A 35-megawatt heat pump designed to pull thermal energy from the Charles River into Vicinity Energy's Boston–Cambridge steam network would rank as the largest unit of its kind in North America once commissioned, according to the company and a parallel feasibility study underway at the Boston Green Ribbon Commission.

Vicinity, the district steam operator serving large institutional customers across the two cities, is mid-retrofit on a system that draws river heat rather than burns fossil fuel to generate steam. The utility's chief sustainability officer, Matt O'Malley, said the design copies European precedents, citing Glasgow's River Clyde network and Malmö's wastewater-fed thermal loop. "We are not trying to prove something," O'Malley said. "We are shamelessly emulating what has worked in leading European sustainability cities."

How large is the water-borne thermal resource?

Boston Harbor alone holds enough thermal energy to heat an estimated 2.4 million households each year, according to calculations by researchers at HEET, a Boston nonprofit that advocates for thermal networks. Ocean turnover refreshes harbor heat every two to ten days, said Archana Dayalu, a scientist who worked on the analysis. "Really, this is a near-infinite source of energy," Dayalu said. "It is a long-term sustainable solution."

HEET executive director Zeyneb Magavi labels the stock "anthrothermal" — heat trapped in urban water bodies largely as a byproduct of climate-warming activity. Three Boston-area projects now aim to draw on it.

What is UMass Boston building on its harbor campus?

The University of Massachusetts Boston has drawn seawater for cooling since the 1970s and now adds equipment to heat classrooms, laboratories, and dormitories. The project is in design and currently going out to bid, said James O'Day, the university's director of utilities, energy management, and laboratories.

When the first phase completes, the seawater-fed loop will satisfy the majority of campus heating load and trim natural gas use by 82 percent. A later phase will push that figure close to 100 percent. "From an environmental standpoint, it's definitely a win," O'Day said. "It's also much more energy-efficient."

The campus retrofit covers 12 buildings on a coastal footprint that already pulls cooling directly from the harbor.

What is BosTEN studying, and when does it report back?

The Boston Area Thermal Energy Network Project, or BosTEN, is the third initiative in flight. The Boston Green Ribbon Commission held its kickoff in late July at UMass Boston and has engaged a technical team to assess four questions:

• How much thermal energy can reliably be extracted from Boston Harbor, the Fort Point Channel, the Charles and Mystic rivers, and the bedrock beneath them
• How the energy should be transported to customers
• Which large institutional customers are most likely to connect
• What permitting and regulatory issues remain

The commission expects to deliver final recommendations in spring or summer 2027. "We really want to know where we can put a shovel in the ground at the end of this," said Lindsey Butler, the commission's executive director. "We're really trying to emphasize the era of implementation."

Why water-source rather than ground-source?

Conventional geothermal is hard to scale inside Boston's footprint, Butler said: the city is old, dense, and built on fill over a crowded underground. Water transfers heat more efficiently because seasonal swings are smaller than air swings — cooler in summer, warmer in winter — making the harbor and Charles more productive heat sources at peak load.

What decides whether the buildout actually happens?

The watchpoints to track are the BosTEN recommendations in spring or summer 2027, the construction-bid awards at UMass Boston, and Vicinity's commissioning schedule for the 35 MW Charles River unit. A separate permitting track through Massachusetts environmental and utility regulators will set the rate-recovery and emissions-credit terms that ultimately decide whether additional thermal loops get laid across the harbor floor in the decade after.