Hartford loop piping schematic for a steam boiler - adapted from ITT's The Steam Book - (C) InspectApedia Daniel Friedman The Hartford Loop on Steam Boilers
Definition, function, safety features, piping for the Hartford Loop

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The Hartford Loop used on steam heating systems, definition, functions, inspection, installation, repair:

This article gives a definition of the Hartford Loop used on steam heating systems, explains why the Hartford Loop is necessary to protect against steam boiler damage, and gives a bit of the history of the development of the Hartford loop.

We include sketches and photographs of the Hartford Loop piping arrangement on steam boilers and list inspection & defect points of interest.

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Hartford Loop safety piping arrangement on steam boilers

Definition of the Hartford Loop - what is a Hartford Loop & Why is it Important on Steam Systems?

Hartford loop piping schematic for a steam boiler - adapted from ITT's The Steam Book - (C) InspectApedia Daniel Friedman Hartford loop piping schematic for a steam boiler - adapted from ITT's The Steam Book - (C) InspectApedia Daniel Friedman

Invented in the U.S. in 1919 by the Hartford Steam Boiler and Inspection Company in response to an unacceptable rate of steam boiler damage, loss, and even explosions, the Hartford Loop is a piping arrangement at the steam boiler condensate return line that allows condensate to re-enter the boiler but prevents complete loss of steam boiler water should a leak develop elsewhere in the condensate return line.

Today the company refers to itself as the Hartford Steam Boiler Inspection and Insurance Company (founded in 1866) - not to be confused with the Hartford Fire Insurance Company (founded in 1810).

[Click to enlarge any image]

As you can infer by looking at our drawing at above left Boiler Without a Hartford Loop, a leak anywhere in the wet return portion of the condensate return piping that is below the boiler water line risks siphoning out all of the water from the heating boiler.

Our illustration at above right titled Hartford Loop shows where the Hartford Loop is located on a steam boiler. Above the drawing is adapted from ITT's The Steam Book.

The 2-inch Rule and the 24-inch Rule for Hartford Loops & Steam Headers

Steam boiler hartford loop improperly piped (C) Daniel Friedman

But the boiler manufacturer may provide additional steam piping specifications - as we illustrate below. For example instead of specifying that the top of the Hartford Loop is two inches below the boiler water line the manufacturer may specify that the pipe fitters maintain at least 24 inches between the boiler water line and the bottom of the steam header.

The risk of steam condensate return water leaks was and can remain significant because of the corrosive acids that form in steam and other heating boilers and because not coincidentally, the more likely place where such leaks would develop is in the last and lowest run of the condensate return line along or close to the floor approaching the steam boiler.

In our discussion of steam condensate systems we illustrate a chemical injection system intended to reduce this corrosivity.

At below right our image is adapted from the piping schematic for a Weil McLain steam boiler - Weil McLain Model 78 (2014)

Steam condensate return leaks (C) D Friedman Hartford loop piping schematic (C) InspectApedia

Boiler manufacturers typically specify that a Hartford Loop piping arrangement and a wet condensate return line are required for steam boilers.

In addition to the installation of Hartford Loop to prevent total boiler water loss from a leak in the condensate piping, [the wet condensate return line] a steam boiler specification will maintain at least 24 inches from the steam boiler water line (see GAUGES, STEAM BOILER) to the bottom of the steam header.

And if a separate condensate receiver is used, a feed pump to move condensate back to the boiler must be energized by a boiler-mounted condensate return pump controller. (see CONDENSATE RETURN PIPES, PUMPS, STEAM) - adapted from Weil McLain 78 boiler instructions.

A separate worry for Hartford-loopless steam boilers has been suggesteed by Steam Boiler Gurur Dan Holohan - paraphrasing

The Hartford loop is needed to prevent steam pressure in the condensate return line from forcing the liquid condensate (water) out of the boiler and back into the return piping system - something that could be a serious problem resulting in boiler water loss and boiler damage or destruction.

The “wet” gravity return line, which returns the condensate from the system, rises up from the floor to join with the equalizer at a point about 2 in. below the boiler’s lowest operating waterline.

[Steam boiler installers ...] didn’t always use this piping arrangement. They used to bring the return directly back into the bottom of the boiler without the benefit of either a Hartford Loop or an equalizer. When they piped a boiler this way, however, the slightest steam pressure would push water out of the boiler and into the return. [Emphasis ours - Ed.]

They solved this problem by using a check valve in the wet return (that’s the pipe below the boiler waterline).

Before long, though, they found the check valve would fill with sediment and get stuck open.

