A gas furnace that will not heat usually has one of a short list of problems: the burners do not light, they light and go out, the blower does not move the heat, or a safety switch has stopped the furnace on purpose. In Menlo Park, West Menlo Park, Atherton and Portola Valley, most of the furnaces we open sit in one of three places in an older ranch home: the attic, a hall closet or the garage. That location shapes both the fault and the repair. We find the failed part with measurements first, then tell you what it costs to fix.
What fails, in the order the furnace runs
Every cycle follows the same sequence, and the control board stops at the first step that fails. Following it finds the fault without guessing at parts.
- Call for heat. The thermostat closes the heating circuit. Dead batteries or a loose wire stop everything here.
- Inducer and pressure switch. A small motor pulls combustion gases through the furnace and out the vent, and a pressure switch proves the draft. A worn inducer, a split hose, a blocked vent or a clogged condensate trap stops it here.
- Ignition. A hot surface igniter glows or a spark igniter fires. Hot surface igniters crack with age; older standing-pilot furnaces wear out thermocouples instead.
- Gas valve and flame sensing. The gas valve opens and the flame sensor has seconds to prove the flame. A coated sensor is the classic “lights, then shuts off” fault. A valve that gets power but never opens is replaced, not adjusted.
- Blower. After a short warm-up the blower pushes air across the heat exchanger. A weak capacitor, a failing motor or a blower wheel caked with dust shows up as little airflow or a furnace that overheats.
- Safety controls. Limit and rollout switches shut the burners off if the furnace runs too hot or flame escapes the burner area. A tripped limit is a signal, not the fault: look for a clogged filter, closed registers or a crushed attic duct.
Many furnaces flash a code on a small status light when they lock out. Count the flashes before you switch the power off, because a restart can clear the code. Electric furnaces skip the burner steps; their faults are heating elements, sequencers and relays.
Symptoms and what they usually take
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Lights, runs a few seconds, shuts off | Coated or failing flame sensor | Cleaning or a new sensor |
| Inducer runs, nothing lights | Cracked igniter, pressure switch or vent fault | Igniter or switch, vent or trap clearing |
| Burners run, vents blow lukewarm | Clogged filter, limit trips, leaky attic duct | Airflow fix, blower parts or duct repair |
| Squeal or grind as the blower starts | Worn motor bearings, or a belt on very old units | Motor service or replacement |
| Water around a high-efficiency furnace | Blocked condensate trap or drain | Trap and drain cleaning |
| Breaker trips when the furnace starts | Shorted motor or wiring | Electrical testing before any reset |
Where the furnace sits changes the job
Attic furnaces lie on their side above the hallway ceiling. On a high-efficiency model the condensate drain runs across the attic, and a blocked drain can show up as a ceiling stain before the furnace stops. A narrow ladder and no walkway add time.
Hall-closet furnaces need air to burn. If the louvered door was swapped for a solid one, or the closet now holds boxes and cleaning supplies, a furnace that takes its combustion air from the closet can run poorly. Clearing the closet is part of the fix.
Garage furnaces are easy to reach, but paint, solvents and gasoline stored nearby are a fire risk.
Heat exchanger inspection
The heat exchanger keeps combustion gases out of the air you breathe. We inspect it with a light and, where the design allows, a camera, and watch the flame as the blower starts: a flame that lifts or wavers then is a warning sign. A confirmed crack means the furnace stays off, and it is the most common reason a repair visit turns into a replacement conversation.
Furnace cleaning: what actually helps
Useful cleaning is targeted: flame sensor, burners, blower wheel, filter and, on high-efficiency models, the condensate trap. It belongs in a yearly furnace tune-up. Dust blowing out of the registers is a duct cleaning question, not a furnace one.
What moves the price
Most furnace repairs fall between $89 and $600. Flame sensors, igniters, capacitors and pressure switches sit near the low end. Inducer motors, blower motors (especially variable-speed ones), gas valves and control boards push toward the top, and a few repairs go past it. Attic access and the furnace’s age also count.
Repair or replace
One failed part on a furnace with a sound heat exchanger is worth repairing. A cracked heat exchanger, two major parts failing on an old unit, or a furnace that needs a visit every winter is the time to compare furnace installation with a heat pump. If the furnace is unsafe right now, go to emergency heating repair.
When you book, and before you approve
Tell us where the furnace is, its rough age, the brand and model from the label, when the problem started and any smell, sound or flashing code. Photos of the furnace, its label and the access help.
Before you approve, ask which symptom was reproduced and which reading confirmed the failed part. Keep today’s repair separate from anything suggested for later. For a larger repair you get the diagnosis, parts, labor and options in writing first.