Plenty big, according to the Census Bureau, which charts expenditures made on behalf of U.S. homes. The report refers mainly to single-family detached structures, but extends to buildings with up to four housing units.

Through 1998, the most recent year measured, expenditures totaled $9.763 billion.

This includes three categories of activity: alterations, major replacements, and maintenance and repairs.

This 10% decline from the previous year is not explained by the Bureau, which just charts the numbers.

Since 1993, each year has exceeded expenditures in the previous year until 1998.

Trades collateral to hvac also showed a dip in 1998. Plumbing, at $7.306 billion, was off 24%. Electrical, at $1.644 billion, was off 28%.

The heating-cooling activity is about 11% of the total expenditures ($90.2 billion) for such things as additions of decks and porches, remodeling, roofing, flooring, fences, and other activities.

In addition to repairs and replacements to the hvac equipment in existing buildings, the new housing sector, which is at 1 million-plus units a year, also represents about $1.5 billion in sales of new hvac equipment, according to Bob Schjerven, president of Lennox Industries. The company is expanding its attack on this market, he said.