The 50 percent rule: when to repair and when to replace

By Dana Whitlock. Updated October 13, 2026. How we work.

Two quick tests and one better one

Nobody wrote the 50 percent rule down in a code book. It goes like this: if a repair costs more than half the price of a new unit, replace. Its cousin multiplies the repair cost by the unit's age in years and says to replace when the result beats the replacement price. Both are fast. Both can send you the wrong way.

A sounder test compares cost per year. Divide the repair bill by the years the repaired unit should keep running. Divide the replacement price by the years a new unit should last. Pick the smaller number. The repair or replace calculator on this site does exactly that.

The two versions of the rule

Some contractors plug a fixed round number into the second version instead of a real replacement price. That number drifts out of date as prices climb. Use your own replacement quote, or the typical figure for your area shown above, and the comparison stays fair.

• Half-price version: when the repair costs more than 50 percent of replacement, replace.
• Age times cost version: multiply the repair cost by the unit's age, and replace when that beats the replacement cost. A 12-year-old furnace with a repair at one-tenth the price of a new furnace scores 1.2 times the replacement price, so this version says replace.

Where the rule came from

Neither version comes from a standard, a code or a government study. You won't find them in DOE's appliance analyses or in ENERGY STAR guidance. They are trade shorthand, passed around because they're easy to say on a service call.

They aren't useless. Two true ideas sit underneath. An expensive repair on an old unit buys only a few more years. And the older a unit gets, the sooner the next failure tends to arrive. The trouble is the cutoffs themselves. Fifty percent, or age times cost, ignores how long each kind of equipment lasts.

Six ways the rule misleads

• Lifespans differ a lot. DOE's analysis puts the average gas tank water heater at about 14.5 years, the average non-weatherized gas furnace at about 21.4 years and the average hot water gas boiler at roughly 27 years. A ten-year-old boiler is middle-aged. A ten-year-old water heater is near the end. The age times cost rule scores them the same.
• It punishes young units. A big repair on a three-year-old system can top half the replacement price and still be the right call, since most of the unit's life remains and the part may be under warranty.
• It leaves out what a new unit brings: lower energy use, a current refrigerant and a fresh warranty on a new heat pump or furnace.
• It can't see the next failure. An old AC with a dead outdoor fan motor may also have a tired compressor, and one repair turns into three.
• It can't judge safety. A suspected cracked furnace heat exchanger needs a licensed HVAC technician's diagnosis before any cost rule applies.
• It ignores timing. A cheap stopgap through a cold snap, then a planned replacement with time to collect quotes, often beats an emergency install at whatever price shows up that day.

The cost-per-remaining-year method

Think of both choices as buying years of service, and find out which one buys them cheaper.

Repair cost per year is the repair price divided by the years the unit should run afterward. Replacement cost per year is the installed price of a new unit divided by its expected life. Lower repair figure, repair. Lower replacement figure, or close to it, and replacing usually comes out ahead once efficiency and warranty are counted.

Estimating remaining years is the hard part. The calculator starts from DOE's average lifetime for the equipment type and your unit's age from its serial number. Remaining years shrink as a unit nears and passes the average. They never hit zero, because a unit that has outlived the average still has some chance of running a while. So the method doesn't declare every old unit dead.

Try it without dollar figures. A 12-year-old gas furnace needs a repair that costs one-fifth of a new furnace. With roughly nine years of expected life left, the repair works out to about 2 percent of a new furnace per year. A new furnace spread over DOE's 21.4-year average runs close to 5 percent of its price per year. Repair wins easily, even though the age times cost rule (12 times one-fifth is 2.4 times the price) says replace.

Now a 13-year-old gas tank water heater with the same one-fifth repair. Against DOE's 14.5-year average, maybe a couple of years remain. The repair runs near 10 percent of a new heater's price per year, against roughly 7 percent for a new tank. Replace. The 50 percent rule would have missed this one entirely, since the repair sits far below half.

Running it on your own unit

• Find the age from the serial number on the rating plate, using our decoder for your brand.
• Get a written repair price that names the part and the labor.
• Get the replacement figure, either a real quote or the typical range for your area on our cost pages.
• Put both into the calculator, then adjust for what it can't see: a safety concern, other worn parts, rebates or a big efficiency jump.
• When the two come out close, lean toward replacing on your own schedule before the next breakdown picks the date for you.