Furnace & Heating Faults

Flame sensor. The burner lights, holds for a few seconds, then shuts down and tries again. A coated sensor rod stops proving the flame it can see perfectly well. Cheap, common, and often the entire call.

Hot-surface igniter. Nothing glows, nothing lights. These are consumables with a service life measured in seasons, and the crack that ends them is usually invisible.

Draft and pressure switches. The inducer runs but the sequence stops there. Sometimes it is the switch; often it is what the switch is correctly complaining about — a blocked flue, a bird, a length of PVC that has sagged and is holding water.

A cracked heat exchanger. This is a combustion product leak into the air you breathe, and it ends the repair conversation. Any technician who offers to weld one and keep going is not doing you a favor; that condition is a red-tag on the equipment.

Soot, yellow lazy flames, or a CO alarm. Turn the system off, open a window if you can do it safely, and get everyone out of the house if the alarm is sounding. Carbon monoxide has no smell. This is one of the very few things on this site that is more urgent than the cost of fixing it.

A furnace that starts and stops every few minutes is usually overheating and shutting itself down on the limit switch — which means airflow. A filter nobody has changed since spring, closed registers in half the house, or ducts too restrictive for the unit will all do it. An oversized furnace does it too, and that one was baked in on the day it was installed.

A large part of San Jose's pre-war stock never had ducts: Naglee Park, the Rose Garden and parts of downtown still run wall heaters and floor furnaces. They are their own trade. Parts for older models are increasingly scarce, replacement in kind is possible but shrinking, and the alternative — a ductless heat pump — is a bigger project with rebates attached to it. Photograph the unit, the grille and any plate you can find; the model is what determines whether parts still exist.

Local published figures put furnace repairs from $199 before parts, with a $69 diagnostic and a $199 labor minimum at one San Jose shop. Replacement is the other conversation: Bay Area published ranges are $7,500–$12,000+ for a furnace alone and $12,000–$20,000+ for a furnace and AC together, and a changeout carries permit and Title 24 verification with it — see the cost guide.

The furnace with its cabinet door off if that is safe, the data plate, the flame through the sight glass if there is one, the flue where it leaves the cabinet, the thermostat, and the filter — the last one answers more questions than people expect.