HOMEOWNERS WHO fiddle with their furnaces may get more heat than they bargained for.

In a recent newsletter, Lennox Industries, maker of heating and various other kinds of equipment, warned against installing on its furnace vents any damper devices aimed at saving energy.

It said, "the field application of any retrofit device that alters the design methods for venting combustion products, or any device or modification that alters the combustion process of a Lennox furnace are considered potentially hazardous and may void the Lennox warranty for that unit."

The problem, it said, was that "such retrofit applications can cause condensation inside the furnace which will corrode the heat exchanger, greatly reducing the life of the furnace and eventually creating a hazardous contition for occupants of the structure."

In fact, Lennox has experienced no such problems with damper devices installed on its furnaces, said Mel Cornillaud, vice president for marketing. Nor could Cornillaud cite any laboratory work to support the claim (Several other major furnace manufacturers also said they had heard of no instances of corrosion caused by retrofitted dampers.)

The problem is a hypothetical one.

But Lennox's threat to void its warranties on altered furnaces neatly expresses manufacturer concers that energy-saving dampers, it improperly installed, could lead to injuries or deaths and involve them in costly product liability suits.

"This is an area that will keep a lot of lawyers in business," said a spokesman for Tappan. Tappan warranties also may be void in cases where equipment has been altered by the owner.

Automatic vent dampers have become a popular addition in recent years of high energy costs, especially since the American Gas Association, the major testing organization for such appliances, began certifying them.

Prices for them range from about $150-$250. The dampers attach to furnace and hotwater heater vents (gas or oil), opening when the appliance is running and closing when it is not to prevent heat from escaping up the flue. Applications in different parts of the country indicate they can, in at least some cases, result in substantial energy savings.

Recent studies by the U.S. Department of Energy showed the devices saved between 7 and 32 percent on fuel costs. Tests by the Michigan Consolidated Gas Company in Detroit bore an average savings of 17 percent in 18 different houses.

Though spokesmen for the Gas Appliance Manufacturers Association say they know of no injuries or health problems resulting from the use of vent dampers, homeowners should take care when installing one.

Said the Tappan spokesman: "I really don't feel we have enough field experience with them yet."

If installed incorrectly, the damper might fail to function, causing noxious exhaust fumes to back up in the house.

Find out which type of damper your furnace manufacturer recommends. Even manufacturers who don't make a damper of their own have tested others' on their equipment to meet AGA standards. Some recommend no damper be placed at all on their older model furnaces. Many, Lennox included, now make furnaces with damper options. Carrier's furnaces are build with printed circuitry that will accept only a Carrier vent damper.

Buy only a damper certified by AGA, for gas furnaces, or by Underwriters Laboratories, for oil.

Be sure the installer is a certified heating or plumbing contractor.

Get a permit. Mechanical permits are required in the Washington area to change furnaces. In some jurisdictions, permits are issued only to licensed contractors. The installation should also be inspected to ensure its safe operation.