Obviously when your old furnace stops working completely, a replacement is on the horizon. But since you probably don't want to go without heat while you await a furnace replacement, it's better to be on the lookout for signs that your old furnace is about to fail -- rather than wait for it to fail completely. Here are four signs that your old furnace needs to be replaced ASAP.

Short-Cycling

If your furnace starts turning on, turning off again, and then turning on again in quick succession, this is often a sign that it's truly on its last legs. The motor may be overheating, causing the furnace to turn off before your home reaches the temperature set forth on the thermostat.

Climbing Heating Bills

If your heating bills seem to be climbing higher and higher even though the cost of fuel has remained about the same, this is a sign that your furnace is suffering from some serious trouble. Various parts may be loose or wearing out, causing the furnace to operate inefficiently. It's only a matter of time before these parts wear out completely and cause a complete lack of heat.

Yellow Pilot Light

If your pilot light is yellow, you should not only call your HVAC repair person, but also find somewhere else to stay until your furnace has been repaired or replaced. A yellow pilot light can often indicate that fuel is not being burned completely, which could lead to dangerous carbon monoxide accumulation in your home. In some cases, your HVAC repair person may be able to just repair the pilot itself, but if your furnace is quite old, you may be better off replacing the entire appliance before something else fails.

Rotten Egg Smells

If you notice the smell of rotten eggs near your furnace, turn it off, find somewhere else to stay, and call your HVAC technician. The rotten egg smell is most likely coming from a gas leak. Once the gas lines or heat exchanger inside an older furnace break, replacement is generally your best option.

Aging furnaces don't usually fail at the spur of the moment. There are usually signs of impending failure, like the ones above, that appear before you face a complete loss of heat. If you're not sure whether or not it's time to replace your aging furnace, speak with a heating contractor in your area. They can take a look at your system and make a personalized recommendation.