The majority of houses here and elsewhere are heated with forced hot air. Inexpensive to install, it shares much of the central plant with summer air conditioning.

Both use the same system of room registers, ducts, filter, furnace blower and controls. That sharing, however, makes it more difficult to adjust the distribution of the air for even and pleasant heating and cooling.

The problem arises from the fact that warm air is relatively light and rises, while cold air falls. The air in an undisturbed room containing no heating or cooling sources will naturally arrange itself so that there is a vertical gradient in temperature, warm at the top, cool at the bottom.

Usually, of course, there are a variety of warm and cool "sources" in a room: windows, exterior walls, people, appliances, and heating or cooling outlets. So the air is usually in motion, circulating in various patterns. But, superimposed on the circulation is the natural tendency of the warm air to rise and the cool air to fall to the floor. One cold day I measured a 15-degree difference between a point one foot from the floor and a point one foot from the ceiling.

Good heating design tries to counter that tendency by placing heating outlets (registers) low on the walls, or on the floors. But cooling design would call for placement high on walls or on ceilings.

A solution to this dilemma, used in better homes some years ago, was to run the duct up inside a room's wall so that two registers could be attached, one low, one high. The occupant opens the lower one and closes the upper one for the winter, then reverses the arrangement for the summer cooling season.

Not many recently built houses have this option. Registers are often placed where construction convenience dictates, and the results are not always satisfactory. Another solution requires that the furnace fan be quite powerful so that air leaves each register at high speed, and utilizes registers with adjustable vanes to aim the air downward in the winter, upward in the summer.

Unfortunately, these adjustable registers cost more and are seldom installed by builders. But the homeowner can do so where needed. Such a register has, in addition to the inner vane which is largely for controlling the amount of flowing air, an outer set of vanes with which one can manually adjust air flow direction. They cost between $10 and $15 each. But remember that air speed, or pressure, must be good in order to take full advantage of the feature.

Air speed may be adjustable in some installations. It might be possible to change pulley sizes on belt-driven furnace fans, or to change electrical taps on the motors, but this had best be determined by a qualified mechanic. A few systems are switched to higher speed by the thermostat for the cooling season, because circulating the heavier cool air requires more power and because drafts are found welcome rather than objectionable during the summer.

Limitations of register placement and air velocity may be difficult to overcome. To remedy undersized ductwork can be very costly; a duct booster fan at a troublesome register may help.

One trick I've seen used in lieu of replacing ceiling registers worked well in a large living room that happened to have four ceiling diffusers, a type of register that spreads the air out horizontally, parallel to the ceiling. Do-it-yourself owners were able to convert two of the registers so that they aimed the air downward.

Now they open those and close the other pair in the winter, vice-versa for the summer. They carefully chose the ones to convert to downflow; these are not directly over where people usually sit or congregate.

This quick fix may be easy to pull off if the register is the rectangular type; its outer vanes are often easy to bend to the vertical position using a pair of long-nosed pliers. But do not expect to bend them back and forth without breakage; purchase a doubly-adjustable replacement instead.

Comfort can be improved in every house by adjusting registers room by room, partially closing registers in rooms that seem to be getting too much of the desired air in favor of those rooms needing it. The settings are best determined during prolonged cold spells, and will be different for the cooling season, so make notes on your final settings before the seasonal change-over.

Take care not to close off more than about a third of the registers, and check to see whether there are branch valves in the basement or attic ductwork. The amount of air flowing through the furnace must not be reduced below a certain point, and balancing a system with branch ductwork valves may be tricky. If in doubt, call on your heating-cooling contractor.

Recently built houses usually have one to three large central return grilles, usually in hallways, so that the air issuing from bedroom registers must pass through the bedroom doors into the hall, to the grille, and thence back to the furnace fan.

What happens when a bedroom door is closed? That air goes under the door, unless the builder forgot to leave enough clearance or the owner installed carpeting later which reduced the clearance to zero. Check to see that there is about half an inch between door and top of carpeting for a one-register room, an inch for a multi-register room. Or leave your door ajar.

Finally, it may help to run your furnace fan continually during extreme weather, probably in very hot weather, possibly in very cold weather. It may give you more uniformity in home temperatures; try it and see. The option is available in newer homes at the thermostat, in older homes perhaps at the furnace, in some not at all. But don't forget that you've set it that way; operating the fan continually adds wear and tear and runs up your electric bill. Worse yet, if your ductwork is leaky, it may run up your heating bill.

William Papian is an engineer with the home-in-spection firm of Claxton Walker and Associates.