07Gas under the street, and why steam comes out of it

The short version: Natural gas reaches buildings through a network of mains under the street and small service lines to each building, with regulators stepping the pressure down along the way. The gas has no smell, so utilities add a sulfur compound that smells like rotten eggs. Older cities are spending billions to replace leaking cast-iron and bare-steel pipe, and the 2018 Merrimack Valley explosions in Massachusetts showed what happens when a pressure system fails. The steam rising from manholes in New York and Boston is something else: district steam, made at central plants and piped under the street to heat buildings.

How does gas get to a building?

In outline, natural gas arrives in a city through high-pressure pipelines and is fed into a local distribution system of gas mains under the streets. A smaller service line runs from the main to each building. Along the way, regulators reduce the pressure so that what reaches a stove or furnace is gentle and steady.

A regulator needs to know the pressure downstream of it, and in many older systems it learns this through a small pipe called a sensing line (or control line). If downstream pressure drops, the regulator opens to let more gas through; if pressure rises, it closes. That simple feedback loop is safe as long as the sensing line reads the right pipe. The Merrimack Valley disaster, described below, happened when it did not.⁷¹

According to the US Pipeline and Hazardous Materials Safety Administration (PHMSA), the handoff from transmission pipeline to local system happens at the city gate, where the pressure is lowered and a sour-smelling odorant is added.¹⁶² From there, gas typically reaches distribution mains from a high-pressure source through a regulator station that lowers the pressure again.¹⁶³ In the Merrimack Valley system described below, 14 regulator stations each used two regulators in series, a "worker" and a backup "monitor."¹⁶³

What does a gas leak smell like?

Natural gas has no smell of its own. Utilities add mercaptan, a sulfur compound with a "rotten egg" smell, so that people notice leaks.¹⁶⁴ The standard advice is the same everywhere: if you smell gas, leave the building and call the utility or emergency services from a safe place.¹⁶⁴ The order matters because a light switch, a phone or a doorbell inside can make the spark that ignites gas, so the call waits until you are outside.

What is leak-prone pipe?

Much of the gas pipe under older American cities is cast iron or bare steel, which utilities and regulators call leak-prone pipe. Cast iron is brittle and can crack as the ground shifts and freezes, and steel without corrosion protection can rust through.

Massachusetts set up a formal program to replace it. A 2014 gas leaks law created the Gas System Enhancement Plan (GSEP), which lets utilities recover the cost of replacing leak-prone pipe through customers' bills.⁷⁹ The price has climbed fast. The cost per mile replaced rose from $1.32 million in 2015 to $3.46 million in 2025.⁷⁹ As of 2025, 4,049 miles (6,500 km) of leak-prone pipe remained in the state, and finishing the job by replacement alone would cost an estimated $13.7 billion more.⁷⁹

In May 2025 the state Department of Public Utilities (DPU) cut the cap on how much GSEP can add to revenue, from 3% toward 1.5% by 2027, and told utilities to consider repairs and non-pipeline alternatives instead of automatically replacing every old main.⁷⁹ Massachusetts had about 1.7 million gas customers in 2019, 91% of them served by National Grid or Eversource.⁷⁹

The debate behind those numbers runs like this. Replacing old pipe makes the current system safer and reduces leaks of methane, a potent greenhouse gas. But the state is also pushing buildings to switch to electric heat. A new main is expected to last for decades, and if fewer customers use gas over that time, the remaining ones may end up paying for pipes that carry less and less. Utilities, regulators, climate groups and ratepayer advocates weigh those two goals differently.

What happened in the Merrimack Valley in 2018?

At about 4 p.m. on September 13, 2018, high-pressure gas poured into the low-pressure distribution system serving Lawrence, Andover and North Andover, Massachusetts, north of Boston.⁷⁰ Fires and explosions damaged 131 structures and destroyed at least five homes. One person was killed and 22 people were taken to hospitals, including three firefighters.⁷⁰ Columbia Gas of Massachusetts shut down the low-pressure system, cutting off 10,894 customers.⁷⁰

The US National Transportation Safety Board (NTSB) traced the cause to a pipe-replacement job. A crew working for Columbia Gas, which was owned by NiSource, was replacing an old cast-iron main. They abandoned the old main while the sensing lines for the area's regulators were still attached to it.⁷⁰ With the old pipe out of service, the sensing lines read falling pressure, and the regulators did what they were designed to do: they opened fully, sending high-pressure gas into the neighborhood system.⁷¹ The work package "did not account for the location of the sensing lines."⁸⁶ Columbia Gas' monitoring center in Columbus, Ohio, received the high-pressure alarms.⁸⁶

On September 24, 2019, the NTSB determined the probable cause: weak engineering management and oversight by Columbia Gas, and a low-pressure system without adequate overpressure protection.⁷¹ The board called systems that rely only on sensing lines and regulators "not optimal" and found NiSource's engineering risk management deficient.⁷⁰

Our reading: the disaster happened during work meant to make the system safer, which is a reminder that replacing old pipe carries risks of its own and depends on accurate records of what is connected to what.

