Manufacturers of heating and air conditioning appliances and even the U.S. Department of Energy suggest that you should automatically replace your HVAC equipment at the end of its expected service life, commonly estimated at about 10 to 15 years for AC systems and 15 to 20 years for a furnace or boiler. Yet most people don't heed this simplistic advice -- and given the cost of replacing these units, who can blame them? Instead, look for these telltale signs that your HVAC system needs replacing.

Furnace: Faulty Heat Exchanger

The heart of any gas-fired furnace is the heat exchanger. It transfers the heat from the gas burners to the air that circulates through the system. If a heat exchanger develops cracks or holes, replacement of the appliance is imminent. Heat exchangers can crack if they get too hot and expand beyond their design limits. This can actually be caused, over time, by dirty air filters, but it's most often just a condition of old age. Heat exchangers are vulnerable to rust if there's excessive moisture in the area ... or if the furnace is tied to a central AC whose interior exchanger has a problem with its condensate pan or drain, resulting in water dripping onto the furnace exchanger.

Symptoms that may indicate a bad heat exchanger include exhaust smell in your living space and soot around the furnace or heat registers in the house. If you see water under your furnace and/or corrosion inside the unit, have the heat exchanger checked for holes. A breach in the heat exchanger allows exhaust gases to mix with the system's circulated air and can result in dangerous levels of carbon monoxide inside the house. If your CO detector sounds off during the heating season, have the furnace checked immediately. 

Boiler

Like furnaces, gas boilers have heat exchangers that are critical to their performance and overall health. And if one goes bad, it's time for a new unit. In boilers, the exchanger is a chamber that holds water, so the telltale sign of a crack is water on the floor below the boiler. Boilers can also be damaged by water outside the appliance, such as by frequent or prolonged flooding. If your boiler unit has a lot of rust on the bottom, have it inspected by a boiler specialist.

Air Conditioners

Central AC systems generally don't last as long as furnaces, and their outdoor parts tend to need more maintenance, partly because they're exposed to the elements and temperature extremes. If you have to have the unit serviced or repaired more than once in a single season, it may be time to call some local HVAC companies to get some quotes on what it will take to replace your air conditioner. A malfunctioning part or improper system setup can cause all sorts of collateral damage to an AC system. Fixing the source problem won't reverse damage already done to other parts. For example, a burned out compressor often is caused by acid circulating in the system. You can replace the compressor, but if there are any leaks in the system or if the acid isn't completely flushed out, the problem might recur.

Other symptoms of very sick or tired AC equipment are excessive noise and reduced efficiency. Failing systems tend to make some terrible sounds, and while this could be due to a number of issues, not all of which support replacement, there's a good chance the problem is serious and should be checked out. Also, if you notice a marked reduction in efficiency, especially if you've followed a reasonable maintenance schedule, your AC may be on its way out.

Consider Operating Cost

Because your HVAC system is the biggest energy user in your home, operating cost is an important factor in determining when to replace it. For example, consider a broken furnace that's about 7 or 8 years old, roughly half of its projected service life. If repairing the unit costs about half the price of a new unit, should you repair or replace? If you repair, you'll have a furnace that runs more or less at the same efficiency for the next 7 or 8 years, at the end of which you'll buy a new unit regardless. If you replace now, you can get a more energy-efficient unit that will save on energy bills over the same 7 or 8 years, which means you'll likely save more in the long run.

Philip Schmidt writes for networx.com.

Updated February 28, 2018.