Furnace Not Heating? What to Check Safely

First: if you smell gas, leave

If you smell rotten eggs or sulfur in the house, don't troubleshoot. Get everyone out, and don't flip switches, light anything or use your phone until you're outside. Call your gas utility's emergency line or 911 from outside. After the utility says the house is safe, we can check the furnace.

If a carbon monoxide alarm goes off, get everyone outside and call 911. Carbon monoxide has no smell, so trust the alarm.

1. Check the thermostat

Set it to Heat, raise the set point a few degrees above the room temperature and set the fan to Auto. If the fan is on On, the blower runs between heating cycles and blows room-temperature air that feels cold. Replace the batteries if the screen is dim or blank.

Smart thermostats can hold an eco or away temperature, and many wait a few minutes before starting the furnace again. Give it five minutes after changing the setting.

2. Check power and the furnace switch

Every furnace, gas or electric, needs electricity. Look for a tripped breaker labeled furnace or air handler and reset it once. Near the furnace there's usually a switch that looks like a light switch; make sure it's on. It's easy to flip off by accident.

Also make sure the furnace's front panel is fully in place. Most furnaces have a door safety switch that won't let them run with the panel loose or off.

3. Check the filter

A clogged filter is one of the most common reasons a furnace shuts itself off. With too little air moving through it, the furnace overheats and a high-limit safety switch shuts the burners off. The blower keeps running to cool things down, so you feel cool air from the vents, and then the cycle repeats.

Replace the filter if you can't see light through it, using the size printed on the frame. Open supply vents and clear the return grilles. If the furnace keeps shutting off with a clean filter, call. Repeated overheating is hard on the heat exchanger.

4. Know the startup sequence

On a modern gas furnace, heat doesn't come on instantly. When the thermostat calls for heat, the furnace goes through a sequence:

• The draft inducer motor starts to clear the burners and venting.
• A pressure switch confirms the venting is clear.
• The hot surface igniter glows, or a spark igniter clicks.
• The gas valve opens and the burners light.
• The flame sensor has to prove the flame within a few seconds, or the furnace shuts the gas off.
• After roughly 30 to 90 seconds, the blower starts and warm air reaches the vents.

Where the sequence stops is a clue

Burners that light and then shut off after a few seconds often point to a dirty flame sensor. An inducer that runs with no ignition often points to a pressure switch or venting problem. Many furnaces also flash a code on a small light you can see through a window in the lower panel. Count the flashes and tell us when you call.

If the furnace locks out after several failed tries, turning its power off for a minute and back on resets it. Do that once. If it locks out again, call instead of resetting it over and over. Repeated resets won't fix the cause and can hide a safety problem.

Why a furnace blows cold air

These are the usual reasons:

• The thermostat fan is set to On instead of Auto.
• The furnace is in the first minute of a cycle, before the blower delay ends.
• The burners shut off on a safety limit, often from a clogged filter, and the blower is still running.
• The furnace locked out after several failed ignition attempts.
• On a heat pump system, the outdoor unit is in a defrost cycle. See our heat pump guide.

Electric furnaces and heat strips

Many Central Kentucky homes heat with electricity. An electric furnace or air handler uses heating elements, often called heat strips, that come on in stages. If only some of them work, the air feels lukewarm and the house heats slowly. A tripped breaker is a common cause, and many air handlers have two. Reset it once, and if it trips again, leave it off and call.

When to stop and call

Turn the system off and call if you notice any of these:

• A gas smell (leave first and call the utility from outside)
• A carbon monoxide alarm (leave and call 911)
• A burning smell that lasts beyond the first few minutes of the season, or any smoke
• Scorch marks, soot or melted wiring near the furnace
• A loud bang when the burners light
• A furnace that locks out again after one reset, or a breaker that trips again

How HVAC Doctor can help

Call or text (859) 684-3259 and one of the owners, Ahmad or Ammar, will answer. Mention our online special and the diagnostic is $39 (regularly $79), credited to the repair if you go ahead, you get a firm price before any work starts, and there are no overtime fees at night or on weekends. More on furnace repair and 24/7 emergency service.

Common questions