Furnace Not Heating in Foster City

Furnace Not Heating in Foster City

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 Foster City is the fill soil settling under the ductwork and the damp lagoon-front air.

Housing age and local climate both matter on a no-heat call. Foster City is built almost entirely on filled marshland reclaimed from the bay starting in the 1960s, laced with the lagoons the city is known for, and the lagoon-adjacent air runs damp even on a mild day. The housing is unusually uniform for the Peninsula: 1960s and 70s tract construction on engineered fill, much of it lagoon-front or a short walk from the water. Sixty years of settling in that fill soil has shifted more than a few slabs, which shows up as duct runs that no longer sit level and condensate lines that no longer drain the way they were installed.

In Foster City, 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. Sixty years of settling on filled bay soil here has left more than a few duct runs out of level or pinched at a joint.

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, bay-adjacent air here is harder on exposed electrical connections than a drier inland spot would be. 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, bay-adjacent air here is harder on exposed electrical connections than a drier inland spot would be. Sixty years of settling on filled bay soil here has left more than a few duct runs out of level or pinched at a joint.

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

Furnace Not Heating in Foster City: common questions

Nearby and related

Furnace Not Heating near Foster City: San Mateo · Belmont · Redwood City .

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