Furnace Not Heating in Alamo

Furnace Not Heating in Alamo

A furnace that quits heating almost always still tries to run. The thermostat calls, the blower may even spin, but the burners never light or the heat never reaches the rooms. In a single-system house that is obvious within an hour. In a larger Alamo home with separate furnaces serving different zones, one unit failing on a cold morning can hide for a while behind the others that are still working.

The cause is rarely the whole system. A gas furnace that won't heat is usually one part. We see cracked hot surface igniters, flame sensors coated with carbon, and limit switches tripped by a dirty filter. On bigger homes with long duct runs and tight filter schedules, the limit-switch trip is a recurring one, because airflow restriction is exactly what protects the heat exchanger by shutting the burners down.

We diagnose the dead zone on its own rather than assuming the problem is shared across systems. In a home where the upstairs and downstairs units are often different ages, different brands, and on different control boards, that distinction matters.

Common causes

Cracked hot surface igniter. The most common modern gas furnace failure. The igniter glows to light the burners and eventually cracks from heat cycling. You hear the furnace try and fail to ignite. We test continuity, confirm the crack, and replace it. On a home with two furnaces we check both igniters since they age at the same rate.

Dirty flame sensor. The flame sensor proves the burners actually lit. Carbon buildup makes it read no flame, so the board shuts gas off within seconds for safety. The furnace lights, then dies, repeatedly. Cleaning the sensor usually fixes it. We replace it only if it is pitted or failing.

Limit switch tripped by airflow restriction. A clogged filter or a closed-down zone damper starves the furnace of airflow, the heat exchanger overheats, and the limit switch cuts the burners to protect it. On long-run Alamo systems this is common. We check the filter, the damper actuators, and confirm the switch resets once airflow is restored.

Control or zone board fault. A multi-zone system runs a zone control board on top of the furnace board. A failed relay or a drifted board can stop one zone from ever getting a heat call. We run the board diagnostics in sequence and check wiring and sensors first, because most suspected board failures are actually wiring or actuator problems.

Failed gas valve. Less common but real on older equipment. The igniter glows, the sensor is clean, but the valve never opens to release gas. We measure the valve coil and inlet pressure before condemning it, since a wiring or board fault upstream can mimic a dead valve.

Blower motor not moving heat. The burners light but no warm air reaches the rooms because the blower motor or its capacitor has failed. We test the motor windings and capacitor and confirm the run on both the heating and cooling speed taps.

How we diagnose it

• Confirm which specific zone or furnace is dead and whether the thermostat is actually calling for heat on that unit.
• Read the control board fault code and watch a full ignition sequence: inducer, igniter glow, gas valve, flame sensing.
• Check filter condition and zone damper position to rule out an airflow-tripped limit switch.
• Run combustion analysis and CO testing on the gas furnace, and inspect the heat exchanger before signing off.
• Put the failed part, the price, and any safety findings on a written estimate before we touch the repair.

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

Furnace Not Heating in Alamo: common questions

Nearby and related

Furnace Not Heating near Alamo: Danville · Blackhawk · Lafayette · Walnut Creek .

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