The core heating elements of a gas furnace are relatively straightforward, although modern furnaces also contain numerous features to help them operate more safely and efficiently. As a result, these appliances can fail for numerous reasons. While some problems originate with the combustion process, others may be due to faulty secondary hardware or sensors.

Since it's hard to know why your furnace isn't working, you should do a few things whenever your heat stops working. These three steps will help you rule out simple problems, confirm that your furnace is safe, and restore heating to your home as quickly as possible.

1. Rule Out Common Issues

If your heat doesn't work, especially when you turn it on for the first time in the fall or winter, you'll want to start by ruling out a few simple issues. First, confirm that your thermostat is on heating mode and that you've set the fan to "auto." Turn the setpoint a few degrees above the ambient temperature in your home and wait a few minutes to see if the heat turns on.

Next, confirm that the emergency shutoff switch is in the "on" position near the furnace. It's easy to bump this switch, so it's common to find it mysteriously shut off. You'll also want to check the pilot light and relight it if necessary for older furnaces. Newer furnaces use alternative ignition methods that don't require any maintenance.

2. Check For Safety Problems

The number one concern with any gas furnace is always a gas leak. Although serious leaks are uncommon, you should never discount the possibility. Check for any gas odors near the furnace, and follow proper gas safety precautions if you notice this telltale smell. Gas leaks can be incredibly dangerous, so don't wait to call your utility company if you notice one.

Less serious (but still potentially dangerous) problems can originate with the heat exchanger. If your furnace control board has an error display, check for any codes and reference them against your manual. Codes that point to overheating (sometimes called "over-limit") or flame rollout conditions can indicate heat exchanger problems, so you'll want to contact a professional immediately.

3. Avoid Making the Situation Worse

It's not fun being without heat in the winter, but attempting to run a furnace that refuses to start or repeatedly shuts down can cause more damage to the unit. Even worse, you may be trying to run a furnace with a dangerous problem, such as a cracked heat exchanger. Instead, switch the furnace off until a professional contractor can diagnose the problem.

If it's an especially cold night, you may want to consider making an emergency call or purchasing some space heaters to use temporarily. These options can be costly, but they're better than attempting to operate a potentially dangerous furnace. 

Reach out to a heating contractor to learn more.