Decision Guide

Furnace Repair vs Replace: Decision Framework with Real LA Math

Q: Should I repair or replace my old furnace?

Five-question framework: (1) age — under 8 years lean repair, 15+ lean replace, 20+ replace almost always; (2) what's failing — ignitor/sensor/pressure switch always repair, heat exchanger crack always replace (CO safety); (3) heat exchanger condition (cracks = condemn the unit, not optional); (4) heat pump conversion option — LADWP territory has $1,250–$2,500/ton heat pump rebates that flip the math; (5) ownership horizon — selling within 2 years lean repair, owning 5+ replacement math usually wins. The 50% rule is a starting point, not the answer.

Q: When does a furnace need to be replaced?

Replace when any of these are true: cracked heat exchanger (carbon monoxide safety, not negotiable at any age); furnace 18+ years old (efficiency gap alone justifies it — old 80% AFUE wastes 16–20% more gas than 96%); multiple major components failing within 12 months; AFUE under 80% (very old units losing money every cycle); LADWP territory homeowner with heat pump conversion eligibility (rebates close the upfront cost gap).

Q: How long should a furnace last in Southern California?

SoCal mild winters mean lighter furnace duty cycle than cold-climate states — 18–25 year service life is typical for gas furnaces if maintained. Heat exchanger expected life is 15–25 years. Inducer motors and control boards typically last 12–18 years. Hot surface ignitors last 4–8 years and are routine replacement items. The mild climate means most LA furnaces die from age-related component failures, not cumulative wear, which is why post-15-year systems start cascading repairs.

Q: Is it worth replacing an 80% AFUE furnace with 96%?

Operating-cost math alone: 80% AFUE wastes 20% of gas energy through the flue; 96% AFUE wastes 4%. That's a 16% reduction on gas heating bills. LA market typical gas heating bill is $400–$800/year (we don't run heat much). Annual savings: $65–$130. 10-year operating savings: $650–$1,300. Replacement premium for 96% over 80% AFUE: $400–$900 typically. So pure efficiency math justifies the upgrade over a 10-year horizon, but barely. The bigger drivers for furnace replacement are usually age + component failures, not efficiency alone.

Q: Should I replace my furnace with a heat pump?

LADWP territory: usually yes. LADWP heat pump rebate $1,250–$2,500/ton typically saves $3,750–$7,500 on a 3–4 ton system, which closes the upfront gap with a gas furnace replacement. Operating cost: heat pumps use 60–70% less energy than 80% AFUE furnaces. SCE territory (most of OC, Riverside, SB, Ventura): TECH Clean California fully reserved November 14, 2025 — no current heat pump rebate stack, so the upfront premium isn't offset. Math is harder to justify in SCE territory until rebates reopen. Federal IRA Section 25C ($2,000 heat pump credit) was terminated December 31, 2025 under OBBBA and is no longer available.

Q: How much does it cost to replace a furnace in Los Angeles?

Practitioner pricing for LA market 2026: standard 80,000 BTU 96% AFUE gas furnace replacement $4,500–$6,500 like-for-like. Premium variable-speed (Carrier 59TN6, Trane S9V2, Lennox SLP99V): $6,200–$8,500. Includes permit, gas line work if needed, removal, code-compliant install. Heat pump replacement (different system entirely): $11,500–$18,500 install, with LADWP rebate $1,250–$2,500/ton. Avoid contractors quoting under $3,800 for a full furnace replacement — they're cutting code compliance, equipment quality, or skipping the permit.

Q: Can my furnace be repaired if the heat exchanger is cracked?

We don't repair cracked heat exchangers. A crack leaks combustion gases, including carbon monoxide, into the air the furnace blows through your house. CO is colorless, odorless, and lethal at concentrations homeowners can't detect without a meter. We condemn the unit, shut off the gas, and quote replacement. If a tech tells you 'it's just a small crack, we can patch it' — get a second opinion and walk away from the first one. This is one of the few cases where the 50% rule doesn't apply: even at 50% of replacement cost, putting a new heart in a 12-year-old body whose other components are also aging is bad math.

Furnace repair vs replace is the second most common big-decision conversation we walk customers through (after AC). The 50% rule—replace if repair exceeds 50% of new—is a starting point, not the answer. After eight years running this calculation in LA homes, here's the framework we actually use, accounting for system age, AFUE rating differential (80% vs 96%), heat exchanger condition, gas-to-electric conversion timing, LADWP heat pump rebates that change the replacement math, and your actual ownership horizon. This is the framework we use on real service calls, not a marketing brochure. CSLB #1138898 (C-20).

