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Building Science

Boilers Don’t Boil

Nearly all boilers used for residential heating just make hot water, not steam

This boiler provides space heating for a home in Maine. But it doesn't actually boil the water. It just makes it hot.
Image Credit: Energy Vanguard

Boilers for heating homes are common in some areas. Not here in Georgia, where I live, but my friends in the colder climates have them.

This type of heating equipment takes a fuel like natural gas or fuel oil, burns it to create heat, and then puts that heat into water circulating through the distribution system. Since it’s called a boiler, naturally it must be heating the incoming water up to the boiling point and creating steam. Right?

Most boilers don’t boil

Some boilers do boil water and make steam. Most don’t. If you live in a home built since the middle of the 20th century, any boiler you find therein almost certainly does not boil the water. Instead of steam, it makes hot water.

The hot water that comes out of the boiler then goes through a distribution system that dumps the heat into the house to keep you warm. Usually it’s a hydronic distribution system that circulates the hot water throughout the house. The water cools as it passes through radiators or or radiant floor tubes, giving up its heat to the surroundings.

Some boilers are part of a heating system that uses forced air distribution. The hot water passes through a hydronic coil in an air handler. The blower blows air over the coil and the warm air is distributed though a duct system. That’s a great way to provide supplemental heat when you have a heat pump.

The demise of steam heating

Since I live in the land of forced air, I don’t deal with boilers and hydronic systems much, so I asked around about steam heating. Dan Holohan, the god of steam heating and the creator of, told me in an e-mail, “Steam was very popular prior to WWII, particularly in the cities. Tract housing, the suburbs, and cheap furnaces killed it after the war.”

I knew that steam engines were dangerous in the past and that a desire to reduce the risk of explosion is often cited as one reason that Robert Stirling invented the hot air engine (now known as the Stirling engine). I asked Holohan about that issue, and he told me, “Steam boiler explosions were common during the 19th Century.”

A couple of changes, however, made them safe for use in homes. They were the Hartford Loop, mandated by Hartford Steam Boiler Insurance & Inspection Company, and McDonnell’s low-water cutoff. According to Holohan, these changes saved a lot of lives.

I also asked another expert in the field, John Barba of Taco Comfort Solutions, about the reasons behind the demise of steam heating. He told me, “expense, complexity, and the virtual cornucopia of alternatives have sent steam the way of the coal stove.” He also explained why it would be difficult to get a new steam heat system today: “The hardest part would be finding someone who could design and install it from scratch.”

Who still uses steam heating?

I also posted my question in the forum and found out a bit more about what steam heating is still out there. Robert O’Brien of Technical Heating on Long Island, New York, said it’s all in older homes. When I asked him if anyone installs steam boilers in new homes, he wrote, “No, no one does. Other than a few eccentrics, most of whom frequent this joint!”

Apparently a lot of older homes that used to have steam heating no longer have it. When the old systems needed to be repaired or replaced, they changed them out for another type. But there’s one big exception. O’Brien looked up some numbers for me and said that 65% of the one- and two-family homes in New York City still have steam heat.

There you have it. The two lessons for today are: (1) Most boilers don’t boil, and (2) Go to New York City if you want steam heat. Or at least hang out over at and find someone who can help you.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.

One Comment

  1. Expert Member
    Dana Dorsett | | #1

    Hydronic boilers DO indeed boil...
    ... but since the system is under pressure it's in the form of micro-bubbles at the surface of the heat exchanger. If the pressure inside the boiler goes too low it becomes an audible sizzle, at which point the heat transfer efficiency begins to drop a bit due to the insulative value of frothy bubble layer.

    Take the pressure down a bit more and that micro-boil goes macro, with larger bubbles that collapse with loud banging when the flow sweeps the larger bubbles away from the heat exchanger. A hydronic system that's losing pressure give plenty of warning in the form of clang-bang & sizzle alarm LONG before it becomes a safety issue.

    Steam systems are really quite simple and easy to debug & maintain. In locations where electricity is expensive (like NYC's ~30cents/kwh) an 80% combustion efficiency steam system can even have a lower operating cost than a mid-efficiency pumped hot water system, due to the cost of the pumping power involved. The fact that steam doesn't need to be pumped to run 5, 10, even 25 stories is also an advantage, since the pressures involved with pumped hot water in high-rise is substantial when compared to 2psi steam. (Even in 100 story buildings steam can be delivered from the basement at only 5psi.) It's not always just irrationally old-school fondness to keep steam systems going (though in low-rise construction it pretty much is.) Steam systems can even be micro-zoned on a radiator by radiator basis with thermostatic radiator vents.

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