No matter how advanced, sophisticated, or expensive your furnace is, in reality, it never will last forever. For the most part though, a furnace is expected to last 15 years before you develop headaches due to constant repairs. But you only can achieve that through proper and regular maintenance.
Now if you plan on taking advantage of your furnace for as long as it's possible, then you at least have to understand and learn the signs that it needs repairs; and with that skill, you'd be able to know when to call in a qualified service technician to fix it for you.
1 - You forgot to have your furnace serviced in a year or even longer.
Can you recall the last time you called a qualified HVAC technician to inspect your furnace? The fact that you no longer can determine when's the last time you have it checked only means that your furnace needs to be inspected for major problems right away. Without inspection or checkups, you probably haven't had the chance to replace your filters, too.
2 - There's water pooling around the furnace.
Furnaces always will be prone to leaks, and it could either be oil, gas, or water. While you wouldn't worry that much if water is leaking, but if the leak is gas or oil, then that's way more serious. As for the case of water leaking, the most obvious sign is pooling near or under the furnace. It is important that you call in a qualified furnace repair technician at http://nelsonsplumbingandheating.com/services/hvac-system-maintenance/ right away if there is a leak because it could be your condensate line failing or clogging, which eventually will result to the furnace's inability to produce heat.
3 - There's a smell of glass leaking.
Luckily, it's very easy to detect a gas leak because you will immediately smell gas. First of all, you need to prevent a major accident from happening by shutting the furnace down right away once you realize there's a leak. Open the furnace and find the gas supply valve and make sure this is turned off, too. Be reminded that lights shouldn't be turned on during a leak, more so that of lighting a match while you're waiting for the local furnace repair technician to arrive.
4 - You observe that your most recent utility bill has increased without doing anything different the last time.
Lastly, in case you see a sudden spike in your utility bills, it could mean that your furnace is working harder than usual to produce heat inside your home, which in turn relates to the use of more fuel than usual. The only way to figure out if there is a problem with the furnace, causing it to work hard is by calling in a qualified service technician at http://nelsonsplumbingandheating.com/services/hvac-system-maintenance/.