Just months before NYCHA’s problem-plagued heating systems suffered from periodic outages amid a sub-freezing cold snap, the agency lost roughly 100 boiler repairmen, officials said.

The departures left the public housing agency with just 243 heating technicians as of December 29 — down from the 347 positions that were allocated in the agency’s budget.

“We had around 100 workers take civil service promotions in the recent call up that created vacancies which we have been working to fill,” said NYCHA spokeswoman Jasmine Blake. “During this extreme cold period to meet the needs of our residents, we have turned to vendors to maintain service levels.”

Officials said they have since hired 47 heating technicians, and plan to hire 55 more.

They noted that 97 percent of the authority’s 178,000 have consistent heat, with chronic issues hitting some locations — but most apartments sufficiently heated in recent weeks.

The agency acknowledged that its current heating technicians are not qualified to manage boilers at 69 developments — which are new and high-tech — but that it is seeking out more specialized workers to manage those boiler plants.

Mayor de Blasio and agency officials also pointed to the unusually long, cold-snap for straining aging boilers and spiking the number of “no heat” work orders that have been opened.

From Dec. 29 to Jan. 2, the agency opened 15,039 work orders — compared to 3,659 over the same stretch a year prior, officials said.

“These last days we have 6 days in a row, subfreezing temperatures, my understanding the last time that happened is the year of my birth, 1961, here in new york city,” Mayor de Blasio said on NY1 late Monday.

“That was really a strain on everyone.”

He pointed to the age of the infrastructure at NYCHA developments — many of which date back to the 1940s and 1950s — for some of the recent heating outages throughout the city.

“I went out to Woodside Houses in Queens during one of the coldest days and I talked to the head of heating and maintenance for all of NYCHA, and he said, ‘Look, we’ve got one of our oldest boilers from 1954. We’ve got a lot of boilers that the companies have gone out of business, you can’t get the parts, we have to manufacture our own parts,’ ” the mayor told NY1.

“This is the reality of public housing for 400,000 people.”

Residents at least 42 projects have filed complaints about lack of heat amid the current cold snap, officials said.