Over the past few decades, two trends -- electric heat pumps and direct-vent, gas-powered fireplaces -- have made chimneys an optional feature for many new houses. But in recent years, the desire to upgrade heat pumps to oil or gas furnaces that actually put out warm-feeling air has raised a problem: How do you get the furnace exhaust out of the house if you don't have a chimney?

Furnaces are pretty simple, really. They burn something (oil, gas, coal, wood) to create heat and then use various means (forced air ducts, radiators) to spread most of that heat around the house. "Most" is a key word -- from 10 percent to 20 percent of the heat produced by an oil or gas furnace gets tied up in combustion gases that contain soot, carbon dioxide, carbon monoxide and other substances that are dangerous and need to be vented outside the house. This is where the chimney comes back into the picture.

Chimneys take advantage of the fact that, when contained to a narrow column, warm air will rise straight up. A chimney that is tall enough and has the right size opening creates a difference in pressure between the outside air and the fireplace that "draws" the smoke and those dangerous combustion gases away from living areas.

A furnace needs that same draw to exhaust soot and combustion gases safely to the outside. Since furnaces produce much more heat than a fireplace, they also create much higher quantities of exhaust gases, making proper ventilation even more critical. In houses with wood-burning fireplaces, a masonry chimney will usually contain two separate paths upward to the roof: a flue for the fireplace and one for the furnace. In the basement, the furnace connects to the chimney through a metal duct and the exhaust gases come out at the top of the chimney above the roof. But what about the many houses that don't have a wood-burning fireplace and chimney?

The solution is pretty straightforward with gas furnaces, said Dan Blum of Integrity Home Inspection Services Inc. in Washington. "The exhaust from gas furnaces can be run through an exhaust flue that can be as narrow as eight inches in diameter," Blum said. This allows a dedicated metal exhaust flue to be run inside the house to the roof much the way plumbing is run between floors. This is common practice for houses in the close-in suburbs where natural gas is available.

However, for new construction in more-remote suburbs where natural gas is often not available, oil heat is more common. The higher exhaust temperature of oil furnaces requires an exhaust flue clearance of at least 18 inches wide -- too bulky to run from the basement to the roof without intruding on living space in most houses. To solve this problem without incurring the expense of building an external masonry chimney just for the oil furnace, the heating industry came up with a new approach -- the side-wall-vented exhaust system.

A side-wall-vented furnace uses an electric fan to suck the combustion gases from the furnace and propel them out an opening in the foundation. The fan is mounted in housing in the exhaust duct between the furnace and the wall, generally in the floor joists in the basement ceiling. Outside, an exhaust vent that looks similar to those used by gas-burning, direct-vent fireplaces is mounted at least one foot above ground and at least four feet away from any window opening.

The fan provides the draw that a chimney would create but adds another layer of complexity to the heating system. Side-wall-vent systems must be wired so that the furnace can never operate unless the fan is running and creating the draw. If the furnace were to come on without the fan, toxic gases would be vented into the basement. Even worse, since the exhaust temperature of an oil furnace can be hotter than the ignition point of wood, heat would build up in the exhaust duct work and create a severe fire hazard to the wood joists and plywood subfloor above.

The leading manufacturers of residential side-wall-venting systems are Tjernlund Products Inc. of White Bear Lake, Minn., and Field Controls LLC of Kinston, N.C. Their systems include electronic controls wired between the thermostat and the furnace. When the thermostat calls for heat, the exhaust system intercepts the call and turns on the exhaust fan. Only after the fan has come up to speed and created a draw will a pressure sensor close the relay that tells the furnace to start up. If the fan or the circuitry fails to operate, the furnace should never receive the signal to start up. The side-wall-vent fan should also run for a few minutes after the furnace shuts down, in order to exhaust remaining heat and exhaust products from the furnace.

Correct installation and wiring of this interlock feature are critical to preventing disaster. A side-wall-exhaust system should always be inspected by a professional heating contractor that is experienced in installing and maintaining such systems. Home inspectors can perform limited visual inspection of side-wall-venting systems, but "many home inspectors may not have seen these systems if they mostly deal with houses with gas furnaces" said Blum, who is also president of the Mid-Atlantic Chapter of the American Association of Home Inspectors. "In that case, a home inspector should always recommend a full inspection by an oil furnace expert."

Oil furnaces should be inspected and tuned up every year, but many maintenance contracts don't automatically include the side-wall-exhaust system. Homeowners should make sure that whoever performs the yearly service is inspecting and testing the side-wall-vent systems thoroughly. During the heating season, you should be alert for signs of trouble and make occasional checks:

* When the heat is running, go outside and make sure the furnace exhaust is being forcibly expelled from the vent.

* Have someone turn the heat on while you are in the basement near the furnace. You should always hear the side-wall-vent fan start before you hear the furnace start up. The fan should run for a minute or two after the furnace stops.

* While you are next to the furnace, shine a flashlight at the exhaust fan assembly and make sure there are no signs of scorching or heat buildup

* If you begin to notice an oil or exhaust smell, turn off the furnace.

* A carbon monoxide detector mounted in the basement will detect if exhaust gases are building up. If you observe any of these problems, turn off the furnace and call for service. Do not run the furnace until the problem is resolved.