Furnace Not Heating in San Jose, CA | Bay Area HVAC Service

Furnace Not Heating in San Jose

San Jose is a cooling-driven city. Summers run hot and the AC does the heavy lifting most of the year, while the heating load stays light. The practical effect is that furnaces here sit unused for months, then get switched on cold for the first time in fall. A part that degraded over the off-season picks that exact moment to fail.

Most no-heat calls trace to one component. A cracked igniter, a fouled flame sensor, a limit switch tripped by a filter nobody touched all summer, or a control board relay. These are repairs. A furnace that won't light is rarely a furnace that needs replacing. We isolate the failed part first.

San Jose's housing mix changes the conversation. A lot of the older Eichler tracts were built with radiant-slab heat and no ducts, and on the homes where that slab has failed, there is nothing to repair under the concrete economically. If you have an Eichler with no real heating left, the practical fix is a ductless mini-split, not a furnace repair. We'll tell you straight which situation you're in.

Common causes

Cracked hot surface igniter after a summer of disuse. Igniters are the most common no-heat failure, and a long idle summer doesn't help a ceramic element that was already aging. We test continuity and watch whether it glows on a call for heat. A cracked igniter gets replaced with the correct part for your furnace, roughly $200 to $350, and you have heat the same visit.

Flame sensor fouled over the off-season. When the furnace hasn't run for months, the first cold-weather cycles can reveal a flame sensor coated in carbon. The burner lights, then dies a few seconds later, over and over. Cleaning the rod fixes most of these. Replacement runs $150 to $200 if it's degraded. We check it early because it's a cheap, frequent cause.

Limit switch tripped by a clogged filter. A filter that went unchanged all summer restricts airflow once the furnace fires, the heat exchanger overheats, and the high-limit switch cuts the burners. You get cold air from the registers. We replace the filter, verify airflow, and reset or replace the limit. Often a same-visit resolution.

Control board relay failure. On the post-2000 furnaces common across San Jose, a failed relay or board fault can stop the furnace from sequencing ignition at all. We meter the board for power and output before condemning it, because boards are expensive and the actual fault is sometimes a sensor feeding bad data to a healthy board.

Dead radiant slab in an Eichler (not a furnace problem). If you're in an Eichler with no heat, the original copper-in-concrete radiant system has often failed for good. There's nothing to repair under the slab economically. The practical answer is a ductless mini-split, which heats and cools without cutting into the open ceilings these homes are known for. We've done a number of these retrofits and will quote it honestly.

Thermostat fault or lost programming. Sometimes the furnace never got the call for heat. A dead thermostat battery, a wiring issue, or a lost schedule looks like a dead furnace. We confirm the thermostat is actually signaling the board before we open the cabinet, so you aren't paying for a board diagnosis on a thermostat problem.

How we diagnose it

Whether the thermostat is actually sending a heat call to the control board
Igniter glow and continuity, then flame sensor signal through a full firing cycle
Filter and airflow, then the high-limit switch for an overheat trip after the summer idle
Control board power and ignition sequencing on the common post-2000 brands here
For Eichlers, whether you have a repairable forced-air system at all or a dead radiant slab that needs a ductless solution

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

Furnace Not Heating in San Jose: common questions

You're in San Ramon. Do you really service San Jose?

Yes, San Jose and the South Bay are part of our regular service area. We're farther from San Jose than from our Tri-Valley home base, so when you call about a no-heat furnace we'll give you a realistic arrival window for that day rather than overpromising. We carry the common South Bay furnace parts on the truck.

Heating barely matters in San Jose. Is it worth keeping an old furnace alive?

It depends on the unit's age and condition. The heating load here is light, and California is steadily tightening rules on new gas furnace equipment, so an old furnace at end of life is often better converted to a heat pump than repaired again. We run repair cost against replacement on the written estimate so the decision is yours with real numbers.

My furnace fires for a few seconds then shuts off, repeatedly. What is that?

That short-cycling pattern usually points to a dirty flame sensor. The burner lights, the sensor fails to confirm the flame, and the board shuts gas off as a safety. Cleaning the sensor fixes most cases. A tripped limit from a clogged filter can cause similar behavior. We diagnose which one before quoting.

Nearby and related

Furnace Not Heating near San Jose: Santa Clara · Milpitas · Cupertino .

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