As temperatures dipped into the single digits last Thursday, the city’s housing agency received a record number of calls for this winter complaining about cold radiators and frozen faucets.

More than 3,300 tenants called the Department of Housing Preservation and Development’s hotline on January 23, more than double the average call rate on a winter day. The No. 1 source of the problem, according to an agency spokesperson: boiler malfunction.

“Nine days of subfreezing weather taxes even boilers in good shape,” said spokesperson Carol Abrams. She added that with about 60 percent of the city’s residential buildings built before 1947, many of their boilers are probably as old, and likely need work. “Boilers are just like cars,” she said. “There’s always a car in the repair shop.”

HPD inspects boilers in the summer before each heat season, Abrams said, and offers low-interest loans to property owners who need to make building improvements.

While they don’t dispute the faulty boiler theory, some tenant advocates wonder if the heaters are really making it to the “repair shop,” and if the city’s inspectors are able to make it to all the buildings. HPD’s team of inspectors has shrunk dramatically over the last several years, as has the agency’s housing litigation bureau.

“It’s cost effective for landlords to ignore things now, and not pay for them later,” said David Powell of the Met Council on Housing. “More and more tenants go without heat, and more and more landlords get away with it.”