When a furnace in Tiburon, Belvedere or Mill Valley stops heating, the cause is usually one part in its start-up sequence rather than the furnace as a whole. A modern gas furnace goes through the same steps every time the thermostat calls for heat, and when one fails, the control board stops the sequence and often flashes a code. The repair is finding the step that failed, and why.
The start-up sequence, and what fails at each step
- Thermostat calls for heat. Dead batteries, the wrong mode or a broken wire stop everything here.
- Inducer motor starts to push exhaust out through the vent. A worn inducer hums, rattles or won’t start.
- Pressure switch proves the draft. A blocked vent, a cracked hose or, in high-efficiency furnaces, a clogged condensate trap keeps the switch open, and the furnace stops.
- Igniter heats up. Hot-surface igniters crack with age; this is one of the most common no-heat repairs.
- Gas valve opens and the burners light. A failed valve or a gas supply problem shows up here.
- Flame sensor proves the flame. A dirty sensor lets the burners light for a few seconds and then shuts them off, again and again.
- Blower starts and sends heat through the ducts. A failed capacitor or motor leaves the heat trapped in the furnace until the high-limit switch trips.
Symptoms and likely causes
| What you notice | Likely cause |
|---|---|
| Burners light, then go out within seconds | Dirty or failing flame sensor |
| Clicking or humming, never lights | Igniter, pressure switch, inducer or gas valve |
| Blower runs but the air is cold | Burners not lighting, or the thermostat fan set to On |
| Heats a few minutes, shuts off, repeats | Overheating from a clogged filter or closed vents, tripping the limit switch |
| Squealing, scraping or rumbling | Blower motor bearings, a belt on older units, or loose panels |
| Lazy yellow flames, soot, stuffy air | Combustion or venting problem: turn it off and call |
| Burning-dust smell on the first cold day | Normal for a few minutes; call if it lingers |
Electric and oil furnaces
Electric furnaces have no burners. Their faults are in the heating elements, the sequencers that switch the elements on in stages, the limit switches and the breakers. Oil furnaces, still found in a few older homes, have nozzles, filters and ignition parts that need regular attention. We repair all three types.
A safety check after every repair
Every furnace repair ends with a safety and operation check: burners and flame, venting, the visible parts of the heat exchanger, the safety switches, and a test for carbon monoxide. A furnace that runs is not necessarily a furnace that is safe, and the heat exchanger in particular is not something you can check yourself.
What you can safely check
- The thermostat is on Heat, set above room temperature, with fresh batteries.
- The switch next to the furnace is on, and the front panel is seated (it presses a safety switch).
- The filter isn’t clogged.
- Supply and return grilles aren’t closed or blocked by furniture.
- On a high-efficiency furnace, the plastic vent pipes outside are not blocked by leaves or a nest.
Don’t keep resetting the furnace. If it locks out again after one reset, the fault is real and needs testing.
Repair or replace?
Most furnace repairs are a single part and worth doing. Replacement is the better conversation when the heat exchanger is cracked, when an older furnace needs a blower motor and a control board in the same season, or when it is oversized and loud for the house. Our furnace installation page explains how we size and choose a replacement.
Local notes
Furnaces work long hours here. In Mill Valley’s shaded homes under the redwoods, heating runs most of the year, which wears igniters, flame sensors and blower motors sooner. Larger homes on the Tiburon peninsula often have two furnaces or several zones, and one cold zone may be a damper or thermostat rather than a furnace fault. The local pages cover furnace repair in Tiburon, Belvedere and Mill Valley. A fall HVAC maintenance visit catches most of these problems before the first cold night, and for other heating systems see heating repair.