The 50% rule and what it misses for furnaces

The conventional advice: if repair cost exceeds 50% of replacement cost, replace. Examples worked through:

$1,200 inducer motor on $4,500 furnace replacement = 27% → rule says repair
$2,800 heat exchanger on $4,500 furnace = 62% → rule says replace

The rule misses several things that matter for furnaces specifically:

Heat exchanger condition—a crack is a carbon monoxide safety issue, not a cost decision
AFUE efficiency differential—old 80% AFUE wastes 16–20% of gas vs modern 96%
Heat pump conversion option—replacing furnace with heat pump opens the LADWP rebate stack
Refrigerant transition—if your AC is paired with the furnace, R-410A vs R-454B factors in
Gas-to-electric trajectory—California regulatory direction favors electrification; gas equipment may face restrictions
Ownership horizon—buying a furnace you'll own 5 years vs 15 years is a different calculation

The five-question framework we actually use

Question 1: Furnace age

0–8 years: lean repair (warranty likely covers components, lots of life left)
8–15 years: case-by-case—it depends on what's failing
15–20 years: lean replace (efficiency gap, components compounding failures)
20+ years: replace almost always (efficiency alone justifies it)

Question 2: What's the actual repair?

Not all furnace repairs are equal. The component drives the decision more than the dollar amount.

Hot surface ignitor failure ($245–$385): always repair regardless of age. Cheap, common, predictable.
Flame sensor cleaning ($245–$385): always repair. Half the cost is the diagnostic, the cleaning itself is 10 minutes.
Pressure switch ($385–$685): always repair. Single-purpose component that doesn't predict other failures.
Gas valve ($585–$1,485): repair under 12 years; lean replace at 12+ (other components likely closing in on end of life).
Inducer motor ($785–$1,485): repair under 12 years; replace at 14+ (motors usually fail near end of system life).
Control board ($585–$1,200): repair under 12 years; case-by-case at 12+.
Heat exchanger ($2,200–$3,500): nearly always replace the unit—see safety section below.
Multiple component failures within 12 months: replace regardless of age. The system is signaling end of life.

Question 3: Heat exchanger condition (the safety question)

A cracked heat exchanger leaks carbon monoxide into the heated air your furnace blows through the house. We don't repair this. We condemn the unit, shut off the gas, and quote replacement. This is the one case where the 50% rule does not matter:

Heat exchanger itself: $1,000–$1,800 in parts
Labor: 4–6 hours
Total repair: $2,200–$3,500
On a 12-year-old furnace, you're paying $3,000+ to put a new heart in an aging body that has 3–5 years left on the rest of components
Replacement furnace: $4,500–$8,500 with 15–20 year design life

The math rarely works. And if a tech tells you "the heat exchanger is fine, we can just patch the small crack"—get a second opinion and walk away from the first one. CO is colorless, odorless, and lethal at levels homeowners can't detect.

Question 4: Heat pump conversion option

Most LA homes facing furnace replacement should evaluate heat pump conversion before defaulting to a gas-to-gas swap. The math depends on territory.

LADWP territory (most of LA city proper):

New heat pump install: $11,500–$18,500
LADWP heat pump rebate: $1,250–$2,500/ton ($3,750–$7,500 typical for 3–4 ton)
Net heat pump install: $7,750–$13,500
Vs gas furnace replacement: $4,500–$8,500
Premium for heat pump after rebate: $3,250–$8,000 over gas furnace
Operating cost: heat pump uses ~60% less energy than 80% AFUE furnace
Trajectory: California regulatory direction favors electrification

SCE territory (most of OC, Riverside, SB, Ventura):

TECH Clean California fully reserved November 14, 2025—no rebate stack on heat pump currently
Heat pump premium not offset by rebates
Math: gas furnace replacement often wins on upfront cost
Long-term operating savings still favor heat pump if you own 8+ years

Federal IRA Section 25C ($2,000 heat pump credit, $600 furnace credit) was terminated December 31, 2025 under OBBBA and is no longer available for 2026 installs anywhere. Don't let any contractor tell you otherwise.

Question 5: Ownership horizon

Selling within 2 years: lean repair (next owner inherits replacement decision; you don't get the operational savings)
Owning 3–5 years: case-by-case; repair often makes sense if no compounding issues
Owning 5+ years: replacement math usually wins on operational savings + reduced repair frequency

Operating cost math: 96% AFUE vs 80%

Modern 96% AFUE furnace vs old 80% AFUE on the same SoCal home:

80% AFUE wastes 20% of gas energy out the flue
96% AFUE wastes 4%
Energy reduction: 16% on gas heating
LA market typical gas heating bill: $400–$800/year (mild winters)
Annual savings on the AFUE upgrade: $65–$130
10-year operating savings: $650–$1,300

Heat pump comparison:

Heat pump efficiency 250–400% (vs 96% gas furnace)
Operating cost reduction vs 80% AFUE: 60–70%
Annual savings: $250–$550
10-year operating savings: $2,500–$5,500

The cases where furnace repair always wins

Even on aging furnaces, repair is the right call when:

Hot surface ignitor failure on any age system. Cheap fix ($245–$385), the rest of the system is still working.
Single-component failure on a furnace under 8 years old. Replace the part, system has many years left.
Customer has a limited ownership horizon (selling within 1–2 years). Replacement won't pay back.
Customer has a budget constraint AND existing furnace has no compounding issues.
Furnace is matched to specific equipment requirements (zoned system with specific BTU pairing) where replacement requires major redesign.

