Repair or Replace? A Calculator for HVAC, Water Heaters & Roofs

You're standing in front of a system that just quit, a technician has handed you a repair quote, and the only question that matters is whether to pay it or put that money toward a new unit instead. Guess wrong and you either throw good money at a machine that's about to die anyway, or you scrap a unit that had years of life left. The calculator below gives you a clear verdict in seconds, and the rest of this guide explains the three rules behind it so you can trust the answer — and know when to override it.

The three rules the calculator uses

There's no shortage of repair-or-replace advice online, and most of it leans on a single rule of thumb. The problem is that every one-line rule fails in some common situation. The calculator above combines three of the most reliable rules so that no single blind spot decides your money. Here's each one, and where it shines.

1. The 50% rule

The oldest and most-quoted rule is simple: if the repair costs more than half of a comparable new unit, replace it. A $400 fix on a $700 dishwasher fails the test — you're better off replacing. A $120 fix on that same dishwasher passes easily — repair it.

The 50% rule works because it ties the decision to value rather than to a fixed dollar amount. A $400 repair sounds expensive, but it's a bargain on a $4,000 furnace and a waste on a $700 dishwasher. The rule scales the threshold to the thing you're fixing.

Its blind spot is age. The 50% rule alone would tell you to repair a 22-year-old roof for any quote under half the replacement price — even though that roof is on borrowed time. That's why the calculator pairs it with the next rule.

2. The service-life check

Every system has a typical lifespan, and a repair only makes sense if there's enough life left to enjoy it. The calculator checks the unit's age against its expected service life and flags anything that's past the back half. Spending $1,200 to fix a furnace with 12 years left is an investment; spending the same on one with 2 years left is a stopgap.

Here are the planning anchors the calculator uses. These track DOE Energy Saver, ENERGY STAR, and equipment-life data, but treat them as ranges, not promises — maintenance, climate, water hardness, and run-time all move the real number.

Replacement ranges are mid-2020s national typicals for a like-for-like, installed swap. Capacity, efficiency tier, brand, and your local labor market move the number a lot, so always price your own job rather than leaning on the table. For a deeper breakdown of how long each appliance lasts and the warning signs of a system on its way out, see appliance lifespans and when to replace.

3. The $5,000 rule (HVAC only)

For heating and cooling equipment, there's a clever shortcut that folds age and cost into one number: multiply the unit's age in years by the repair quote. If the result is over $5,000, replace it.

The beauty of the $5,000 rule is that it weights age automatically. Watch how the same $700 repair flips as the unit gets older:

A young AC clears the bar easily — fix it. An older one fails, because at that age you're one expensive failure away from replacing the whole unit anyway. The calculator applies this rule only to HVAC systems (furnaces, central AC, and heat pumps), where it's well established, and shows you the score so you can see exactly why the verdict landed where it did.

How the verdict is decided

The calculator doesn't just average the three rules — it follows a clear priority so you get a defensible answer:

REPLACE  if the repair is over 50% of replacement AND the unit is
         past half its service life,
         OR the $5,000 HVAC rule is exceeded.

REPAIR   if the repair is well under a third of replacement
         AND there are 3+ years of service life left.

BORDERLINE  everything in between — let the tie-breakers decide.

When the answer comes back borderline, that's not the calculator giving up — it's telling you the numbers genuinely don't decide it, and the next section matters most.

Three worked examples

Rules are easier to trust when you watch them resolve a real case. Here are one of each verdict, run exactly the way the calculator does.

Replace — a 12-year-old central AC, $1,400 repair, $7,000 replacement. The 50% rule looks fine on its own (the repair is only 20% of replacement), but the $5,000 HVAC rule settles it: 12 years times $1,400 is $16,800, far over the $5,000 line. At 12 of about 15 typical years, you'd be sinking $1,400 into a unit that's one failure from the curb. Replace.

Repair — a 6-year-old dishwasher, $120 repair, $700 replacement. The repair is just 17% of a new unit, and at 6 of about 12 typical years it has real life left. Both the 50% rule and the service-life check point the same way, so there's no reason to scrap it. Repair, and bank the $580 you didn't spend.

Borderline — a 10-year-old water heater, $500 repair, $1,500 replacement. The repair is 33% of replacement, so the 50% rule says repair — but at 10 of about 11 typical years, this tank is on the doorstep. The numbers don't agree, so it's a coin toss the tie-breakers decide: if the tank itself is sound and you're short on cash this month, the repair buys time; if you're staying put, replacing now with an efficient unit usually wins. Borderline.

