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Central A/C Systems, Equipment, Maintenance and More

If you've been thinking of upgrading your old central air conditioning system, or installing central air in a house that has never had it, there's a lot to consider. Every manufacturer today offers a wide range of products, with one suited to nearly every situation. A few even make systems for houses that won't accommodate conventional ductwork. As you might expect, the two most important considerations are efficiency and cooling capacity, but there's more to it than buying the biggest, most efficient system you can afford. In fact, there's quite a bit more.

An Equipment Overview

Central air conditioning is, of necessity, a split system, with some components installed outdoors and others indoors. The heaviest, noisiest, heat-shedding components—the compressor and condenser coil—are installed outdoors, while the evaporator coil is installed indoors, usually in the form of an A-frame in the plenum of a forced-air furnace. In this case, the furnace's blower moves warm air over the coils and distributes the chilled air. The indoor and outdoor segments of the system are typically joined by two refrigeration lines and a low-voltage relay cable.

If a home is heated by some means other than forced air—baseboard or radiant floor heat, for example—the evaporator coil is typically mounted in a dedicated blower unit, which pushes the cooled air through conventional ductwork. Most blower units are installed in attics and crawlspaces and are connected to flexible, insulated ductwork, which is the easiest and least costly to install, especially in retrofit situations.

In houses built without ductwork and where conventional ductwork would be too costly or too inefficient to install, a ductless central-air system is now possible. Ductless systems have long been popular in Europe and Asia, where building methods discourage ductwork, but are relatively new in the United States. The Carrier Corp. is one of several companies now making ductless systems for the U.S. market.

In these systems, a single outdoor compressor serves several smaller evaporator coils located indoors, each in its own box and each with its own blower fan. These components are installed on exterior walls, usually on the upper half, where much of the heat accumulates. They're finished unobtrusively, but they can be quite large, often measuring 6 x 18 x 24 in. Condensation lines are routed outside, along with the refrigeration and electrical lines.

Ductless systems can also provide heat, either through resistance coils in the wall units or in heat pump fashion. The advantages of ductless air over window air are that ductless systems move the noisiest components outdoors, they can provide heat, they distribute the air more evenly, and they don't block the better half of a window. Installation costs vary widely, but ductless systems can be more expensive than ducted systems. It's a sliding scale, with each home's variables—primary building material, number of rooms, house size, and layout—coming into play. They're reliable and efficient, but they're not likely to replace ducted systems in the United States. They're considered problem solvers, with the problem being fairly rare.

Evaporator Coils

Evaporator coils come in a variety of shapes and sizes, depending on the type of installation, the amount of cooling capacity needed, and the manufacturer. It is the source of cooling as air passes through the furnace or air handler. They are constructed of aluminum-finned copper tubing. The copper tubing runs perpendicular to the aluminum fins, making U-turns back and forth until the desired coil size is achieved. Added cooling capacity without an increase in length and width is accomplished by adding more rows of copper tubing.

Slant coils and horizontal coils have a slab appearance, similar to the radiator in an automobile. They can be installed in ductwork running horizontally or in an air handler. An A-coil is shaped like a capital A without the crossbar. It can be installed on top of a fuel burning furnace heat exchanger or in an air handler. The newest design is the multi-flex coil which is a series of A-coils linked together at the base. The multi-flex coil can be installed in any position when encased in a special cabinet. All evaporator coils must have a drain pan to collect the water that condenses as the air flowing across the coil cools. The water can drain away by gravity or be pumped away.

The cooling effect that takes place inside the coil requires a pressure drop in the refrigerant. This drop can be accomplished in a number of ways: capillary tube, piston or orifice, or thermostatic expansion valve.

A capillary tube is a thin copper tube of predetermined length into which the compressed liquid refrigerant is pumped. The length of the tubing causes the pressure to drop and subsequent cooling effect of the refrigerant.

A piston or orifice blocks the flow of refrigerant and forces it through a tiny hole, creating the needed pressure drop.