The person killed was Leonel Rondon, 18.¹⁶⁵ According to PHMSA, the low-pressure system was meant to run at 0.5 pounds per square inch (psi), about 12 inches of water column, and its regulator stations stepped gas down from roughly 75 psi.¹⁶³

Why does steam come out of the street?

In Manhattan, and on a smaller scale in Boston, the white plumes rising from manholes and from tall orange-and-white striped tubes are usually not a gas leak or a sewer problem. They come from district steam: steam made at central plants and piped under the streets to heat many buildings.

New York's system is among the oldest and largest in the US. The New York Steam Company began service on March 3, 1882, and merged into Con Edison in 1954.¹⁶⁶ As of 2025, Con Ed's steam system had about 105 miles (169 km) of mains running from 96th Street to Battery Park and serves about 1,500 customers.¹⁶⁷ About 27 billion pounds of steam flow through it each year.¹⁶⁶

The visible vapor has two usual sources. Groundwater or rainwater that touches a hot steam pipe boils off, and small leaks in the pipes themselves release steam.¹⁶⁶ Con Ed vents that vapor through 10-foot (3 m) orange-and-white stacks placed over manholes, so it rises above pedestrians and drivers instead of blinding them at street level.¹⁶⁶

The pipes can also fail badly. Steam pipe explosions hit Midtown Manhattan in 2007 and the Flatiron District in 2018, when dozens of buildings were evacuated (Wikipedia gives both 49 and 28).¹⁶⁶

Steam does more than heat. In New York, Con Ed steam is used in restaurant kitchens and to power absorption chillers for air conditioning.¹⁶⁸ US district energy supplier Vicinity Energy says hospitals on its systems use its high-pressure steam for space humidification, to support infection control.¹⁶⁹ Steam systems like New York's belong to the first generation of district heating, introduced in the US in the 1880s; later generations, starting in the 1930s, switched to pressurized hot water, and the common heat carrier today is water rather than steam.¹⁶⁸

In Boston

Gas. Boston's gas history begins with light. The Boston Gas Light Company was chartered on January 22, 1823, and the city's first public gas street lamp was lit in Dock Square, near Faneuil Hall, on January 1, 1829.¹⁷⁰ Today the city's gas customers are part of the Massachusetts system described above, where National Grid and Eversource serve most of the state's roughly 1.7 million gas customers and the GSEP program pays to replace leak-prone mains.⁷⁹ The city still burns gas in about 2,800 street lamps; that story is in How streetlights work.

Steam. Boston and Cambridge share a district steam system now run by Vicinity Energy. Vicinity says it has served Boston for "over 90 years."⁹⁷ The system's size depends on which source you read, and they disagree:

Vicinity's own page gives both sets of pipe and building figures.⁹⁷ The larger numbers may describe Boston and the smaller ones Cambridge, but the page does not say so. Canary's figures come from its reporting on the system.⁵⁵ Either way, roughly 70 million square feet (6.5 million square meters) of buildings take heat from these pipes.

Much of that steam has come from the Kendall Cogeneration Station in Cambridge, a plant about 75 years old.⁵⁵ Emerson College and the life-science developer IQHQ have committed to buying its lower-carbon steam.⁵⁵

How it changed

Gas networks were built first for light, not heat. In London the Gas Light and Coke Company, the world's first gas company, received a royal charter in 1812, and by the 1820s London had about 50,000 gas lamps fed by roughly 300 miles of pipe.¹⁷¹ Boston followed with its 1823 charter and 1829 Dock Square lamp, but grew slowly: by 1839 it had only about 180 gas street lamps.¹⁷⁰

District steam arrived in the 1880s with the New York Steam Company in 1882.¹⁶⁶

The modern regulatory era in Massachusetts runs through two events: the 2014 gas leaks law that created GSEP,⁷⁹ and the 2018 Merrimack Valley explosions, followed by the NTSB's 2019 finding on cause.⁷¹

What's changing now (2026)

Gas: replace, repair or retire. Massachusetts regulators are no longer assuming that every old main should be replaced with a new one. The DPU's May 2025 order lowering the GSEP cap and pressing utilities to consider repairs and non-pipeline alternatives reflects the rising cost of replacement - $3.46 million a mile in 2025 - and the $13.7 billion still needed to finish.⁷⁹ That leaves a question for the next decade: how much to spend on a gas network that the state's climate policies aim to shrink.

Steam: moving to electricity. Both big US steam systems are trying to stop burning so much fuel.

• In Boston and Cambridge, Vicinity finished a 42 MW electric boiler at Kendall in 2024 and is selling what it calls eSteam, steam made with electricity rather than by burning fuel on site.⁵⁵⁹⁷ It has begun building a 35 MW heat pump that will draw heat from the Charles River, expected in mid-2028.⁵⁵⁹⁷
• In New York, Con Ed says direct emissions from its steam system are down 60% since 2005, and it is piloting heat pumps, electric boilers and thermal storage, aiming for measurable emissions cuts around 2035.¹⁶⁷

Electric steam shifts the emissions question onto the power grid (see How electricity gets to your building). A 42 MW boiler is a large new load on a grid that is also absorbing heat pumps and electric cars.

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