Russian fireplaces. They're more European than Russian and more stove than fireplace; but the name has stuck, because they warm like a bear coat and eat only as much wood as you'd find on a Siberian tundra.

GUS McNEIL of Shelton, Wash., was skeptical. How could an armload of twigs and sticks keep an 1,800-square-foot house warm for 12 hours? Not only that, but emit a heat so even that it would only vary 4 degrees from one end of his house to the other? All this from something known as a ``Russian fireplace.''

``It took a little convincing,'' says his wife, Merle. But eventually he was persuaded to have this massive brick and clay structure installed when they were remodeling their family room.

Now the McNeils have two fireplaces. One is the fieldstone-covered open hearth that's dear to the American heart as a symbol of home and warmth. Never mind that most of the heat goes right up the chimney and must be augmented by other heating methods.

The other, the Russian fireplace, is a big brick structure with a little iron damper door on one side that keeps their house warm at an even 68 to 70 degrees.

``It costs us half as much to heat our home using the Russian fireplace than it did to use electricity,'' says Mrs. McNeil. ``Plus, our water is pre-heated to 150 degrees through copper tubing that runs from the fireplace to the water heater.''

People have been using masonry heating for centuries in Northeastern Europe and Asia. Ancient Chinese and Koreans used a system of underground flues to heat their floors, as did the Romans. People of the tree-sparse plains of northern Russia, Poland, and the Baltic needed a way of keeping warm during the long, bitter winters without using much wood. The result was a massive masonry heating unit, which evolved, through some ingenious variations, into the modern masonry stove.

Actually, the term ``Russian fireplace'' is misleading. The brick and clay heating unit in the McNeils' family room is a combination of the best elements of centuries-old wood-burning masonry stove designs of the Eastern Europeans, Finnish, and Austro-Germans.

It's also not a fireplace, in the strict sense of the word. Generally, you won't see open flames roaring and crackling before your eyes, although glass doors can be installed for diehard fire-gazers. And, even though it's more efficient to maintain a closed system, Americans prefer an open fire, and many models are available here with an open-faced hearth.

A masonry stove will achieve energy efficiency of 70 to 90 percent - if properly built and maintained - so nearly all the wood's energy is used. By comparison, the typical masonry fireplace has an efficiency of 10 percent to minus 10 percent; a radiant-type woodstove has 50 to 70 percent efficiency.

There are many different types of masonry heating, and including variations such as cooking stoves, closed fireplaces, or some combination thereof.

Masonry heating has probably been advanced most efficiently and beautifully by the Austrians and Germans, in the development of the tile stove, or Kachelofen. The interior is based on the same principle as masonry stoves, but the exterior is covered with specially-fired, heat-retaining tiles, making use of the fact that a greater surface area will retain more heat. These convex, hollow block tiles come in varying shapes and colors with relief patterns or indentations to add extra surface area for heat retention. Many are hand-painted and quite lovely, making this popular European heating method an elegant architectural addition.

As with every heating system, there are some drawbacks. First, from its massive nature, the masonry stove lends itself best to installation only during new home building or remodeling.

Nevertheless, one of the leading masonry stove contracting companies in the United States, Deitmeyer, Ward and Stroud of Vashon Island, Wash., says a new masonry stove can be built almost anywhere in an existing house, if structural support can be provided.

Architect Gordon Cultum of Seattle agrees.

``The main problem in constructing a masonry stove would be ensuring sufficient support for the massive weight of the bricks.''

For that reason, the Russian fireplace is ``largely a new home phenomenon,'' says David Lyle, who runs Heating Research in Alstead, N.H., and is author of numerous articles and a book on masonry heating. ``Masonry is a bigger investment than an appliance,'' he says. ``It is a piece of architecture that adds value to a home, like a fireplace, and pays off in the long run.''

In fact, a recent survey by a remodeling magazine cited fireplace additions as having the top remodeling (that is, resale) return of 147 percent.

The second inconvenience of the masonry stove is that due to its size, it has a delayed warmth response. This isn't a problem in late fall and winter, when the weather is predictably cold, but in spring and early fall, weather must be closely watched. At worst, a window may have to be opened if the day becomes unseasonably warm.

Cost is another important factor. A custom model built by a master mason might run $5,000 for a standard brick stove and quite a bit more for a tile model.

As an alternative to the custom masonry stoves, there are also pre-fabricated models, Mr. Lyle notes. ``I think we have to go to modular construction in the US, because we lack the tradition in masonry for building these stoves,'' he says. ``It's a way to ensure quality in design in a world where there are not enough craftsmen to go around.''

For more information, contact the Association for Mason Heating Professionals, Dave Holland, P.O. Box 669, Englewood, CO 81051.