A thermostatic expansion valve meters the flow of refrigerant to meet the cooling demand of the coil. It determines this demand by way of a sensing bulb attached to the outlet tube on the coil. Because it can meter the flow to meet demand, the expansion valve can keep the coil at optimum cooling potential.

How Conventional Systems Work

All heating and cooling technology grows from the law of thermal dynamics that says when hot and cold spaces are separated by a medium, the transfer through the medium will always be from hot to cold. Heat follows cold, whether the medium is human skin, an exterior wall, or the metal surface of an evaporator coil. When warm, humid air is blown across the evaporator coil in your furnace, the heat in the air is drawn to and impinges on the cold metal surface of the coil's fin tubes. In the process, the moisture in the air condenses on the cold metal and drips into a pan below the coil where it's drained off, thus lowering the humidity in the house.

It's a neat trick—a double trick—but it requires a repeatable cycle, over and over, every time the system's thermostat demands it. The evaporator coil needs to be constantly recooled, and the heat it absorbs needs to be carried outdoors. This is accomplished with a liquid/gas refrigerant, which undergoes a pressure-induced state change. An outdoor compressor pressurizes the refrigerant, heating it to a gas state, then sends it through an adjacent condenser coil to be cooled and returned back to the evaporator coil. There it picks up household heat and carries it back to the compressor. When this cycle is repeated often enough, our homes become a lot more comfortable.

Understanding Energy Ratings

Air conditioners have always been prodigious users of electricity, but efficiencies have improved considerably in the past few years. Part of this improvement was federally mandated in 1990, through the culmination of the National Energy Conservation Policy Act of 1971. These regulations established minimum efficiency standards for heating and cooling equipment. As a result, nearly all models manufactured today are more efficient than those made just 10 or 15 years ago.

How are systems rated? Central air conditioners—the condenser units—are given a Seasonal Energy Efficiency Rating, or SEER. In simple terms, SEER is calculated by dividing the cooling capacity of a continuously operating air conditioner by the electrical input required to run it. The value is expressed in numbers. A SEER 10, for example, is now the lowest number allowed, and any number larger than that is accordingly more efficient and will cost less to operate. Along with the yellow Energy Guide tags attached to each appliance, these ratings give consumers a benchmark sense of where their choices fall on the energy-efficiency scale.

Most manufacturers now offer SEER 10, 11, 12, and 13 models, and some offer SEER 14. This gives you five separate efficiency options, with model numbers usually keyed to the SEER numbers, so they're easy to recognize. Lennox's Value 12 system, for example, is a SEER 12.

How Better Efficiencies Are Achieved

Like the auto industry in its quest for better mileage, cooling equipment manufacturers have combined some minor tweaking with some major re-engineering. Because the compressor is the biggest energy user, that's where they have focused much of their attention. The first step was to improve the internal components of standard, reciprocating compressors so that less pressure—and therefore, energy—was lost to internal leaks. Another step was to increase the size of the condenser coil. With more fin-tube surface area, the returning refrigerant could be brought to the compressor with less heat, reducing the compressor's load.

These two steps yielded substantial savings, bringing condensers into federal compliance, but a complete retooling was needed to achieve significant improvements beyond this level. Part of the answer was a multispeed compressor. With two or more speeds, the system doesn't have to run full out on days when only mild cooling and dehumidification are needed. Full-speed use is still available for those really miserable days, but the compressor doesn't have to run wide open all the time. At low speed, the practical effect is that of a small compressor matched with an oversize condenser coil. The savings can be substantial, and most manufacturers offer multispeed compressors in their lineups.

At the same time, engineers began testing a radically different kind of compressor—the scroll. Scroll compressors are so different that they practically defy description. But their mechanical advantage is clear. Because they generate much less friction, they experience much less wear. The final product is a compressor that is very efficient and lasts longer. Today, most companies offer some multispeed and some scroll compressors, though a few, like Ruud/Rheem, have gone to scroll compressors exclusively as a statement of across-the-line quality. Any way you look at it, today's condenser units are better than those made just a few years ago.

