The main thing you need to retrofit central air conditioning is a deep desire for it.

Even in a gut rehab, central air ductwork can be obtrusive and play havoc with a design.

There are three areas where problems arise:

* Air supply. A truly efficient air-conditioning system needs a lot of ductwork. Generally, each room needs at least one air supply, or outlet. Most supplies feed from the basement trunk line, with a separate duct all the way up for each one. (It is possible to feed more than one supply by installing trunk lines at each floor, but it requires chases and bulkheads. If you're concerned about the historic appearance of your house, or just don't want a lot of protrusions in the walls and ceilings, such a system is out of the question.)

The ducts are oval or square, about 3 by 11 inches for each outlet. If your house is undergoing substantial reconstruction, it may be possible to hide supply ducts in the framing.

If your house is already substantially rehabbed, or doesn't have a lot of framed walls, ductwork may need to be concealed in chases -- boxes framed and covered with drywall. The problem is, it's difficult to hide the chases, and they can interfere with ceiling moldings, old flooring and plaster walls and ceilings.

Sometimes a clever design will come to the rescue, and the chase can be concealed in a closet, behind a stair, next to a fireplace or in an odd corner. Randy once worked on a house where a deep coat closet was positioned directly behind the first-floor powder room. The closet opened to another room and the back part was a perfect space to run ducts. In the same house, several ducts were run through vestibule walls that had been framed out and insulated, to avoid disturbing moldings on the parlor side of the space.

The most efficient supply system has high and low outlets on each duct. In the cooling season, the top ones are open, because cold air falls. In the heating season, the bottom ones are open, because hot air rises. Efficient as it is, a dual duct system may be a jarring note in a historically sensitive interior.

* Cold-air returns. This part of the system can cause problems even in houses that already have forced-air heat. For central air to work efficiently, it must return hot air from the top of the house to the furnace to be recooled. If too little hot air is returned, the house will be hotter on the top floors than on the lower ones.

Most building codes require separate cold air returns for each floor -- you can't feed more than one floor from the same duct. Since these tend to be pretty large metal boxes, they're hard to fit in without disruption.

Since hot air will always head for the upper levels, that's where it needs to be recaptured. If your house has a fairly simple stacked floor plan, the return should be located high up on the wall in as central a location as possible on each of the upper floors. If your house has different levels on a single floor -- a second-floor front room that's up a few steps from the back room, for instance -- you should have a separate return for each area. An ideal system would have returns in every room, for maximum circulation.

If you already have central air and have had problems with uneven cooling, it may be because the return system is #i inadequate. The returns can be enlarged, but again, it may be intrusive in an old house.

(If your house has just one level, the returns should be as close to the ceiling as possible for cooling. And all houses with &L; forced-air systems need low returns to capture sinking cold air and return it to the furnace for reheating in the winter.)

* Equipment. Cooling systems have two parts, two large pieces of equipment: the cooling unit, which is usually installed on top of the furnace, and the compressor unit, which is usually placed outside. The compressor has a fan that can be noisy. You can ask for quieter equipment (which may be more energy efficient, and may be more expensive); you can also make sure that the unit is not sitting right next to the patio or under the window of someone sensitive to noise.

The compressor and the cooling unit are connected by a pipe carrying refrigerant. The connection can be pretty ugly, so you probably don't want it to be highly visible.

If you're thinking of installing the compressor unit on the roof, it should be placed so the surface underneath can still be maintained.

If you think you'd like central air but aren't sure how much disruption it will cause, by all means shop around. Installers have widely varying degrees of sensitivity to the structure. A contractor who fires up a chain saw to cut walls for duct work probably has no business in an old house.

Sometimes the duct work installer will design a system that's perfect for moving air, but is obtrusive and ugly. The best system cools the house with a minimum of temperature difference between floors -- and it's invisible.

You need to participate in the design of your system. There are lots of options, including deciding, as Karol did recently, that installing an efficient central-air system (in a 75-year-old bungalow) is just too intrusive.

Next: A trip to the Shenandoah Valley.

Mr. Johnson is construction manager for Neighborhood Housing Services of Baltimore. Ms. Menzie is Home Editor of The Sun.

If you have questions, comments, tips or experiences to share about working on houses, write to us c/o HOME WORK, The Sun, 501 N. Calvert St., Baltimore, Md. 21278. Questions of general interest will be answered in the column; comments, tips and experiences will be reported in occasional columns.

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