He stood at his kitchen sink Saturday morning, with all of New England in a deep freeze, watching a thermometer that showed the temperature outside: 8 degrees below zero. Inside, it was 68, just where he’d set the thermostat. “I saw that and thought, ‘OK, it does do what it’s supposed to do.’ ”

Across the region, where thousands have already converted to heat pumps and millions more are expected to in coming decades, the weekend’s plunge into sub-zero cold represented a critical first test of a kind of heat still considered new and unproven by many homeowners.

The pumps are a cornerstone of Massachusetts’ and some other states’ plans to slash carbon emissions. But many have wondered whether they can handle the worst of New England winter; contractors, in fact, often recommended that customers converting to heat pumps keep fossil fuel as a backup for such times.

In Massachusetts, where climate plans call for 1 million buildings to be converted from fossil fuel by 2030, homeowners reported that heat pumps, for the most part, did well on their own.

But there were exceptions. Heat pumps tended to struggle where temperatures sank much below minus 10 degrees, and some users reported difficulty keeping their living spaces well-heated when outside temperatures were slightly higher.

Katherine Tarca’s experience at her 2,700-square-foot Colonial home in Malden was similar to what several others reported. At 9 a.m. Saturday, when the temperature was 9 below zero, a cold-climate heat pump couldn’t get the temperature much above 60 degrees.

Such experiences appeared to be fairly rare, though, and many heat pump users said they were able to stay comfortably warm.

“I think as a stress test, this went really well,” said Justin Evans, whose home in Whitman stayed around 70 degrees — crucial with an infant at home — even as the temperature outside sank to 9 below. “It was as comfortable inside as it is any day,” he said.

As temperatures began to fall on Friday, the Globe asked heat pump users via Twitter and an online survey to report how their pumps performed as the worst of the cold hit.More than 450 responded, and most said their pumps kept their living spaces as warm as they wanted. That was particularly true in Massachusetts, where temperatures fell below zero but not to the extreme depths they hit farther north. And the best experiences were reported by people who had installed heat pumps intended for cold climates and whose homes were well-insulated.

In parts of New Hampshire and Maine, where temperatures plunged beyond 20 degrees below zero, a number of homeowners said their heat pumps couldn’t keep up, allowing indoor temperatures to fall into the low 60s or high 50s. Several who had that happen said their pumps were able to bring inside temperatures back up quickly after the sun rose and outside temperatures climbed back to 0 and above.

The reason, experts said, is that even cold-climate heat pumps become less effective at very low temperatures and can stop warming altogether when the mercury dips below the systems’ low temperature rating, about 13 degrees below zero without wind chill for most recent models. Before the weekend’s cold blast, ReVision Energy, which installs home solar panels and heat pumps in New England, tweeted a reminder to heat pump owners that their systems could shut down in the extreme cold.

In Jackson, N.H., where temps plunged to minus 25 degrees overnight Saturday (minus 50 with the wind chill), Mark Paul said his house dropped to about 50 degrees inside. He was saved by a wood stove, and Paul said it indicated to him that in especially cold places, it’s important to have a back-up heating source.

“That said, multiple people in town reported that their [gas or oil] furnaces went down or had other issues, and therefore I do not think this event should be used to single out heat pumps, though the challenges with them are undeniable,” he said.

Pump installers said they received very few calls for service on broken systems. “Our region experienced record-breaking low temperatures with essentially zero service issues as a result,” said Phoebe Hunt, vice president of service at ReVision Energy. “For us, this was a clear indication that air source heat pumps can maintain in New England weather.”

The extreme cold caused pipes to burst across New England, but just one of the heat pump owners who responded to the Globe’s survey said they had had that problem.

Climate advocates and heat pump contractors were carefully watching to see how the pumps would fare during the extreme cold, the first such spell since the pumps have been more widely adopted. Last year, 18,158 heat pumps were installed in Massachusetts — 2.5 times as many as in 2021, according to Mass Save, a state-sponsored energy efficiency program that offers rebates to encourage homeowners to switch.

“We can’t imagine a better demonstration of how heat pump technologies can perform than during last weekend’s weather,” a Mass Save spokesperson said.

Greenhouse gases produced by heating buildings, including homes, account for about a third of Massachusetts’ climate-warming emissions.

Ben Butterworth, the director of climate, energy, and equity analysis at the clean energy advocacy organization Acadia Center, said he wasn’t surprised that heat pumps performed well. “After years of falsely being told that heat pumps weren’t suitable for extreme cold, I do think this moment was critical for instilling confidence in heat pump users,” he said. “Continued reliance on fossil fuels to heat our buildings is simply incompatible with the state’s climate targets.”

Sabrina Shankman can be reached at sabrina.shankman@globe.com. Follow her @shankman.