Furnace Not Heating in South San Francisco

Furnace Not Heating in South San Francisco

No heat from a gas furnace almost always traces to one step in the ignition sequence failing. The inducer has to run, the igniter has to glow or spark, the gas valve has to open, and the flame sensor has to confirm the flame is lit, and a fault at any one of those steps shuts the whole cycle down as a safety measure. Add a tripped limit switch from restricted airflow, a dead thermocouple on an older wall or gravity furnace, or simply no call for heat reaching the board, and that covers nearly every no-heat call we run. We watch the sequence directly rather than swapping parts on a guess. The detail that matters in South San Francisco is the fog-line split between the old flatland neighborhoods and hillside Westborough.

Housing age and local climate both matter on a no-heat call. South San Francisco sits under heavy fog for much of the year, especially in the older flatland neighborhoods below Sign Hill, while newer construction up in Westborough sits above the fog line and runs noticeably warmer. Neighborhoods like Winston Manor and Sunshine Gardens carry a lot of older housing built without ductwork, since nobody needed central air under that much fog. Westborough's newer hillside construction is a different story, built later and with more real cooling load to plan around.

In South San Francisco, every gas furnace call includes a carbon monoxide reading and a heat exchanger check, regardless of what brought us out, because that's a safety step we don't skip. The written estimate covers the actual repair, and on an older furnace we'll tell you honestly whether a heat pump conversion is worth putting on the table too. The $99 diagnostic fee is credited toward the repair.

Common causes

Cracked hot surface igniter. The igniter is a thin ceramic element that eventually cracks from repeated heating and cooling cycles. Once it can no longer glow hot enough, the gas valve never gets confirmation to stay open.

Dirty flame sensor. A flame sensor coated with a thin layer of carbon can no longer read the flame properly, so the board shuts the gas valve seconds after ignition, over and over, until the furnace locks out.

Limit switch tripped by a dirty filter. Restricted airflow, often from a dirty filter, lets the heat exchanger run hotter than it should and trips the high-limit switch as a safety response, shutting the burners down. The fog-belt tract homes here were built with minimal or no ductwork, so a retrofit duct run is often undersized or improvised.

Failed thermocouple on a wall or gravity furnace. Older wall and gravity furnaces use a thermocouple rather than a flame sensor, and a weakening one shuts the pilot's gas valve even when the pilot is actually lit.

Corroded gas valve or board connection. Age and moisture take a toll on gas valve terminals and board connectors over time. The damp, fog-heavy air here still works its way into loose or aging connections over time, even without direct salt exposure. That can cause an intermittent or complete no-heat fault.

Thermostat or no call for heat. If the call for heat never reaches the board, nothing downstream matters yet. We confirm 24 volts and a real heat call reach the furnace before assuming a mechanical failure.

How we diagnose it

• Confirm a real heat call reaches the control board, so we're diagnosing the furnace and not a dead thermostat or broken low-voltage wire.
• Watch a full ignition sequence: inducer, igniter glow, gas valve, flame sensing, and where in that chain it stops.
• Test the igniter, flame sensor, and limit switch directly with meters rather than swapping parts on a guess.
• On every gas furnace, meter carbon monoxide and inspect the heat exchanger; on older wall and gravity units this is non-negotiable.
• Check filter, airflow, and the gas valve and board connections. The damp, fog-heavy air here still works its way into loose or aging connections over time, even without direct salt exposure. The fog-belt tract homes here were built with minimal or no ductwork, so a retrofit duct run is often undersized or improvised.

$99 diagnostic, credited toward the repair. You get a written quote before any work begins.

Furnace Not Heating in South San Francisco: common questions

Nearby and related

Furnace Not Heating near South San Francisco: San Bruno · Daly City · Millbrae .

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