect filter and measure static pressure to rule out an airflow-driven limit trip.
• Confirm thermostat fan on AUTO and check gas valve pressure with a manometer.

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

Furnace Blowing Cold Air in Danville: common questions

Nearby and related

Furnace Blowing Cold Air near Danville: San Ramon · Alamo · Blackhawk · Walnut Creek · Pleasanton .

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