Signs Your Furnace Needs Replacing (and When Repair Still Wins)

A residential gas furnace typically lasts 15 to 20 years in a southern New Hampshire home. As the system ages, efficiency drops, repair frequency climbs, and the likelihood of a failure on the coldest day of the year goes up. There are six common warning signs that point to end-of-life and that should prompt a serious replace-versus-repair conversation. Knowing them helps you decide on your schedule rather than during an emergency.

At A.J. LeBlanc Heating, here is what we look for during inspections and what we tell NH homeowners.

1. Age

Age is the most important single factor. Equipment expected lifespans for NH installations:

Gas furnaces: 15 to 20 years
Oil furnaces: 20 to 25 years (often longer with diligent service)
Cast-iron boilers: 25 to 30+ years
Condensing boilers: 15 to 20 years
Heat pumps: 12 to 15 years

If your furnace is past 15 years old, replacement is on the table even if it is currently working. Past 20 years, continued repair rarely pays off.

2. Frequent repairs

The general rule we use: if the cost of an upcoming repair is more than half the cost of a new system, replacement is usually the better value. Some specific patterns that signal end-of-life:

Multiple service calls within a 12 to 18 month period
Repeated control board, ignitor, or capacitor replacements
A repair that requires parts that are increasingly hard to source
Diagnosed heat exchanger cracks or leaks

A heat exchanger crack is a hard-stop replacement. There is no safe repair for a cracked heat exchanger on a fuel-burning furnace.

3. Rising energy bills with no change in behavior

If your gas, propane, or oil bills have crept up year over year (after adjusting for fuel price changes) while your thermostat habits and household occupancy stayed the same, the furnace is likely losing efficiency. Symptoms that confirm this:

Combustion analysis showing higher flue gas temperatures than spec
Visible scorching, soot, or rust at the burner
A combustion CO reading higher than the manufacturer's range
Longer cycle times to reach the same setpoint

4. Uneven heating throughout the home

If certain rooms are noticeably colder than others, the furnace itself may be the cause:

A weak blower motor that cannot push air to the far rooms
A failing variable-speed control that no longer modulates correctly
A degraded heat exchanger that is not transferring heat fully to the air

Uneven heating can also be caused by leaky or undersized ducts. A good inspection identifies which.

5. Unusual noises

A furnace should run quietly with consistent burner ignition and steady blower operation. Sounds that signal trouble:

Banging or booming at startup: delayed ignition, dirty burner, or gas pressure issue
Rattling or vibration: loose panels, failing blower motor or bearings, or fan blade damage
Squealing: blower belt or motor bearings
Clicking that does not lead to ignition: failing ignitor or flame sensor
Whistling: often a return duct or filter restriction

6. Yellow burner flame or visible soot

A properly running gas or propane burner has a steady blue flame. A yellow, flickering, or sooty flame indicates incomplete combustion, which can mean:

Blocked combustion air intake
Cracked heat exchanger drawing return air into the combustion side
Dirty burner
Improper gas pressure

Yellow flame on a fuel-burning furnace is a safety issue. Stop using the system and call for service.

Repair vs. replace decision framework

When deciding:

Under 10 years old: repair almost always wins. Most components are replaceable individually.
10 to 15 years old: case by case. Repair small items, get a replacement quote for major repairs.
15 to 20 years old: replacement is on the table. Major repairs (heat exchanger, blower assembly, control system) usually trigger replacement.
Past 20 years: replacement usually wins unless the repair is minor and inexpensive.

What replacement actually involves

For a typical NH gas furnace replacement:

Manual J load calculation to size the new equipment correctly
Removal of the old furnace
Inspection and any modifications to the existing ductwork
Installation of the new furnace, gas line, condensate drain (for condensing models), and venting
Permits and inspections
Commissioning, including combustion analysis
Owner orientation on the new system

For homeowners thinking about a replacement, this is also the right time to consider:

Whether to switch to a heat pump or dual-fuel system
Whether to add a high-MERV media filter cabinet
Whether to address any duct sealing or insulation issues
What NHSaves rebates and financing options apply to your replacement (the federal 25C tax credit ended December 31, 2025)

Schedule a furnace assessment

If your furnace is showing any of the warning signs above, or is past 15 years old, contact A.J. LeBlanc Heating for an honest assessment of repair vs. replacement options. Serving New Hampshire families since 1928.

Frequently Asked Questions

How long does a furnace last in New Hampshire?

Gas furnaces typically last 15 to 20 years; oil furnaces often longer (20 to 25 years) with diligent service. Cast-iron boilers last 25 to 30+ years.

What is the most common reason a furnace needs replacing?

Age and accumulated wear. Specific failure modes that trigger replacement: cracked heat exchanger (most common safety-driven replacement), failed blower assembly on an old system, and accumulating repair frequency on aging electronics.

Is a yellow flame on my furnace a safety issue?

Yes. A yellow, flickering, or sooty flame on a gas or propane burner indicates incomplete combustion and possible CO production. Stop using the system and call for service.

When does it make sense to repair vs. replace?

Rough rule: if the repair cost exceeds half the cost of a new system, replacement is usually the better value, especially on equipment past 15 years old. A licensed contractor can walk you through the math for your specific situation.

Should I replace my furnace with a heat pump?

For most NH homes, it is at least worth running the numbers. A heat pump (or dual-fuel system using the existing fuel source as backup) often produces lower year-round operating cost, eliminates the need for separate AC, and qualifies for NHSaves rebates. The federal heat pump tax credit ended December 31, 2025; we track current incentives and can walk you through what applies to your home.