Furnace Not Heating in Sunnyvale

Furnace Not Heating in Sunnyvale

Sunnyvale winters are mild, so the furnace sits idle most of the year and then gets asked to run hard the first week it turns cold. That long dormant stretch is when problems show up. A flame sensor carbons over while the system sleeps through spring and summer. An ignitor that was already weak at shutdown finally cracks. Whatever the part, the furnace tries to fire and either never lights or lights and cuts out after a minute.

Most of the housing here is 1950s and 60s orchard-tract ranches, and a lot of them have had garage conversions or second-story pop-ups added without the heating ever being resized. When a furnace is undersized for the space it now serves, it runs longer cycles, the heat exchanger and limit switch see more stress, and a marginal part fails sooner. The no-heat call is usually the symptom of that wear, not the system giving up.

In most cases this comes down to a single replaceable component. The point of the diagnostic is to find which one, confirm it on a meter, and rule out anything unsafe before we put a price on it. Our $75 diagnostic gets credited toward the repair if it runs over $200.

Common causes

Cracked or weak hot surface ignitor. The most common modern no-heat failure. The ignitor glows but no longer reaches light-off temperature, or it has hairline-cracked and shows an open circuit. We ohm it out and watch a startup sequence, then quote the exact ignitor for your unit on the written estimate.

Dirty flame sensor. The sensor proves a flame is present. Carbon buildup makes it read no flame, so the board shuts the gas off seconds after ignition. You hear it light and then quit. Cleaning often restores it; if the rod is pitted we replace it.

Limit switch tripped from low airflow. A clogged filter or restricted return starves the blower, the heat exchanger overheats, and the high-limit trips to protect the unit. Common in added-on Sunnyvale ranches where return air never got upsized. We restore airflow, check the switch, and find why it tripped so it does not repeat.

Failed gas valve. If the ignitor glows correctly but no gas reaches the burners, the valve may not be opening. We check the coil and the voltage the board sends it before condemning the valve, since a wiring or board fault can look identical from the outside.

Control board or thermostat fault. A board that does not sequence the ignition, or a thermostat not calling for heat, leaves you cold with a furnace that looks fine. We meter the call signal at the board to separate a thermostat issue from a board issue before quoting either.

Blower motor not running. If the furnace lights but no warm air moves, the blower motor or its capacitor may have failed. The limit will trip on the trapped heat and lock the unit out. We test the motor and capacitor under load and replace the failed part.

How we diagnose it

• Watch a full ignition sequence end to end and note exactly where it fails: no spark, no flame proof, or flame dropout.
• Meter the ignitor, flame sensor, gas valve coil, and the 24-volt call signal at the control board to isolate the failed part.
• Pull and inspect the filter and check return airflow, since a tripped limit on an added-on ranch usually traces back to restriction.
• Run a CO test at the registers and inspect the heat exchanger before clearing the system to run.
• Confirm the thermostat is actually calling and reading correctly so we do not replace a furnace part for a thermostat fault.

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

Furnace Not Heating in Sunnyvale: common questions

Nearby and related

Furnace Not Heating near Sunnyvale: Mountain View · Santa Clara · Cupertino .

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