President and Mrs. Reagan are planning to be away from the White House for three weeks in late August and early September, and they have the best possible excuse: The mansion's air-conditioning system will be shut off during some of what is predictably Washington's hottest season.

The system, which is being renovated and partly replaced, was installed between 1948 and 1952 as part of the White House reconstruction during the Truman administration.

While the total $5 million project will be done over a three-year period with both cooling and heating systems usually functioning, officials said the first phase requires a total shutdown from Aug. 26 through Sept. 7.

The work, to be overseen by the National Park Service, will replace deteriorated ducts, air-handling equipment, smoke dampers and temperature and humidity controls.

The Park Service said every effort will be made to continue tours for as many as 8,000 daily visitors during the shutdown. However, the announcement said decisions will be made on a day-to-day basis out of concern for heat and humidity damage to the White House art collection and the health and comfort of visitors.

According to the announcement, the president and first lady have agreed to be away during the shutdown. The White House press office had no early comment on their plans, but Metro Scene would bet they include some time on the California ranch.