Twenty years is the age where furnaces start getting judged unfairly in both directions. One contractor tells you it's a ticking time bomb. Your neighbor tells you his ran for 31 years and he only replaced it because he remodeled. Both stories are true, and neither one is about your furnace.
Here's how we actually work through this with homeowners in Federal Way, Kent, Puyallup and Olympia — with the numbers, and with the honest admission that sometimes the right answer is "leave it alone for now."
What 20 years actually means for a gas furnace
Most gas furnaces are engineered for a 15-to-20-year service life. In Western Washington they often beat that, because our winters are mild — a marine climate that rarely drops below 25°F asks far less of a furnace than the Midwest does. Fewer run hours, fewer ignition cycles, less thermal stress on the heat exchanger.
So a 20-year-old furnace in Puget Sound isn't automatically finished. What it is, without exception, is past the point where you should be surprised by anything. Specifically:
- The manufacturer's parts warranty is long gone. Every repair is out of pocket.
- Parts availability starts thinning. Control boards, specific ignitors, and inducer assemblies for 2006-era models are still findable, but not always same-day. That matters when it's 34°F outside.
- The heat exchanger has done 20 seasons of heating and cooling cycles. Metal fatigue is cumulative, not sudden.
- The blower is probably a single-speed PSC motor. Loud, and the least efficient part of the whole system when it runs.
Age is a risk factor, not a verdict. Condition decides.
The five things that turn "keep it" into "replace it"
1. The heat exchanger has rust, scorching, or a crack
This is the one non-negotiable. A cracked heat exchanger is a combustion safety issue, not a repair line item — on a 20-year-old unit it's effectively the end of the road, because the part cost plus labor lands close to a new furnace with no warranty to show for it. If a technician red-tags your furnace for this, get a second set of eyes if you want, but don't run it in the meantime.
2. You've paid for two or more repairs in the last two winters
One ignitor at the low end of the typical $150 to $1,500 repair range is maintenance. An ignitor, then a pressure switch, then a control board is a pattern. Money spent on a 20-year-old furnace buys you time, not value — it doesn't extend the heat exchanger's life or come with a warranty.
3. Rooms are uneven and the blower is straining
On older systems this is sometimes the ductwork, not the furnace. Worth diagnosing separately. But a tired single-speed blower pushing air through 20-year-old flex duct is a comfort problem no repair fixes.
4. You want air conditioning
This changes the math completely, and it's the most common reason a still-working furnace gets replaced in our service area. If you're going to spend money on cooling anyway, you're no longer comparing "repair vs. replace." You're comparing a furnace plus a separate AC system against one piece of equipment that does both.
5. Yellow burner flames, soot, or a CO alarm event
Stop and call. That's a diagnostic visit today, not a planning conversation.
The three real options, priced
| Path | Installed cost (standard markets) | What you get |
|---|---|---|
| PathKeep it, maintain it | Installed cost (standard markets)Tune-up cost only | What you getHeat this winter, no warranty, no cooling |
| PathReplace with a gas furnace | Installed cost (standard markets)$4,000 – $8,000 | What you getNew heating only, 10-year parts and labor |
| PathFurnace + central AC together | Installed cost (standard markets)$10,000 – $16,000 | What you getHeating and cooling, two pieces of equipment |
| PathDucted heat pump | Installed cost (standard markets)$12,000 – $18,000 | What you getHeating and cooling in one system, rebate-eligible |
Affluent markets and larger homes run higher: $5,000 to $10,000 for a furnace, $13,800 to $21,600 for a ducted heat pump.
Notice the gap between a furnace swap and a heat pump is smaller than most people assume once cooling is in the picture — and the heat pump is the only path on that table that can qualify for local utility rebate money. Rebates apply to qualifying heat pump installations when they replace an existing electric or fossil-fuel system; the amount depends on what you're replacing and what you install. Gas furnace replacements don't get that money. Rebate rules change, so we confirm what your utility actually pays before we quote and handle the paperwork.
Two things people still ask about, answered plainly: the federal 25C tax credit ended December 31, 2025 — no 2026 install qualifies for it. And utility rebate programs pay only through their own contractor networks, and the rules change. If a bid promises you rebate money, ask which program it comes from and whether that company can actually file it, because otherwise the rebate quietly disappears at the end.
When keeping it is the right call
We say this out loud on estimates and it costs us jobs sometimes. Keep the 20-year-old furnace if:
- It passes a combustion analysis and heat exchanger inspection clean.
- You've had no repairs in the last two seasons.
- You don't want or need cooling, and your rooms are comfortable.
- You're planning to sell within a year or two — buyers rarely pay a premium for a furnace, and disclosure is straightforward either way.
- The budget genuinely isn't there yet. A furnace that works is not an emergency. Get the safety check, get carbon monoxide alarms on every level, and plan the replacement for spring.
What we'd push back on is waiting for failure. When a 20-year-old furnace quits, it usually quits during the first real cold snap, which is exactly when every HVAC company in Pierce and King County is booked. You get one quote instead of three, whatever equipment is available instead of what fits your home, and zero time to think about the heat pump question.
How to plan the replacement without panic
- Get a real inspection now, in the shoulder season. Combustion numbers, heat exchanger condition, flue condition, blower amp draw. You want data, not a hunch.
- Price both paths in the same visit. Furnace replacement and a ducted heat pump, with a Manual J load calculation so the sizing is real and not a rule of thumb off square footage.
- Check your ductwork condition. Twenty-year-old duct in a damp crawl space is often the hidden half of a comfort complaint. If it needs work, better to know before you pick equipment.
- Ask about the warranty in writing. Every Varsity install carries 10 years on parts and labor. Parts-only warranties are common in this industry; they're not the same thing.
- Decide on your own timeline. Most of our furnace and heat pump installs are completed in one day, so scheduling in March or September is easy.
The bottom line
A 20-year-old furnace that heats safely and hasn't needed repairs has earned another season. A 20-year-old furnace with rust at the exchanger, a repair history, or a homeowner who's tired of sweating through July has told you what to do.
The deciding question usually isn't the furnace's age at all — it's whether you want cooling. If you do, price the heat pump before you spend a dollar on gas-only equipment. Our post on replacing a gas furnace with a heat pump walks through that trade-off, and the local utility rebate guide shows exactly what qualifies.
Want a starting number before anyone comes out? The instant estimate tool gives you Good, Better and Best pricing in about 60 seconds. When you're ready for real numbers on your actual house, request a free in-home estimate or call (253) 785-4622, Monday through Friday, 8 to 6. Owner-operated, Federal Way based — and we'll tell you if your old furnace has another winter in it.