Most furnaces last between 15 and 20 years. However, that figure can vary depending on how well the furnace is maintained, how often it is used, whether or not it is the proper capacity for your home, and the quality of the furnace itself, among other factors.

While your furnace will make some noise as it operates, it shouldn’t make much. You should hear the burners of your furnace (which sound similar to a gas stovetop), as well as the fan, and possibly the electronic ignition (if you don’t have a pilot light). However, you should be wary that other noises can indicate an issue. Keep an ear out for any and all of the following indicators that your furnace is failing:

• Rattling: Rattling can occur in your furnace if a component has come loose. You should have a professional take a look to locate the component and to secure it, before it causes additional damage to your furnace.
• Banging: Banging noises often mean that a component has become completely detached. Again, call a professional right away to take a look.
• Clicking: If you have an electronic ignition, it will click once or twice every time your furnace turns on. However, if you have a bad ignition switch, dirty burners, or restricted gas flow, your igniters may be slow to create a flame. Have an expert take a look at your ignition source. You may need a new ignition switch or another repair.
• Loud bangs: Loud banging is an indication that gas has built up before ignition. Gas can build up and late ignition will cause a small explosion — which is an obvious hazard and may damage your furnace even more. Get help from an HVAC technician, and shut off your furnace and its gas source if you smell gas near the furnace (more on that below).

It’s concerning if you smell gas near your furnace. Natural gas smells like rotten eggs (thanks to an additive in the gas), which is an indication that there is a leak in your furnace or its gas source. While it’s okay to smell a tiny bit of gas near your furnace during its ignition sequence (if you have an electronic ignition), you shouldn’t smell gas when your furnace is off, or if it has been running for a minute. If you smell gas, then you may have a dangerous leak, and you should act right away to close off the source of the leak. Here’s what you should do if you smell gas leaking from your furnace:

1. Turn off your furnace at the thermostat — there should be an “off” setting.
2. Open nearby windows to dissipate any gas that has built up. Leave these windows open for several minutes to ensure that the space is safe.
3. Turn off the gas valve to your furnace. You should be able to locate the gas line that approaches your furnace, and the gas valve switch. Turn the switch off.
4. If your gas valve is malfunctioning, shut off the gas main for your home.
5. Contact an HVAC technician to take a look at your furnace and to assess the source of the leak.

Take note, you should not breathe in natural gas, since it is dangerous to your health. In addition, if you note that the gas levels are high and you think it is unsafe to turn off the gas yourself, you should vacate your home and contact emergency services to report a gas leak.

If your heating bill is constantly on the rise, then your furnace and HVAC system may be losing their efficiency, which can be an indication of a number of problems. You may have issues with your vents and air ducts, you may have a bad furnace burner, and you may have a clogged air filter. If some of your vents are closed, you can create a cool zone in your home, which — if the thermostat is in this zone — could trigger your furnace to work overtime. In addition, you may have a blockage in your air ducts (e.g. an animal nest, dust, etc.), which is hindering the performance of your furnace. If you have bad burners in your furnace, then your furnace won’t heat up as efficiently as it was designed. Your furnace’s burners can become dirty and clogged over time, which will restrict the flow of gas to create a flame in your furnace. You may also have a clogged air filter. Your air filter will, over time, capture dust and debris from the air that is forced through the filter (which is the purpose of the filter). Eventually, however, this debris can build up and choke the air supply to your furnace, which means that your HVAC system won’t have the flow that it depends on.

In general, you should change your filters once every 30 to 90 days. That said, you may want to change your filters more or less often depending on a few factors. For example, if you are in a highly polluted area, or an area that has a lot of wind and airborne dirt, you’ll have to swap out your air filter more often. You should also replace your filter more often if you have animals in the house (whose dander and hair can clog the air filter), or if you have lots of activity in the house. Likewise, you should replace your filter more often if you or any other residents in the house have allergic sensitivities. On the other hand, if you don’t spend much time at home (perhaps it is a vacation home), or you don’t turn on your furnace all that often, then you won’t have to replace the filter at those intervals. You can always check the box for the manufacturer’s recommended air filter change interval. It’s also fairly easy to inspect your filter to see if it is gunked up. If you can locate your filter near your heating unit, you should be able to pull it out and see if it is covered with dust and debris.

Your furnace is outfitted with either an electronic ignition or a pilot light. Your furnace relies on these components to ignite the burners when the gas clicks on for a heating cycle. If you can’t tell whether or not you have a pilot light or an electronic ignition, you should be able to either see the pilot light (which will be a small blue flame), or hear the ignition clicking on when your furnace begins to heat. Both of these components can fail, and your thermocoupler (which is a failsafe that closes off the gas source to your pilot light) can also fail — which means that your furnace won’t be able to heat up. If you have a pilot light that is out, you may be able to reignite it, following the instructions in the next section.

If your pilot light is out, you should follow the following steps to reignite the pilot — you’ll need a long-stem lighter:

1. First things first, you should shut off your furnace. You should be able to locate a switch with three settings: On, Off, and Pilot. Turn the switch to the Off position for the time being.
2. Let any residual gas dissipate. If you smell natural gas (which smells like rotten eggs), then it is dangerous to light the pilot. Open nearby windows, and wait for several minutes until you’re certain that there is no gas in the air. If you still smell gas after some time, you should turn off the gas to your furnace and contact a professional — do not attempt to ignite your pilot light.
3. Once the gas has dissipated, you can turn the control knob to Pilot. Also, you should be able to locate a button that says Reset. This primes the pilot and thermocouple so that your light will stay on after it is lit. Hold the Reset button down and light the pilot with your lighter. Release the Reset button after you have a flame coming from the pilot. If it remains on, you’re set!
4. Finally, turn the control knob to the On position. Monitor your furnace to make sure it works the first time it comes on. If your pilot light goes out again, you may need to maintain or repair the pilot.

Once again, be wary of gas buildup. If your pilot light doesn’t ignite after your first try, be sure to let any gas dissipate before trying again, or call a professional.