Most gas furnaces in Novato, San Rafael, San Anselmo, Larkspur and Sausalito that fail to heat have stopped at one point in their start-up sequence: the igniter did not glow, a pressure switch did not close, the flame sensor could not prove the flame, or a limit switch shut the burners off because the furnace overheated. A furnace repair visit finds that point first, then the reason behind it, so the fix lasts.
How a gas furnace starts, and where it fails
- The thermostat calls for heat over a low-voltage wire.
- The inducer motor starts, pulling air through the burners and pushing exhaust up the vent.
- The pressure switch closes, proving the vent is clear. A blocked vent, a cracked hose or a weak inducer stops the sequence here.
- The igniter heats (or a spark or pilot lights). Hot surface igniters are brittle and crack with age.
- The gas valve opens and the burners light.
- The flame sensor proves the flame within seconds. A sooty sensor is the classic cause of a furnace that lights, then shuts off.
- The blower starts after a short delay and sends warm air through the ducts.
- The limit switch watches the temperature. If air cannot move - a clogged filter, closed registers, a weak blower - it shuts the burners off to protect the heat exchanger.
Many newer furnaces flash a code on a small light visible through the door window. Write down the pattern before you call; it points straight at one of these steps.
The parts that fail most
| Part | Typical sign | What it usually takes |
|---|---|---|
| Igniter or pilot | Inducer runs, burners never light | Igniter or pilot assembly replacement |
| Flame sensor | Burners light, then go out | Cleaning or a new sensor |
| Pressure switch | Inducer runs, nothing else happens | Clearing the vent or hose, or a new switch |
| Limit switch | Heats a while, then shuts off and restarts | Fixing the airflow cause, then the switch if worn |
| Blower motor or capacitor | Burners run but no air at the vents | Capacitor, motor or control module |
| Gas valve | No gas to the burners with power present | Valve replacement after the other checks |
| Control board | Erratic operation, no code or a board fault code | Board replacement after testing inputs |
| Leaking ductwork | Furnace runs long, far rooms stay cold | Duct repair |
Carbon monoxide and the heat exchanger
Every furnace visit starts with a safety and carbon monoxide check. The heat exchanger keeps burning gases apart from the air that circulates through your home. Cracks develop with age and overheating, often after years of a clogged filter tripping the limit switch. Warning signs include a CO alarm, soot around the burner area, yellow lazy flames and flames that waver or roll when the blower starts. We inspect the exchanger and test for CO at the vent and in the supply air, and if we find a problem we show you, not just tell you.
Electric furnaces
An electric furnace has no burners or vent. It heats with resistance elements switched on in stages by sequencers or relays. When it blows cool air, one or more elements or their controls usually failed, or a breaker tripped. We test each element and its circuit, check the limit switches and the blower, and look at the wiring for heat damage.
Older furnaces in Marin crawlspaces and attics
In many older Marin homes the furnace sits in a tight crawlspace or an attic, often installed long before current equipment sizes. Damp air under a house rusts burners, igniter mounts and wiring connections, and ducts in the same crawlspace work loose or get crushed over the years. We bring the right tools for tight access and check the duct connections near the furnace while we are there. On hillside homes in San Anselmo and Larkspur, a furnace under the house can be reachable only through a small hatch, so tell us about the access when you book.
Repair or replace?
We recommend only what the furnace needs. A failed igniter, sensor, switch or motor on a sound furnace is a repair. A cracked heat exchanger on an older furnace, or a string of failures in a few winters, is when furnace installation is worth a conversation. If your heat runs on a heat pump or a wall furnace, start with heating repair, and for no-heat troubleshooting see furnace not heating. Local pages: furnace repair in Novato and in Larkspur.