Should you repair or replace your furnace? Rules of thumb from ENERGY STAR and the DOE

By Jason Dearmin · Last reviewed November 14, 2026 · How we check facts

The ENERGY STAR checklist

ENERGY STAR says it may be time to call a professional contractor if:

• Your furnace or boiler is more than 15 years old (ENERGY STAR says a qualified furnace is 15% more efficient than a conventional furnace, and a qualified boiler 5% more efficient than a new standard model).
• Your heat pump or air conditioner is more than 10 years old.
• Your equipment needs frequent repairs and your energy bills are going up.
• Some rooms are too hot or too cold; ENERGY STAR notes the cause could be improper operation, duct problems or inadequate insulation.
• Your system is noisy, or your home has dust or humidity problems; ENERGY STAR says leaky ducts or inadequate equipment can be the cause.

The DOE flowchart

The DOE's Building America guide on HVAC energy renovations includes a basic decision chart for replacing a heating system:

1. If the system is not broken: replace only if it is over 15 years old; otherwise consider home improvements like insulation and air sealing, unless the system is very inefficient.
2. If it is broken: if repairs will cost more than half the cost of replacement, replace; if not, repair.
3. Very inefficient systems (the guide lists a draft-hood equipped gas furnace, AFUE below 80%, an oil boiler below 80% AFUE, or all-electric resistance heat) point toward replacement.

The DOE guide also says older combustion furnaces have efficiencies of about 56% to 70%, while modern conventional systems can reach 98%, and that upgrading from a 60% to a 90% efficient furnace or boiler in an average cold-climate house can cut fuel bills substantially when combined with duct sealing and insulation. Its savings table shows, for example, about $320 per year on a $1,000 fuel spend when going from a 65% AFUE furnace to a 95% AFUE furnace.

Get safety and sizing checked

The DOE guide lists items a furnace tune-up should include: testing for flue gas spillage, checking the vent connector and chimney, and checking the heat exchanger for cracks or leaks. It also says a new furnace may still be oversized, so ask your contractor to size equipment properly (Manual J) rather than matching the old unit.

Try the repair vs replace calculator to apply these rules to your own numbers, then get written quotes from licensed contractors.

Frequently asked questions

Is a 15-year-old furnace always worth replacing?

ENERGY STAR says to consider replacement at more than 15 years. The DOE flowchart replaces a working system over 15 years old; a technician can inspect the heat exchanger and efficiency.

What is the 50% rule?

In the DOE flowchart, if a system is broken and repairs would cost more than half the cost of replacement, it points toward replacing it.

Sources

• When is it time to replace? (ENERGY STAR)
• Building America Best Practices Series Vol. 14: Energy Renovations, HVAC (repair or replace flowchart) (U.S. Department of Energy)

Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.