Furnace Not Heating in Los Altos, CA

Furnace Not Heating in Los Altos

When a furnace stops making heat, the instinct is to assume the whole system died. It almost never has. A gas furnace lights in a set order. The thermostat calls, the inducer spins up, the igniter glows, the gas valve opens, and the flame sensor has to confirm the burn. If any one of those steps fails, heat stops, and the part that broke is usually small and replaceable.

Los Altos adds a wrinkle most cities don't. A lot of the larger ranch homes here are run as dual-zone systems, and many have been added onto over the years without the heating ever being resized. So 'no heat' sometimes means one zone is cold while the other works, which points at a zone damper or control board rather than the furnace itself. We sort out which it is before quoting anything.

Winters here are mild, so a furnace that quits in December rarely leaves you in danger. That buys time to do the diagnosis right and fix the actual fault instead of guessing at parts.

Common causes

Cracked hot surface igniter. The most common modern no-heat failure. These thin ceramic igniters glow to light the burners and eventually crack from thermal cycling. The blower may run but no flame ever lights. We test resistance and inspect for the hairline crack, then replace it. Igniter replacement typically runs a couple hundred dollars depending on the furnace, and the exact number goes on your written estimate.

Dirty flame sensor. If the furnace lights for a few seconds then shuts off, the flame sensor is usually the culprit. Carbon builds on the rod and the board stops seeing flame, so it cuts gas as a safety. Cleaning the sensor fixes it most of the time. A worn sensor that needs replacing is a modest part cost we'll show you before doing it.

Limit switch tripped by a clogged filter. Homes with additions often have undersized return airflow for the current footprint. A dirty filter chokes airflow, the furnace overheats, and the high-limit switch shuts it down. We check the filter and static pressure, replace the filter, and confirm the limit resets. If airflow is chronically short we flag the duct issue on the estimate.

Zone control board or stuck damper. On dual-zone systems, a failed zone board or a damper stuck closed makes one area go cold while the rest of the house heats normally. People read that as a broken furnace. We meter the board outputs and cycle the dampers manually to isolate it. Often the furnace is fine and a board or actuator is the actual repair.

Failed gas valve. If the igniter glows and the inducer runs but no gas reaches the burners, the gas valve may not be opening. We verify gas pressure at the valve and check the control signal before condemning it, since a wiring or board fault can mimic a bad valve.

Thermostat or wiring fault. Smart thermostats added during a remodel sometimes lose the common wire or get miswired, so the furnace never gets the call for heat. We confirm the thermostat is actually sending the W signal before opening the furnace, which saves you a parts bill on a wiring problem.

How we diagnose it

Confirm the thermostat is calling for heat and sending voltage to the furnace control board.
Watch a full ignition sequence: inducer, igniter glow, gas valve, flame, blower, to see exactly where it stops.
Pull and inspect the filter, then measure static pressure to catch airflow-driven limit trips common in added-onto homes.
On dual-zone systems, meter the zone board and cycle each damper to find a cold zone caused by controls, not the furnace.
Run a CO test and inspect the heat exchanger before we sign off on any gas furnace.

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

Furnace Not Heating in Los Altos: common questions

Do you actually cover Los Altos from San Ramon?

Yes. We're based in San Ramon and run furnace calls across the Peninsula and South Bay, Los Altos included. Same-day is best effort depending on where the crew is that morning. Call and we'll give you an honest window, not a vague 'sometime today.'

It's barely cold here. Is a no-heat call worth the $75?

Mild winters mean a dead furnace is rarely an emergency, but the $75 diagnostic gets credited toward any repair over $200, so you're not paying just to find out what's wrong. And a furnace that short-cycles or fails the CO test is worth catching before it gets cold, not after.

The blower runs but I get no warm air. What's that?

Usually ignition. The igniter cracked, the flame sensor is dirty, or the gas valve isn't opening, so the burners never light while the blower still moves air. That's a diagnosable, fixable part in most cases, not a replacement furnace.

Nearby and related

Furnace Not Heating near Los Altos: Palo Alto · Mountain View · Cupertino .

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