Furnace not turning on

Start here: the four free checks

Do these before anything else. They account for a large share of no-heat calls, and every one of them costs nothing.

1. Check the thermostat properly. Set it to HEAT, not AUTO or COOL, and push the target temperature at least five degrees above the current room temperature. If the display is blank or faint, replace the batteries even if it seemed fine yesterday. A thermostat with weak batteries can hold a display but fail to close the circuit that calls for heat.
2. Find the furnace power switch. It looks exactly like an ordinary light switch, usually on or beside the furnace itself, often at the top of basement stairs or on a garage wall. It gets flipped off by accident constantly, most often by someone reaching for a light. If it is off, turn it on and wait a few minutes.
3. Check the breaker. Look for a breaker labelled furnace, heater or HVAC. A tripped breaker sometimes sits in a middle position rather than fully off. Push it firmly to OFF, then back to ON.
4. Close the blower door. Every modern furnace has a safety interlock switch behind the removable front panel. If that panel is not seated all the way, the furnace will not run at all, by design. If someone changed a filter recently, this is a very common cause. Press the panel until it sits flush and the latch engages.

If the furnace starts after any of these, nothing is broken. You are done.

If it still will not start

These are the next five, roughly in the order a technician checks them. The first two you can do yourself. The last three need someone with a meter and the right parts.

5. A badly clogged air filter. A filter that has not been changed in a year restricts airflow enough that the furnace overheats and its high limit switch shuts it down to protect the heat exchanger. Slide the filter out and hold it up to a light. If you cannot see light through it, replace it. This is worth doing before you call anyone.
6. A blocked condensate drain. High efficiency furnaces, the ones with white PVC pipe running outside rather than a metal flue, produce water as they burn. That water leaves through a drain line with a float switch in it. If the line clogs, the float rises and cuts the furnace off deliberately. If you see water pooling near the base of the unit, this is likely your problem.
7. A dirty flame sensor. The signature is unmistakable: the furnace lights, runs for roughly three to seven seconds, then shuts off, and may repeat this two or three times before locking out. The sensor is a thin metal rod that proves a flame is present, and a light coating of residue is enough to make it stop reading. This is a cheap fix, but it involves the burner assembly and is a technician job.
8. A failed hot surface ignitor. You hear the furnace try to start, the inducer motor runs, but no flame ever appears. Ignitors are brittle and fail from normal thermal cycling, usually after several years.
9. No gas reaching the furnace. Check whether your gas stove and water heater are working. If nothing gas powered in the house works, the problem is upstream of the furnace and it is a utility issue, not a furnace repair.

Match the symptom to the cause

What the furnace does before it fails narrows things down faster than any other clue.

Most furnaces also flash a diagnostic code on a small LED behind the front panel. Many units have the code key printed on the inside of that panel. Reading it before you call saves the technician time, and often saves you money.

Stop and call for help if any of this is true

These are not troubleshooting steps. They are reasons to stop.

• You smell gas. Do not flip switches, do not use your phone indoors. Leave the house, then call PG&E at 1-800-743-5000 or 911 from outside. Rotten egg smell near a furnace is never something to diagnose yourself.
• Your carbon monoxide alarm has sounded. Get everyone outside first and call 911. A cracked heat exchanger is not visible without dismantling the unit.
• You see soot, scorching or melted wiring anywhere on or around the furnace.
• The breaker trips again after you reset it. A breaker doing its job twice is telling you something real.

Beyond those, resetting a furnace repeatedly is worth avoiding. Lockouts exist to stop a unit running in a state that could damage it, and forcing repeated ignition attempts can turn an inexpensive part into an expensive one.

What is different about East Bay furnaces

Three things come up locally that national troubleshooting articles will not mention.

The first cold week is when everything fails. Around here a furnace can sit unused from April to October. The failures were already there, and switching on in the first cold snap is simply when you discover them. Running your heat for fifteen minutes in early autumn, while it is still easy to book a technician, is the cheapest insurance available.

A lot of older East Bay housing stock has wall or floor furnaces. Much of Alameda, San Leandro and older parts of Oakland still runs gravity wall units rather than a central forced air system. Those have no blower door, no filter and often a standing pilot light instead of an electronic ignitor, so several checks above simply do not apply. If your heater is a grate in the floor or a panel in a hallway wall, the relevant question is usually whether the pilot is lit.

Replacement means permits. Alameda County jurisdictions require a permit for furnace replacement, and it exists for good reason: it forces an inspection of the venting and gas connections. Any contractor who suggests skipping it is telling you something useful about how they work.