Furnace Not Heating in Redwood City

Furnace Not Heating in Redwood 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 Redwood City is the split between filled-bay Redwood Shores and the older hill neighborhoods.

Housing age and local climate both matter on a no-heat call. Redwood City spans a wide range of microclimates, from the filled bay flats of Redwood Shores to the warmer hills around Farm Hill and Emerald Hills, so two houses a few miles apart can carry genuinely different loads. Redwood Shores was built on filled bay land starting in the 1970s and laced with its own canals, similar to Foster City, while the older downtown core and the hill neighborhoods run a more varied, older housing stock with original HVAC well past service life in plenty of homes.

In Redwood 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 Redwood City: common questions

Nearby and related

Furnace Not Heating near Redwood City: San Carlos · Menlo Park · Atherton .

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