Furnace Not Turning On? 9 Causes New Jersey Homeowners Should Check First
The first cold morning of the season is when most New Jersey furnaces reveal that something failed over the summer. Before you call for service, a few quick checks can save you a visit, and knowing what to look for helps you describe the problem accurately when you do call. Here are the nine most common reasons a furnace will not start, roughly in the order our technicians check them.
1. The thermostat is not calling for heat
It sounds obvious, but a thermostat set to Cool or Off, a schedule that lowered the setpoint, or dead batteries in a battery-powered thermostat account for a surprising number of no-heat calls. Set the mode to Heat, raise the setpoint five degrees above room temperature and listen for the furnace to respond within a minute or two. If the display is blank, replace the batteries or check that the furnace power switch has not been turned off, which cuts power to the thermostat on many systems.
2. The furnace switch or breaker is off
Every furnace has a switch that looks like a light switch, usually on the unit or a nearby wall or joist. Painters, electricians and curious children flip it off. Check it, then check the breaker panel for a tripped breaker. If a breaker trips again after you reset it, stop and call for service; a motor or board is likely shorting.
3. The filter is clogged
A filter packed with dust restricts airflow until the furnace overheats and the high-limit switch shuts it down. After several cycles the control board may lock out entirely. Replace a dirty filter, wait 30 minutes and try again. If the furnace starts, keep a note to change filters every one to three months through the heating season.
4. The flame sensor is dirty
On gas furnaces the single most common no-heat failure is a flame sensor coated with a thin layer of oxidation. The furnace lights, runs for three to five seconds and shuts off, then repeats three times and locks out. A technician cleans or replaces the sensor in minutes. If you hear the furnace light and then shut down quickly, this is the likely cause.
5. The igniter has cracked
Hot surface igniters glow orange to light the burners and are brittle. When one cracks the furnace goes through its startup, the inducer motor runs, but the burners never light. This repair requires the correct igniter for your model, which our trucks carry for most common furnaces.
6. The pressure switch is not closing
Before the gas valve opens, the furnace confirms that the inducer motor is drawing exhaust properly. A blocked flue, a bird nest in the intake, a cracked hose, a clogged condensate trap on a high-efficiency furnace or a weak inducer motor will keep the pressure switch open and the furnace will not light. Look at the PVC pipes outside the house for obstructions, but leave internal diagnosis to a technician.
7. The condensate line is frozen or clogged
High-efficiency furnaces produce water. If the drain line clogs with sludge or freezes where it exits the house, a float switch or pressure switch stops the furnace. This is a frequent problem in New Jersey during deep cold snaps when condensate lines run through unheated spaces.
8. The limit switch tripped
The high-limit switch protects the heat exchanger from overheating. It trips because of a dirty filter, a failing blower motor, closed supply registers or a dirty evaporator coil sitting on top of the furnace. Repeated limit trips can crack a heat exchanger, so the underlying airflow problem needs to be found.
9. The control board or transformer failed
If there is no power at the thermostat and the furnace shows no lights at all with the switch and breaker on, the transformer or control board may have failed, sometimes because of a shorted thermostat wire. This is a diagnosis for a technician with a meter.
A note on high-efficiency furnaces
Condensing furnaces have more safeties than older models: a blocked intake or exhaust pipe, a frozen condensate drain or a failed pressure switch hose will each stop the furnace. If yours has a small LED that blinks a code on the control board, note the pattern before you call; it tells the technician where to start.
When to call for furnace repair
If you have checked the thermostat, switch, breaker and filter and the furnace still will not start, call us. If you smell gas, leave the house and call the gas utility first. If a carbon monoxide detector is sounding, get everyone outside and call 911. Heating Cooling NJ provides 24/7 furnace repair in every New Jersey county with flat-rate pricing approved before any work starts. Call 862-278-1141.
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