Don't sacrifice safety for fireplace efficiency

Recent articles on the energy page about increasing the efficiency of fireplaces have raised a number of questions that need some clarification.

The oxygen supply is one of them.

Most of the houses built prior to the energy crises of the early 1970s had so many loose-fitting doors, windows, and other cracks that several complete air changes would take place in the house every hour.

The tighter construction of newer homes and the retrofitting of older ones for energy efficiency, such as insulation in the walls, weatherstripping, and caulking around windows and doors, have substantially reduced the infiltration of air from the outside.

In these circumstances, then, a roaring fire might consume oxygen in the air faster than it can be replaced from outside -- with potential danger to anyone in the house.

The solution is simple: crack a window about one-quarter of an inch every time you light the fire.

If you plan to use the fireplace consistently, it would pay to duct outside air directly to the fireplace. This would generally be done under the floor, surfacing directly in front of the fire. That way, too, no already-warmed inside air would be lost up the flue and the oxygen supply to the fire would be assured. This applies to wood- and coal-burning stoves, as well as the open hearth. Damper

In one article, headlined ''How to make a grate fire really great,'' the writer, Stanislav Jakuba, recommends ''keeping the damper open to a minimum'' to avoid wasting too much already-warmed inside air up the chimney. This is a long-established practice and is the reason, in fact, why dampers are made so that they can be adjusted to several levels, rather than simply opening wide or shutting tight. However, one reader has questioned the safety of this tactic.

Chief Andrew E. O'Brien of the fire-prevention division of the Boston Fire Department accepts the validity of reducing the damper opening, but he cautions that care must be exercised. If you close the damper too much, he points out, some of the smoke and escaping gases that should exit up the flue may come into the house instead.

According to Mr. Jakuba, his chimney damper has five notches. He starts the fire with the damper on the fourth notch (one lower than wide open) and, once the fire is burning well, reduces the opening to the third notch. When the fire has been reduced to brightly burning coals (no roaring flames, in other words), he cuts the damper down to the second notch.

The adjustment can be maintained as long as no smoke comes out into the room. One approach is to stand with your head by the mantle. If you can smell any hint of smoke, open the damper a notch. Sloping shield

The same writer suggests attaching a sloping shield close above the logs to reflect more heat back at the logs and force the flames closer to the back of the fire. Here, too, the advice is the same: lower the shield so long as the flames and smoke exit at the back of the shield.

If any smoke comes out in front of the grate, the shield has been lowered too far and should be raised up a fraction.

The shield should never actually rest on the logs. Doors and screens

A frequent tip for improving fireplace efficiency is to open such obstacles to radiation as glass doors or metal screens once the fire has become established. This is a fairly common practice because screens exclude an estimated one-fifth of all incoming heat. But fire prevention experts feel this practice is dangerous.

An open log fire can always spit glowing embers into a room and, on a windy night, a sudden draft can come down the chimney and blow sparks into the room. Chief O'Brien also cites another reason: Sturdy fire screens are particularly valuable safety aids when young children are in the house.

When it's time to retire at night and there is still a low fire burning in the grate, the damper has to be left open. Later on, with the fire out, the cold air tumbles down the chimney into the house -- unless you do something to prevent it.

The solution is to block the entire fireplace opening with a solid cover, but not with a wooden panel covered with aluminum foil as was suggested in a Monitor article recently.

''Don't use any combustible material in this situation,'' Chief O'Brien cautions.

While aluminum reflects radiant heat to a remarkable degree, it also conducts heat well. For this reason a cover that contains wood could be vulnerable if hot coals were to roll up against it. ''Stick to sheet metal,'' Chief O'Brien suggests.

While aluminium reflects radiant heat to a remarkable degree, it also conducts heat well. For this reason a cover that contains wood could be vulnerable if hot coals were to roll up against it. ''Stick to sheet metal,'' Chief O'Brien suggests.

The fire official also warns against using foam products between the fireplace and the metal cover. The reason is that the foam plastic can give off toxic fumes at high temperatures.