The tie-breakers that decide a borderline call

When the math is close, four factors usually break the tie:

• Efficiency. A new high-efficiency furnace, heat pump, or AC can cut the energy used for heating and cooling well below a worn 15-year-old unit, and an ENERGY STAR water heater uses noticeably less energy than an older standard model. Over a long ownership window, those savings can offset much of the price gap.
• Reliability history. If a system has needed two or three repairs in a single season, the next failure is likely already forming. A string of repairs is a strong replace signal even when each one passes the 50% rule on its own.
• Parts availability. Once a manufacturer discontinues parts for an old unit, a future repair can mean a long wait or no fix at all. A system you can't get parts for is effectively at the end of its life.
• How long you'll stay. If you're moving within a year or two, a sound repair that gets you through the sale is often the cheaper move, since buyers rarely repay the full cost of a brand-new system in the sale price.

Repair or replace, system by system

The rules above work for everything, but each system has its own tells. Here's the shortcut for the six most common decisions.

Water heater. A tank water heater lasts about 10–12 years. A leak from the tank body means the steel shell has rusted through — replace, no exception. But a bad heating element, thermostat, anode rod, or temperature-and-pressure valve on a unit under 8 years old is a cheap, worthwhile fix. The deciding question on when to replace a water heater versus repair is almost always age plus where the water is coming from. Run the math, then read water heater leaking from the bottom to tell a tank leak from a fitting leak.

Furnace. Furnaces run 15–20 years. A cracked heat exchanger is an automatic replacement on safety grounds. Short of that, whether it's worth repairing an old furnace comes down to age and quote: a blower motor or igniter on a 10-year-old unit is an easy yes, while a $1,500-plus repair on a furnace past 15 years usually loses to a new, far more efficient one.

Central air conditioner. AC runs 12–15 years. Capacitors, contactors, and fan motors are cheap, sensible repairs at almost any age. A failed compressor or a leaking condenser coil is a different story — on a unit past 10 years those repairs often approach replacement cost, and the $5,000 rule usually says replace.

Heat pump. A heat pump lasts 12–15 years and follows the same logic as AC, because it is one running in both directions. Minor electrical parts are worth fixing; a failed compressor or reversing valve on an older unit tips firmly toward replacement, especially since a new heat pump is often markedly more efficient.

Roof. An asphalt-shingle roof lasts 20–25 years. The 50% rule barely applies here — a roof decision is driven by age and extent. A localized leak or some lifted flashing on an otherwise sound roof is a clear repair. Widespread granule loss, curling shingles, or multiple leaks on a roof past 20 years means you're patching a roof that's failing everywhere — replace.

Major appliances. Refrigerators, dishwashers, washers, dryers, and ranges last 10–13 years and are the heart of the 50% rule for appliances. A control board or sealed-system failure on a unit past the halfway mark usually fails the test; a hose, valve, or igniter on a newer one passes easily. See appliance lifespans and when to replace for the warning signs on each one.

When to replace immediately — no calculator needed

A few failures override every rule on this page. Replace right away, regardless of the math, if you see any of these:

• A cracked furnace heat exchanger. This can leak carbon monoxide into your home. It's a safety replacement, full stop.
• A water heater leaking from the tank itself (not a fitting or valve). The steel shell has corroded through and will only get worse — see water heater leaking from the bottom to tell a tank leak from a fixable one.
• Any gas smell or carbon-monoxide alarm. Leave, then call your gas utility or a pro. Safety comes before any cost comparison.
• A major refrigerant-side repair on an older AC using a phased-out refrigerant. As the industry transitions to newer refrigerants, repairs on legacy systems get costlier and harder to source, which pushes the replace decision earlier than it used to be.

How maintenance changes the math

The cheapest version of this decision is the one you delay for years through upkeep. A water heater flushed annually resists the sediment that corrodes the tank. A furnace and AC that get yearly tune-ups and fresh filters run closer to the top of their service-life range, not the bottom. A roof with cleared gutters and prompt flashing repairs reaches its full lifespan instead of failing early.

The repair-or-replace decision is really a question of when, and good maintenance buys you a later, cheaper when. For the full picture of routine upkeep that extends every system's life, walk through the repair-or-replace home systems guide, and to plan for the replacement before it arrives, the home maintenance budget calculator turns your home's value, size, and age into the annual reserve you should be setting aside.

The two concepts worth internalizing: service life is the typical number of years a system lasts, and a capital expense is the big, lumpy replacement you should be funding a little each month through a sinking fund — so the day the calculator says "replace," it's a planned withdrawal, not a crisis.