The Freon Question

As a result of the Montreal Protocol, a conference that grew out of international concern over the ozone-depleting qualities of CFC chemicals, the EPA is mandating the gradual phaseout of Freon, or R22 refrigerant. The new, non-ozone-depleting replacement will be R4-10A. In fact, some manufacturers have switched to R4-10A in some models already. While this new refrigerant works just as well, it requires pressures up to 50% greater than Freon, so it can't be used in existing equipment. Interestingly, the higher operating pressure actually improves efficiency slightly. In any case, there's no practical way to convert existing equipment.

So where does this leave the tens of millions of us with Freon-based R22 systems? The short answer is that the Freon phaseout is stretched so far into the future that nearly all of today's air conditioners will have been replaced by then. The EPA will require a substantial reduction by 2004, and all products containing R22 must stop production by 2010. The production of R22 itself must cease by 2020. For those few R22 units still in service at that time, recycled R22 will be available, though it will probably cost a small fortune.

Sizing for Capacity and Efficiency

Your heating, ventilation, and air conditioning (HVAC) contractor will size your equipment to meet the specific needs of your home. Factored into the equation will be the age of your house, the number and quality of its windows, how well it's insulated, how many stories it has, its size, and, of course, local energy rates. Contractors use industry sizing models, such as Model J, but most use them as a reference, modifying the results to accommodate their own years of experience. A 1,500-sq.-ft. ranch-style home, for example, might normally require a 2 1/2-ton air conditioner, but if it's not well insulated, or if a good many windows have western exposure, or if the trees offer little direct shade, then a 3-ton unit might be more appropriate.

In any case, sizing is critical. If sized too small, the system will struggle, and even freeze over, on the warmest days. If sized too large, the system will cycle on and off too frequently, greatly reducing its ability to control humidity. It will also be less efficient. Keep in mind that efficiency ratings are measured at the factory, under conditions that may have little to do with your house. In any case, sizing is a job best left to seasoned professionals, and it's a good idea to seek out more than one opinion.

General Installation Costs

What do air conditioners cost, installed? Again, local prices will vary significantly, but in a typical Midwestern town, a nonunion shop might charge between $1,200 and $1,700 to replace an old 3-ton air conditioner with a new SEER 10 system. That's assuming a 1,500-sq.-ft. house, 20 years old, with ductwork in place. For a similar home that's new, with a gas furnace and equivalent air conditioning, the price would be $5,000 to $7,000, gas and electrical connections included.

The Price Versus Efficiency Question

Of course, you'll always pay more for high-efficiency appliances, so the critical question is whether you'll save enough in the long haul to come out ahead. And that, unfortunately, requires a region-by-region, even a house-by-house, assessment. You'll need to work closely with your contractor to make an informed decision. Don't assume that high efficiency always pays. It may from a good-citizenship perspective, but real dollars should drive the rest of the equation. And don't forget to add the cost of interest on the money gained or lost.

All we can do here is provide some context. If, for example, your electricity costs you a low 6 cents per kilowatt-hour (kwh) and you live in a reasonably well-insulated home in the northern one-third of the nation, using your air conditioner 200 to 400 hours per year, a basic SEER 10 system is probably your best choice. There's little chance that you'd recover the several hundred extra dollars a SEER 11 or SEER 12 system would cost, spread over a 12- to 15-year projected service life. You may hope for longer service, and you'll probably get it, but don't count on it. A SEER 10 is also a reasonable choice if you plan to move in the next few years.

On the other hand, if you live in Yuma, Arizona, and run your air conditioner 2,000 hours per year, then it makes sense to buy the most efficient model you can afford. The same might be true if you live on the Eastern Seaboard and pay 11 or 12 cents per kwh, or if you live in a house with stone or brick exterior walls, where insulation is simply not feasible. In these cases, high efficiency really is a good investment.

