When a furnace in San Leandro runs the blower but no heat comes out, or does nothing at all on the first cold morning of the season, the cause is usually easy to name once you look: a thermostat setting, a clogged filter that trips a safety switch, an igniter or flame sensor that fails, or a problem with power or gas. Heating calls here bunch up between November and March, often on the first cold snap after the furnace has sat unused since spring.
Safety first
- Gas smell (rotten eggs): do not relight anything, do not flip light switches and do not use your phone inside. Leave the house and call 911 or your gas company from outside.
- Carbon monoxide alarm: get everyone outside and call 911.
- Yellow flame or soot: a yellow, lazy or flickering burner flame (or a yellow pilot on an older furnace) instead of a steady blue one, soot around the furnace, or an exhaust smell means poor combustion. Switch the furnace off at the thermostat and call.
- Normal: a dusty burning smell for the first few minutes of the season is dust burning off and fades quickly. A smell that lingers, or smells electrical, is not normal.
Safe checks before you call
- Thermostat. Set to Heat, a few degrees above the room temperature, with fresh batteries and no schedule overriding it.
- Fan setting. If the fan is set to On, the blower runs all the time and blows room-temperature air between heating cycles. Set it to Auto.
- Filter. A clogged filter makes the furnace overheat. A safety switch then shuts the burners off while the blower keeps running, so you feel cold air. Replace it.
- Power. Check the breaker and the service switch on or near the furnace, which looks like an ordinary light switch.
- The door. The lower panel presses a safety switch. If the panel is not fully seated, the furnace will not run.
- Gas. If your stove burners light, gas is reaching the house. The shutoff valve on the furnace’s gas line should be open, with the handle in line with the pipe.
- The status light. Many furnaces have a blinking light you can see through a small window in the lower door. Count the blinks and compare them with the chart on the door label, and write the code down before you turn the power off.
- High-efficiency furnaces (white plastic vent pipes): make sure the condensate drain tube is not kinked or clogged and the vent pipes outside are not blocked by leaves or a nest.
Most furnaces built in recent decades use electronic ignition and have no pilot to relight. On an older furnace with a standing pilot, relight it only by following the instructions on the furnace label, and call if it will not stay lit. One power reset is fine; if the furnace locks out again, call.
What the symptom points to
Blower runs, air stays cold. The fan setting, an overheating furnace shut down by its limit switch, or burners that light and then drop out.
Burners light for a few seconds, then go out. Most often a dirty flame sensor that cannot prove the flame, so the control shuts off the gas. Gas pressure and wiring are the other suspects.
You hear the small vent motor, but there is no ignition. A pressure switch that is not closing (a blocked vent or, on high-efficiency furnaces, a clogged condensate trap) or a cracked igniter that no longer glows.
Nothing happens at all. No power, the thermostat or its wiring, the door switch, a blown fuse on the control board, or a failed transformer or board.
Short cycling (on and off quickly). A dirty filter causing overheating, a weak flame-sensor signal, a thermostat problem, or a furnace that is too large for the house.
New noises. A boom when the burners light points to delayed ignition and needs attention soon; squealing or grinding usually comes from the blower motor.
What a technician checks
The technician watches the full start-up sequence, from the thermostat call through the vent motor, pressure switch, igniter and gas valve to the flame and the blower, and finds where it stops. From there: the flame-sensor signal, gas pressure, venting, the burner flame and heat exchanger, the limit switch, the blower motor, and a carbon monoxide check. You hear whether the furnace is safe to use, with parts, labor and any safety items priced separately before work starts. See furnace repair for the visit and the furnace repair cost guide for typical costs. If the furnace is old and needs a major part, the repair-or-replace guide walks through the decision.
Heating with a heat pump instead? See heat pump repair. An old or miswired thermostat? See thermostat installation. No heat on a cold night? See emergency HVAC.
Local notes and prevention
Many furnaces in San Leandro, San Lorenzo and Castro Valley sit in a hall closet or the garage, and a hall closet often turns into storage. Keep boxes, paint and cleaning products away so the furnace gets air and nothing flammable sits near the burners. Keep a working carbon monoxide alarm near the bedrooms, change the filter regularly, and book furnace maintenance in the fall, before the first cold week.