The cases where replacement wins

Cracked heat exchanger at any age—safety issue, not optional.
Furnace 18+ years old—efficiency gap alone justifies it.
Multiple major components failing within 12 months—system signaling end of life.
AFUE under 80% (very old units)—losing money on gas every cycle.
Customer in LADWP territory with heat pump conversion eligibility—rebate stack closes the upfront gap.
R-410A AC paired with furnace replacement—opportunity to replace both with a heat pump (one project, R-454B compatible).

Real-world examples

Example 1: Repair wins

Sherman Oaks home, 7-year-old Carrier 58CTX 80,000 BTU furnace. Hot surface ignitor failed during the November cold snap. Repair cost $295. System has 10+ years of expected life left, no compounding issues, AFUE acceptable for age. Decision: ignitor replacement, no further action. Customer joined our Silver Comfort Club ($349/year) for fall tune-up coverage going forward.

Example 2: Replacement wins (gas-to-gas, SCE territory)

Glendale home, 17-year-old Goodman GMS80 80% AFUE 80,000 BTU. Customer reported reduced heating, clicking sounds, gradual decline. Diagnosis: control board failing AND inducer motor showing wear AND heat exchanger showing surface rust patterns (not cracked but flagged for monitoring).

Repair option A: control board + inducer motor = $1,500
Repair option B: same $1,500 + monitor heat exchanger = same cost + safety risk
New equivalent: Carrier 59TN6 96% AFUE 80,000 BTU = $5,800 installed
Replacement = 38% of repair cost on this 17-year-old system
17-year-old AFUE 80% → new 96%: 16% gas reduction = $80–$130/year savings
Customer planned to own home 8+ more years
Glendale is GWP territory (Glendale Water and Power) with its own rebate program; SCE territory neighbors had TECH waitlisted
Decision: replacement won despite a 38% repair-vs-replace ratio because of system age and component cascade
Federal IRA 25C terminated December 31, 2025—not in this math

Example 3: Replacement wins (gas-to-heat-pump, LADWP territory)

Brentwood home (LADWP territory), 14-year-old Carrier 58TN6 80% AFUE 100,000 BTU furnace + paired Carrier 24ANB7 4-ton AC. Both reaching end of life. Customer wanted electrification.

Option A: replace both with same equipment, $11,800 (gas furnace + AC)
Option B: replace with heat pump system, $14,800 install
LADWP heat pump rebate: $1,250/ton × 4 = -$5,000
Net heat pump: $9,800
LADWP rebate makes heat pump cheaper than gas-to-gas replacement
Operating cost: heat pump 60% less than the old 80% AFUE furnace
10-year operating savings: $3,500–$5,000
Decision: heat pump won on both upfront cost AND operational savings
Federal IRA 25C terminated December 31, 2025—not in this math

Example 4: Heat exchanger crack edge case

Pasadena home, 12-year-old Lennox 80% AFUE furnace. Fall tune-up combustion analyzer found CO levels rising during the run cycle, visual inspection confirmed a small crack in the heat exchanger.

Heat exchanger replacement cost: $2,800
New equivalent furnace: $5,200
Repair = 54% of replacement—just over the 50% threshold
System is 12 years old; rest of components have 5–7 years of typical life left
Investing $2,800 in a 12-year-old system that will need full replacement in 5 years anyway = bad math
Customer chose replacement (gas-to-gas given budget; Pasadena is PWP territory with its own program)

Honest opinion: when heat exchanger is involved, age and trajectory matter more than the 50% rule. We always discuss replacement on heat exchanger calls, even if the technical 50% math sits right at the threshold.

Refrigerant transition factor (if AC is paired)

If your gas furnace is paired with an AC system that's also aging, refrigerant transition matters:

R-410A (older AC systems, pre-2025): still serviceable, but supply tightening and repair costs rising year over year.
R-454B (new AC systems 2025+): A2L refrigerant, requires updated equipment and revised line-set practices.
You can't retrofit R-410A to R-454B. When major refrigerant work is needed, it's a binary repair-or-replace decision.

If your AC is also 12+ years old and on R-410A, replacing both furnace + AC together (or converting to a heat pump) is more cost-effective than replacing them in separate years. For complete refrigerant transition context, see our California HVAC Code 2026 guide.

How we handle this conversation

Honest practitioner approach, in order:

Diagnose the actual failure first—not "replace your furnace" sales pitch on arrival.
Heat exchanger inspection always—combustion analyzer + visual on every furnace service.
Quote both options when the call is borderline—repair vs replace side-by-side with real numbers.
Consider heat pump conversion—especially in LADWP territory where the rebate stack makes the math obvious.
No pressure tactics. Most LA HVAC chains push replacement on furnace calls because that's where the margin is. We don't.

If you want a real diagnostic with both options quoted—repair and replacement—call (424) 766-1020 for West LA dispatch, or see our six regional contacts. Diagnostic visit $89 standard / $149 after-hours, waived if you proceed with the work. Free written estimates on installation and replacement.

Frequently asked questions

Should I repair or replace my old furnace? +

When does a furnace need to be replaced? +

How long should a furnace last in Southern California? +

Is it worth replacing an 80% AFUE furnace with 96%? +

Should I replace my furnace with a heat pump? +

How much does it cost to replace a furnace in Los Angeles? +

Can my furnace be repaired if the heat exchanger is cracked? +