Prices vary by manufacturer and with local market pressures, but it's probably safe to say that each step up in efficiency will cost about $200. This may seem a paltry sum, and it is for many people, but when you consider that the equipment costs for an entire SEER 10 system might run $800 to $900, an increase of $200 to $800 is significant.

Quality and Warranties

Most manufacturers offer two quality levels for each SEER number. What you get in return is a better-made unit that runs three to six times quieter and lasts longer. A quieter-running unit can be a real plus if you or your neighbors are particularly annoyed by a loud air conditioner. You'll also get a better warranty: a 10-year compressor warranty instead of five, a five-year warranty against leaks in the coils instead of one year. As with all step-up warranties, you're betting against yourself, which sometimes pays big dividends. If it paid more than about 40% of the time, however, the manufacturer couldn't afford to offer it.

Air Conditioner Maintenance

Whether you have an older air conditioner or a newer, high-efficiency system, maintenance is critical. Older units need all the help they can get, and high-efficiency models won't deliver all that high-dollar efficiency without routine care. The primary culprit is dirt and debris, which clogs coils and taxes equipment, but even something as simple as an out-of-level condenser unit can reduce efficiency and burn bearings. Here, more than most places, maintenance pays. And all it takes is about an hour a season. In most cases, you won't even need to buy tools and materials.

Most of the work you'll be doing will be outdoors, on the condenser unit. To eliminate any chance of an electrical hazard, begin by opening the unit's electrical disconnect panel and pulling the disconnect block from its slot. Most such panels come with a reversible on/off block. Just flip the block over, so that off reads correctly, and return it to its slot.

In order for the condenser fan to do its job, its louvered panel needs to be fairly open. The tradeoff is that plenty of leaves and debris get into the housing when the fan's not running. To remove the debris from this compartment, undo the screws at the top of the unit and tip the panel upward. Then, lift out any leaves, twigs, and debris you find in the coil enclosure.

To clean the outside of the coil, undo the screws from one of the side panels, or from all three sides if the design of your air conditioner makes that more convenient. Then, use a soft-bristle paintbrush to sweep the fin tubes clean. Always brush vertically, in line with the fins. Because the fan pulls air through these fins, you can expect to find a blanket of dust and lint clinging to the fins, which can really reduce efficiency.

The condenser coil's aluminum fins are paper thin and very delicate, enough so that you may find several areas where the fins are smashed together. Virtually anything can damage them. Of course, smashed fins won't remove much heat, so it's a good idea to straighten them whenever possible. For a minor crush, you can use a toothpick, but for professional results, nothing beats a fin-tube comb. They're only sold by HVAC suppliers, but they're inexpensive and will last a lifetime. We paid $11 for a six-comb set, with each plastic comb sized to fit two fin spacings. Our kit, called Super Comb Model T-400, covers spacings from eight to 20 fins per inch and was made by Wagner Products Corp., 5190 N.W. 165 St., Miami, FL 33014. Be sure to match the tool to the spacing. Then carefully insert the teeth and comb through the damaged area. You'll be impressed by the result.

After you've reinstalled all the panels, check the condenser unit for level. Condensers are often set on backfilled soil, which tends to drop like a rock, especially through the first few seasons. If the condenser has settled out of level, the strain can wear out bearings and reduce efficiency by as much as 10%. Check for level, in both directions, and if needed, pry up one end of the support pad and add soil or gravel until the pad is level. Slate shims can also be used for minor adjustments.

And, finally, one of the most frequent air conditioner maintenance problems is a clogged condensate line. The culprit is a bacterial slime that grows in condensed water. To keep this line flowing freely, pour a 1:9 mixture of household bleach and water through the line every month or so during cooling season. Just pull the hose from its A-frame fitting and flush the line, all the way back to the floor drain.

Copyright © Popular Mechanics 2001. Reprinted by permission.

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