Heating/Cooling Systems... When to repair and when to replace

by Michael Chotiner

When your home's heating or cooling system breaks down and you're faced with a big repair bill, it's not unreasonable to wonder whether it might be better to replace it now or risk future outages and associated repair costs. Unfortunately, it's not always a simple calculation, and the right answer depends on what you hope to achieve through your decision. Do you want to:

Not all of these goals can be achieved with the same solution. Important factors in the decision to repair or replace also include:

Some of the most common heating and cooling systems are:

The reason these systems tend to be the most common in the heating and cooling market is they're the most affordable to install and operate over time, and they have all proven to be extremely dependable. Operating failures that may occur during the first five to ten of service rarely require replacement of the furnace, boiler, heat pump or air conditioner.

What Can Go Wrong

Heating and cooling systems, by definition, function with a number of compatible, interoperable parts. Each type of system has its own set of strengths and vulnerabilities.

Gas furnace/forced hot-air systems

If a gas furnace fails to turn on or stay on, the problem is usually electrical—a bad thermostat, igniter module, thermocouple, solenoid or transformer. On older gas furnaces, the problem might be in the pilot burner or a related safety control.

For example, in furnaces with standing pilot lights, a thermocouple acts as a heat sensor that determines whether the pilot is lit and hot enough to ignite the gas burner. When heated to a certain point, wires in the thermocouple convert the heat into electrical energy. That energy is sent to a solenoid, which is basically an electromagnet wrapped around a gas valve's piston. When the solenoid is energized to the right level, it moves the piston to open the valve so that gas flows to the burner, which will light when the thermostat is set to call for heat. These interrelated devices operate at low voltage, so a transformer is required to step-down the house current (usually 120 AC in the U.S.) that is fed to them.

If the system fails to distribute heat effectively, the problem could also be with the blower motor or, more likely, a clogged filter. Any of these can be repaired or replaced relatively inexpensively.

One of the reasons to bear the expense of replacing a gas furnace would be if the heat exchanger were cracked. Heat exchangers are metal chambers that essentially connect that connect the burner to the flue. Hot gases flow through the heat exchanger passageways and heat the metal. Air is heated as it is blown across the exterior surfaces of the heat exchanger and distributed throughout the house via ductwork.

If left in service long enough, all heat exchangers will eventually crack as a result of metal fatigue caused by the continual cycle of expansion and contraction as the heat exchanger heats up and cools down. The actual duration of a heat exchanger's useful life depends on its design, the intensity of its use and how well it was manufactured. A table published by the American Society of Home Inspectors shows the life expectancy and warranty periods of many major brands, which fall within a range of five years lifetime.

The point is, however, that a cracked heat exchanger releases hot, poisonous gases into the home. Heat exchangers should be visually inspected on a regular basis by a professional. Visible corrosion is an indicator of trouble. When inspecting forced-air furnaces, old-school technicians place a light inside the heat exchanger; a light leak is the telltale sign of a crack. More modern operators use infrared video inspection systems, which detect breaches that the light test cannot. In hydronic systems, fluid leaks from the heat exchanger give evidence of a dangerous condition that needs to be addressed immediately.

Whether a cracked heat exchanger should be repaired or replaced, or whether the whole system should be replaced as a result of a failed heat exchanger, depends on the warranty. A licensed, reliable professional will be your best guide. Make sure that whomever you consult on the matter has a good reputation.

Gas boiler/hydronic heating systems

Gas boilers offer many of the same advantages as gas furnaces—namely, they're relatively inexpensive to operate and little typically goes wrong with boilers themselves. Despite this, hydronic systems are more complicated than forced-air. In addition to electric and/or electronic controls that can fail, boilers rely on a circulating pump and a number of valves to distribute heated water and control pressure within the system. Any of these might fail before the actual boiler, and may need replacement or repair. Boilers are susceptible to corrosion but are usually only replaced when internal metal parts rust through.

Oil-fired furnace/forced hot-air systems

Oil-fired furnaces require more maintenance than gas furnaces. In addition to the vulnerabilities of their electrical controls and safety devices, the fuel supply pump and valves need regular cleaning to prevent clogs. The air filter needs changing regularly. The blower motor may need to be replaced before the furnace itself. A cracked heat exchanger is the most common reason to replace an oil-fired furnace.

Oil-fired boiler/hydronic heating systems

Oil-fired boilers are extremely reliable if properly maintained. However, electric parts and valves, as well as the circulating pump may need repair or replacement during the boiler's useful service life. Hydronic systems are prone to leaking and trapping air, which can compromise even heat distribution. Boilers themselves require replacement only when internal metal parts become badly corroded.

Electric heat pump/forced-air heating and cooling

Although they use the most expensive type of energy to operate, heat pumps do double-duty. A single piece of equipment provides warmth and cooling when needed. When heat pumps don't work properly, it can usually be traced to a problem with the thermostat or an electrical issue. Broken belts and burnt-out blower motors need to be replaced.

Central air conditioner/packaged or split system

If a central air conditioning system isn't working properly, suspect the thermostat first, then have the system checked for proper refrigerant level and leaks. Make sure coils are clean and coil fins are straight. A clogged filter will obstruct airflow. Controls for the compressor and blower controls can fail. Replacement of the whole system may be in order if you're unable to get replacement parts or refrigerant to keep it working.

Repair is usually better than replacement for your wallet. BUT...

The facts about what really goes wrong with various heating systems support those leaning toward repair over replacement—better to fix what's broken and save the big bucks for a rainy day. Be that as it may, it might start raining sooner than you'd imagine. See the table below:

A look at the median life expectancy for common heating and cooling equipment tells us that no appliance lasts forever. While we consider the ASRAE Equipment Life Expectancies to be reliable, they are very general; in cold climates where equipment gets intense use over long periods, life expectancy can be as little as eight to ten years. Experts say the most frequent and expensive repairs occur during the last two years of a system's service life. With the high cost for repair work, frequent repairs of a system toward the end of its service life can yield diminishing returns. If you're looking for reliability, take the life cycle of any unit that's acting up into account before spending money on repairs.

Will a replacement save you money in the long run? The answer is yes—but not much. Heating and cooling equipment manufactured today is more efficient than it was 10 years ago. But the most efficient technology—namely condensing units—cost much more and are likely to require more maintenance over their lifetime. As a rule of thumb, if you replace a conventional furnace that's 80% efficient with a new one that's 95% efficient, you should save enough on energy costs to pay yourself back on the initial investment in 17 to 20 years. (See the High-Efficiency Furnace Payback Calculator for a more specific appraisal of the payback period for replacing your furnace in your area.)

Furthermore, installing more efficient heating and cooling equipment will likely reduce your home's carbon footprint, so even though the return on investment period is longer than with other appliances and types of equipment, you'll be making a down payment toward the wellbeing of the Earth.

About the author:  Michael Chotiner is a home repair expert who writes about heating and cooling systems and other core house infrastructure for The Home Depot. Michael Chotiner is a skilled construction craftsman and has managed and owned his own construction business. To research Home Depot's HVAC installation services offerings, you can visit the Home Depot website.