Nelson Stream tried not to fidget, as a long-awaited moment approached. For three years, he had lived with his wife and three children in a ramshackle three-bedroom house built from a mismatch of plywood and rough-sawed lumber. Heat came from a kerosene stove and the only water was from a well.

Yesterday was moving day -- but first there were speeches by politicians and questions from reporters as Montgomery County government turned over the keys of a new $54,000 brown-shuttered house to the 41-year-old Stream.

The new house is next to the Streams' old home in rural Clarksburg and is the first in an unusual county program to replace the houses of families living in badly deteriorating dwellings.

"They are sort of delirious," said G. Henk Smittenaar, the Rockville builder who assembled the ranch-style prefabricated modular house on the hilly woodside of the Streams' one-acre lot. "They said it was about the best Christmas present they could get."

Stream was a little more contained with the reporters who gathered in the morning cold and peppered him with questions. "Yes, it feels good . . . Naturally, we are looking forward . . . Real pleased, we are," said Stream.

Asked when his family would move their belongings up the hill into their new house, Stream said, "Just as soon as you all leave, not to rush you." Their old house and the outhouse that served it will be demolished.

Under the program, which Montgomery officials say is unique, the county identifies substandard houses that face demolition and helps their owners -- who must meet income standards and must occupy the houses -- receive low-interest mortgages. The Streams received a 6 percent, 30-year loan and will have monthly payments of $360.

Stream used to work for the state highway department and was disabled when he was hit by a truck. He receives Social Security benefits and his wife, Cynthia, works at a small neighborhood grocery store. Their monthly payments on the property they inherited from Cynthia Stream's mother, who reared her children there, had been $186.

The county has identified an estimated 200 run-down houses, mostly in its rural regions, as needing replacement, said Richard J. Ferrara, director of Housing and Community Development. Under the $500,000-a-year program, the county plans to replace five houses this fiscal year and five the next, then, it is hoped, 10 houses a year.

Ferrara said many of the deteriorated houses cannot be replaced because they are not on sewer lines and soil conditions make it impossible to build a septic system.

A replacement home program was started in the late 1960s but ended in 1982 after 45 houses had been completed and funding ran out. The current program came about through the interest of County Council member William E. Hanna Jr., a Democrat.

Hanna said he went on a housing tour several years ago and was astounded to discover large numbers of houses without indoor plumbing and in substandard condition. "I just thought it was a travesty in this county, of all counties, to have homes where people lived in the cold and without indoor plumbing," Hanna said.

The dilemma for the county, said Vic Brescia, deputy director of housing and community development, is how to help people of limited means who live in condemned property -- without disrupting their lives. "You don't want to drive them off the land that has been in their family for generations," he said.

The Streams had applied for a loan to rehabilitate their property after they had been cited for housing code violations. Stream said they were a little hesitant to get involved with the government, but the more they heard, the more they liked.

Stream said he and his wife, who was at work yesterday, were able to make most of the decorating decisions about their new property. There are two bathrooms, a full basement, brown linoleum in the kitchen and a fireplace (a gift from his parents) in the living room.

Already there were Christmas lights on the windows of the new house, and a creche out in front. "We'll have a party tonight," Stream said. Hanna, as he left, held out his arm for a final handshake and said, "Live well and enjoy."