That caused the water to back out of the boiler again, so they developed the equalizer pipe to replace the check valve. ... - Dan Holohan,PM Mag, online, un-dated, retrieved 4/7/2017 cited at REFERENCES

Dan continues to point out that leaks at the return line were catastrophic for steam boilers and adds that the low water cutoff valve (LWCO) was not invented until E.N. McDonnell invented it in 1923.

Really? The pressure in a residential steam heating system is normally less than 0.5 psi. At WATER PRESSURE MEASUREMENT (not specifically a heating system article) we describe how to compute the pressure exerted by water at the bottom of any vertical column of water - say a condensate return pipe, for example.

Even without a Hartford loop, and assuming for argument that the steam boiler is in a building basement and that the normal or "safe" water level in the boiler itself is at most 4 ft. above the floor, we can calculate that

The pressure of a column of water is about 0.433 psi per foot of column height.

The pressure of a 4 ft. height of water in the steam boiler and also in the riser of the condensate return pipe wherever it mounts to upper floors, measured at the bottom of that column of water, will be

Water pressure in the steam system at floor level (WPfl) = 4 x 0.433 psi = 1.732 psi.

If my calculations are right (I'm not a boiler engineer) we have about 1.7 psi of resistance to water leaving the steam boiler and 0.5 psi or less of steam pressure trying to push water out of the boiler.

So I think that for low-pressure residential steam systems the greater risk is not steam pressure pushing much water out of the boiler, but rather a leak anywhere in that close-to-floor-level condensate return line. (However the risk of boiler water push-out at high pressure steam boilers would be another and still more serious matter entirely. )

Leaks Do Occur in the Condensate Return Line

Leak in steam heat condensate return line at the boiler (C) Daniel FriedmanDo leaks actually occur in the wet return portion of a steam heat condensate return piping system? You bet they do. Depending on what's done to manage steam boiler chemistry, the condensate may be quite corrosive. Our photo at left shows two leaks at the condensate wet return at a boiler located in in the U.S. in Poughkeepsie, NY.

Watch out: at CONDENSATE RETURN PIPES, PUMPS, STEAM we explain that a leaky condensate line such as the one shown in our photo at above left can be lead to dangerous BLEVE EXPLOSIONS.

You can check the water level in your steam boiler quite easily by using the built-in STEAM BOILER SIGHT GLASS.

The Hartford Loop is also discussed at STEAM HEAT DEFECTS LIST


History of the Hartford Loop & the Hartford Steam Boiler & Insurance Company

Comment: the Hartford Steam Boiler & Inspection Company

2017/04/07 Nancy said:

Quick correction. The loop was invented by the The Hartford Steam Boiler Insurance and Inspection Company (Where I work - we're 150 years old!). The Hartford Insurance co. is a different entity all together.

Nice job with a basic, easy to understand explanation!

Reply: The Hartford Steam Boiler Inspection and Insurance Company is not the Hartford Fire Insurance Company

Thanks for the clarification, Nancy. Indeed it's easy to be confused about Hartford Insurance and Hartford Fire Insurance companies. The Hartford Insurance and Hartford Boiler company were of a piece.

In sum it would be fair to say that the Hartford Loop was invented by and a key feature of The Hartford Steam Boiler Inspection and Insurance Company founded on June 30, 1866.

While the Hartford FIRE Insurance company was and remains a different company.

Munich RE, parent company of the HSB group, publishing as the Hartford Steam Boiler Inspection and Insurance Company at

The Hartford Steam Boiler Inspection and Insurance Company was founded on June 30, 1866. Our first president considered HSB as “the first company in America devoted primarily to industrial safety.” “Hartford Standards” quickly became the specifications for boiler design, manufacture and maintenance.


Insurance incentive for boiler inspection

The Polytechnic Club spent much time debating about an idea that combined insurance with a boiler inspection. They reasoned that inspections would increase boiler safety and the insurance would function as an incentive to inspect and a guarantee of a quality inspection.

Though the insurance offered financial interests, it was secondary to safety and loss prevention – a totally new concept for an insurance offering.

Under that premise, The Hartford Steam Boiler Inspection and Insurance Company was founded in 1866.

Separately at the Hartford Insurance Comany's website one sees that the company was founded as the Hartford Fire Insurance Company.

The Hartford was founded in 1810 in Hartford, Connecticut. A group of local merchants gathered in a Hartford inn and, with working capital of $15,000, founded the Hartford Fire Insurance Company. ...

In 1810, we started as a fire insurance company, employing our own fire department to protect customers. Fifteen years later, we wrote the first insurance policy for an institution of higher learning – Yale University.


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HARTFORD LOOP at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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