Agony is being told that a major household appliance is broken beyond repair. The job you thought might set you back $100 or so is going to cost many hundreds or even thousands.

"Can't you just fix it?" is the plaintive wail we hear from homeowners in this situation. Sometimes we can, but often we can't. Or we can, but it's not in the homeowner's best interest to do so. As with radios, TVs, VCRs and shoes, the cost of a major repair for many home fixtures and appliances is creeping ever closer to the cost of replacement.

Here are some things to consider in deciding whether repair or replacement might be the better option.

Furnace/Heat Pump/Boiler. These are the most costly systems to replace, so naturally you want to get as many years of use out of this equipment as you possibly can. The best way to do this is to make sure you have your system professionally serviced as least once a year. Many heating and cooling contractors offer service agreements that assure routine inspection and cleaning at least. Prices usually are a real bargain considering that a new home heating, ventilating and cooling (HVAC) system will cost many thousands.

Myriad repairs can be made on HVAC systems to keep them running for decades. Yet when a boiler section cracks, there's little that can be done except replace the entire unit. Likewise, central air conditioners and heat pumps have two major components - the indoor evaporator coil and the outdoor condensing unit (compressor) - that when they fail, cannot readily be fixed.

When one needs to be replaced, it is best to replace the other with a compatible unit. Unmatched evaporator coils and condensers usually will operate together for a time, but with a steep penalty in performance, energy usage and premature system failure. So even though it entails higher initial cost, replacing both components at once is the smart thing to do in the long run. Unfortunately, studies have shown that about half the time, people opt for the cheaper but shortsighted single unit replacement.

While HVAC systems ought to last for decades, it's not necessarily a bargain to keep them running that long. Tremendous strides have been made in energy efficiency in recent years, leading to quick cost paybacks from lower energy bills, The United Homeowners Association (UHA) is a Washington-based consumer organization that offers the following advice:

"If your furnace is over 15 years of age, it's probably time to boot it out the door...If your furnace's efficiency comes in somewhere in between 50-75%, you ought to begin investigating rebate offers for buying a high-efficiency new furnace."

This is not a contractor talking. This advice comes from an organization dedicated to protecting consumer interests. UHA can be reached at 1511 K Street NW, Suite 345, Washington, DC 20005. Membership cost $18 a year.

Water heater. Industry statistics show that the average water heater lasts 12 years. With regular maintenance and routine repairs, some keep operating two or three times as long. As with HVAC systems, however, it's not always to your advantage to hang on to older units. Modern high-efficiency water heaters often can pay for themselves in energy savings within 3-5 years.

Almost all components on a water heater can be fixed or replaced except for the tank. Once the tank rusts through, there is no way to rescue the water heater. Replacement is the only solution.

Water heaters come with internal sacrificial anode rods to protect against rusting. An anode's sole purpose is to corrode away so the steel of the tank can't. Replacing the anodes every 3-4 years (more frequently if water is softened) will add considerably to the life of a water heater.

Another main cause of failure is overheating from sediment build-up inside the tank. Ask your plumber to inspect the anodes and sediment periodically. Sometimes this can be done as part of an annual service agreement.

Some plumbing firms also offer extended water heater warranties lasting 10 years or even a lifetime. If you plan to live in your home for quite some time, these warranties may be worth looking into.

Dishwasher. Automatic dishwashers are another appliance that should last a decade or more - though here, too, you often can save money by buying a newer energy-efficiency unit.

Brand new units can be bought for $400-$600, while repairs of various operating mechanisms typically run $150 and up. If your dishwasher is getting near the 10 year mark, a major repair may be a signal that other components are also on their last legs. It won't take many service calls to pay for a brand new unit.

Disposal. Stoppages and minor malfunctions are worth repairing. But if the motor goes out, or the blades break, you are better off replacing the entire unit. Especially so if you deal with a plumbing company that warrants the product for 5-10 years or even longer.

Toilets. Unless you crack the porcelain, a toilet can easily last a lifetime. What will wear out are the flushing mechanisms comprised of moving parts. Leakage may occur from the wax ring seal by the floor, but that can be fixed short of replacement.

Toilets commonly get replaced for reasons other than malfunction. Water conservation is one. Modern toilets operate with 1.6 gallons per flush or less, compared with 3.5 gallons for older standard models. (A few 5-gal. and 7.5-gal. flush versions from many decades ago also are still in operation here and there.) Depending on water rates, sometimes you can save money by replacing a toilet.

Styling and quieter flushing are two other reasons to replace. This is a matter of homeowner choice more than necessity.

Faucets. Leaking faucets can be repaired by replacing a cartridge, washer or other internal component. Tarnishes and nicks are harder to fix.

Good faucets will give at least 5 and often 10 or more years of trouble-free operation. Plumbers can keep them operating almost indefinitely, but here too most people would rather pay a few more bucks for a replacement that offers better styling and convenience.

Decades ago plumbers repaired more faucets than they replaced. For most companies the opposite now holds true.