Two similar accidental house fires in prefabricated fireplaces within three years in Quince Orchard Manor have prompted Montgomery County Fire Marshal Dennis McLaughlin to issue a warning to homeowners in that Gaithersburg subdivision.

McLaughlin sent a memo three weeks ago to the 89 Quince Orchard Manor households advising that the fireplaces shouldn't be used again until they have been inspected for gaps between the flue and the surrounding chimney chase.

The first fire occurred in March 1975 in the house of Louis and Mary Willett on Cheyenne Road and caused $13,000 in damage. The Willetts had to move out for 10 weeks while their house underwent repair. A third of the roof trusses were burned.

The second fire did $6,600 in damage last month to the home on Apache Court of Richard and Mary Bromwell. The back wall, paneling and half the living room ceiling were affected, McLaughlin said. The fire apparently smouldered in the chimney for four days before it broke out. A smoke detector 20 feet away did not sound an alarm.

In both case, fire officials said, it appeared that some of the hot air went up the chimney chase instead of the flue itself, because there was a space between the brick fireplance facing and the metal surface of the pre-fab unit. The timbers above the metal box were continually exposed to the hot air and finally ignited, spreading up to the roof, fire officials said.

Adding to the problem at one house, according to the fire marshal, was the fact that the owners had installed a glass door on their fireplace that served to push more heat into the chase.

Quince Orchard Manor was developed by the Presley East Co.; the houses were built in 1972 by the William Fisher Development Corp. Thulman Majestic brand pre-fab fireplaces were installed in each house, as well as in Parkridge, a nearby Presley development on Clopper Road. The houses are now said to be worth between $80,000 ad $90,000.

Because there were no county standards covering pre-fab fireplaces in 1972, they were not inspected during construction, according to Ralph Graber, chief of the construction codes enforcement division of the county's Environmental Protection Department. The pre-fab code was instituted in June 1977.

Before 1977, said Graber, fireplace inspection was lax. It was "up to the individual inspector" whether to look at the installation, he said.

Sandy Buckstein,a Quince Orchard resident, maintains that the county should have inspected the fireplaces after the first fire, but Graber said that inspection regulations are not retroactive.

The lack of a construction code is disputed by both McLaughlin and Philip Mercer of Thulman Eastern Co., distributor of the fireplaces. Mercer noted that Montgomery County has long had a basic Building Officials and Code Administrators code. Moreover, he added, it is well known within the trade that spaces have to be filled to prevent fire from spreading upward in a house. A mason put on the brick facing after the fireplace box was installed.

In May 1975, owners received letters from Phil Mercer of Thulman Eastern advising that a component was missing from some of the Quince Orchard Manor and park Ridge fireplaces. He said the company would inspect the homes and report back to the owners. Mercer said in a recent interview that all homes were inspected form the exterior but said he did not recall if any repairs were made.

He said an exterior inspection was made because at the time of the first fire, which apparently started high up in the chimney chase, the existence of gaps above the fire box was not suspected.

Legal responsibility for the first fire has not yet been established. The Willetts' insurance company, Government Employees Insurance Co., is suing builder Fisher and subcontractor Jimstar Construction Co., which installed some of the Quince Orchard fireplaces. Jimstar in turn is suing Thulman Eastern, which also installed some of the units, as well as Majestic Co., the manufacturer.

Robert Cox, vice president of marketing for Majestic Co., a division of American Standard, said there have been obly a few accidental fires in the hundreds of thousands fireplaces his company has installed over the past 25 years. Asked about lawsuits, Cox said he didn't remember any large settlement made over the past few years.

Most of the accidents were due to improper usage, he said. There are a few cases now being litigated where improper installation is alleged; the manufacturer has never been found to be at fault, he added.

Lawyers for Fisher and Jimstar declined comment. The trial is set for Jan. 3. But GEICO's spokesman said the insurer hopes to settle out of court.

According to Fire Marshal McLaughlin, some of the Quince Orchard Manor fireplaces have gaps varying from one-eighth of an inch to an inch wide. A homeowner can check for gaps by looking to see if there is space between the brick facing and the metal surface of the firebox.

McLaughlin has instructed homeowners who find similar gaps to buy sheets of mineral wool safing from an insulation manufacturer. These sheets should be pushed into the space to seal access to the wood beams, he said. Materials cost about $10 and the installation can be done by the homeowner, he said. If properly installed and maintained, pre-fab fireplaces present no more hazards than masonry-constructed fireplaces, he said.