When Should a Gas Furnace Be Replaced?

A gas furnace offers one of the most efficient ways to heat your home. Modern types are so efficient that they don't even have a regular chimney. The exhaust is usually vented through a PVC pipe that exits at the side of the building, and virtually no heat is lost through this process. Although efficiency is one reason to replace a gas furnace, there are other reasons that are equally important.

Hot water heater and furnace in basement
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A good rule of thumb is to replace your gas furnace if it is 15 years old.

15-Year Mark

Although a gas furnace is designed to run for a minimum of 20 years or more, it will begin to break down as it gets nearer to the end of its service life. Couple that with how modern furnace efficiency technology is advancing every year, and by the 15-year mark, your furnace has become obsolete. Even if it is running right, the 15-year mark tells you that you should, at least, begin the process of looking for a new furnace.


The best modern gas furnaces convert nearly 97 percent of the fuel burned into usable heat for your home. With spiraling gas prices, the potential for savings, just by having a more fuel-efficient furnace alone, may warrant a change of furnaces. A 97-percent-efficient furnace will cut about 20 percent or more off of your gas heating bill every month if you have an older, 80-percent-efficient furnace. An upgrade would pay for itself in only a few years.

Thermostat Fluctuations

Thermostat fluctuations result from the furnace not being able to keep up with the heating demands of the house. Some rooms may be cool, and others may be hot, so repeatedly adjusting the thermostat for these problems may signal that your furnace is no longer up to the task. This is especially true if there is an added on room that was not part of the original floor plan. The original furnace was sized for a particular amount of square footage, and by making the home larger, the furnace will be struggling to keep up.

The 25-Percent Repair Consideration

During the life of your furnace, you can expect that it will break down a time or two. Most of the time it is a simple repair that costs less than $200 or so. After that, the furnace may work for years without another hitch. If the repair is serious and will cost 25 percent or more toward the cost of a new furnace, however, consider investing in a new one before getting the old one repaired. This is especially true if it is an older furnace to begin with because breakdowns will happen more often as it nears the end of its service life.