Furnace Not Working? A Step-by-Step Checklist for New Jersey Winters

It always seems to happen on the coldest night of the year: the house is getting chilly, the thermostat says the heat is on, and nothing is coming out of the vents. Before you panic, a few safe checks can sometimes get the heat back on, and they'll help you give the technician useful information if you do need a repair.

Safety first: if you smell gas

If you smell rotten eggs or sulfur near the furnace or anywhere in the house:

1. Don't flip light switches, use phones inside, or try to find the leak.
2. Get everyone out of the house.
3. Call your gas utility's emergency line or 911 from outside.

Also, if your carbon monoxide alarm sounds, leave the house and call 911. Read our guide to carbon monoxide and furnace safety for more.

If there's no gas smell and no alarm, work through the checklist below.

Step 1: Check the thermostat

• Is it set to Heat?
• Is the set point at least a few degrees above the current room temperature?
• Is the display blank? Replace the batteries.
• Smart thermostat? Check for schedules, "away" modes or a lost Wi-Fi connection that may be holding the temperature down.

If you have a heat pump, make sure it's not stuck in an unusual mode, and note whether "Emergency Heat" or "Aux Heat" is showing.

Step 2: Check the furnace power switch and breaker

Most furnaces have a switch that looks like a regular light switch, mounted on or near the unit, often at the top of the basement stairs. It's easy to flip off by accident. Make sure it's on. Then check your electrical panel for a tripped breaker labeled furnace, air handler or HVAC.

Step 3: Replace the air filter

A severely clogged filter restricts airflow so much that the furnace can overheat. A safety switch then shuts the burners off. The blower may keep running and blow cool air, or the furnace may cycle on and off repeatedly. Swapping the filter takes two minutes and fixes more no-heat calls than you'd think. See our filter guide.

Step 4: Make sure the furnace door is closed

The blower compartment door has a safety switch. If the panel isn't seated properly, the furnace won't run. After changing the filter, make sure the door is firmly in place.

Step 5: Check the gas supply

Is the gas valve to the furnace turned on (the handle runs parallel with the pipe)? Are other gas appliances, like the stove or water heater, working? If nothing gas is working, contact your gas utility.

Step 6: Look at the status light

Most modern furnaces have a small window with a blinking LED on the control board. The blink pattern is a diagnostic code, often explained on a label inside the panel. Write down the pattern (for example, "3 blinks, pause") before calling. It tells a technician a lot.

Step 7: Check vents and the exhaust pipe

High-efficiency furnaces vent through white PVC pipes on the side of the house. Snow drifts, ice or nests can block these pipes and shut the furnace down. Carefully clear snow away from intake and exhaust pipes after storms. Inside, make sure supply vents and return grilles aren't blocked.

Step 8: Try one reset

Turn the thermostat down, switch off the furnace power for 30 seconds, then turn it back on and raise the thermostat. If the furnace lights and runs, keep an eye on it. If it locks out again, stop resetting it. Repeated resets can mask a safety issue.

Common causes we find on no-heat calls

Heat pump owners: what's normal

Heat pumps feel different than furnaces. The air at the vents is warm but not hot, and the outdoor unit periodically goes into defrost, sometimes with steam rising from it. That's normal. What isn't normal: ice building up and staying on the outdoor coil, the unit running constantly without warming the house, or the auxiliary heat running all the time. Learn more in our guide to heat pumps in New Jersey winters.

While you wait for a repair

• Close curtains and doors to unused rooms to hold heat where people are.
• Use space heaters carefully: keep them three feet from anything flammable, plug them directly into the wall, and never leave them unattended.
• Never use a gas oven, grill or generator indoors for heat.
• If temperatures are dropping well below freezing, open cabinet doors under sinks on exterior walls and let faucets drip to protect pipes.

When to call for heating repair

Call us for furnace and heating repair if the checklist doesn't restore heat, the furnace keeps locking out, you hear banging or squealing, or you notice a burning smell that lasts more than the first few minutes of the season. We offer same day service and 24/7 emergency help across Ocean, Monmouth and Burlington counties.

Prevent the next breakdown

Most mid-winter failures, like dirty flame sensors, worn igniters and clogged filters, are exactly what a fall tune-up catches. Our $89 heating tune-up special and the Prime Comfort Club ($19/month, with one heating and one AC tune-up every year plus priority scheduling) keep your system ready before the cold arrives. Inland towns like Jackson, Manchester and Medford see the coldest nights in our area, so don't wait for the first freeze.