Furnace Blowing Cold Air in Mountain View, CA

Furnace Blowing Cold Air in Mountain View

A good share of older Mountain View housing, especially the mid-century starter homes near the downtown core, was built with a gas furnace and no central AC. Those furnaces are the original heating systems in a lot of cases, and because the marine summers are mild they often sit idle for long stretches. When one runs but blows cold, that idle time is frequently the reason: an igniter or flame sensor that has not worked in months fails on the first real cold call.

The blower spinning while the air stays cool is actually reassuring. It means the motor and control board are alive and the problem is upstream at ignition or flame sensing. On the older heat-only furnaces we mostly see cracked igniters, dirty flame sensors, and overheating limit switches tripped by neglected filters. Newer infill and remodeled homes run modern code-built furnaces, where condensate blockage on the high-efficiency units becomes the more likely cold-air cause.

Cold air from a running furnace is almost always one fixable part, not a dead system. We confirm what failed with meter readings, then put the part and the price on a written estimate before we start.

Common causes

Cracked hot surface igniter on a long-idle furnace. The mild Mountain View climate means the original heat-only furnaces run few hours, so the igniter often cracks and fails on its first hard cycle of the season. Burners never light, blower still runs. We test it electrically and replace it.

Oxidized or dirty flame sensor. A sensor that has sat idle for months builds up enough oxide to stop reading flame. Burners light, then the board cuts gas within seconds and you get cold air. We clean the rod and check the microamp signal, replacing it if pitted.

Overheating limit switch from a clogged filter. In older homes with tight original ductwork, a dirty filter restricts airflow, the furnace overheats, and the high-limit shuts the burners while the blower keeps running. You feel cold air. We measure static pressure to tell airflow from a failed switch.

Thermostat fan on ON instead of AUTO. The blower runs continuously and pushes unheated air between heating cycles, which reads as a broken furnace. We confirm the fan setting and watch the actual burner cycle before condemning a part.

Condensate blockage on high-efficiency units. Newer infill and remodeled homes often run condensing furnaces that lock out the burners when the condensate trap or line clogs and the pressure switch opens. We clear the drain and verify the switch closes.

Gas supply or valve issue. No gas means no heat even though the blower energizes. We confirm the valve is open, the valve is getting its signal, and supply pressure is correct with a manometer before assuming a control fault.

How we diagnose it

Watch a full heating cycle to confirm whether burners light and stay lit or drop out.
Test the igniter and flame sensor with a meter, including the sensor's microamp reading.
Inspect the filter and measure static pressure, which matters on the tight original ductwork in older homes.
On the newer code-built units, check the condensate trap, line, and pressure switch.
Confirm gas valve operation and supply pressure with a manometer before condemning the control board.

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

Furnace Blowing Cold Air in Mountain View: common questions

Do you serve Mountain View from the East Bay?

Yes. We run the South Bay including Mountain View, Palo Alto, and Los Altos from our San Ramon base. Same-day is best effort depending on the schedule, and no-heat calls get priority in cold weather. Call (925) 999-4095 and we will give you an honest window.

My old furnace barely runs and now blows cold. Is that the mild climate's fault?

Partly, yes. Mountain View's mild marine summers mean these original heat-only furnaces run very few hours, and parts like igniters and flame sensors are most likely to fail when they finally have to work after months idle. It is a common pattern here, not bad equipment. A fall tune-up catches most of it before the first cold night.

My ADU furnace blows cold but the main house is fine. What is going on?

They are separate systems, so a cold ADU usually means that unit's own igniter or flame sensor, or, if it is a mini-split set to heat, a different fault entirely. A lot of ADUs and garage conversions run ductless mini-splits rather than gas furnaces, which we diagnose differently. We check whichever system serves the space and isolate the failed part.

Nearby and related

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

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