1. Strange Noises. While it is normal for a furnace to make a bit of noise, it shouldn’t make extremely loud banging or grinding noises. If your furnace is making much noise, then it might be in need of a belt change, might have an ignition problem, or might need another component replaced.
2. Raising Temperature with no Changes. If you raise the temperature of your furnace’s thermostat, but the temperature in the room doesn’t get any higher, then it could be in need of repair. Usually, you only have to bump a furnace’s thermostat up a couple of notches to warm the room, but if doing so doesn’t reap any results, then there might be a problem with your pilot light.