A Hartford Loop is an arrangement of piping between a steam boiler’s header and its gravity-return piping. The end of the header drops vertically below the boiler’s waterline and connects into the bottom of the boiler. We call this pipe the "equalizer" because it balances the pressure between the boiler’s steam outlet and condensate-return inlet. 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 two inches below the boiler’s lowest operating waterline.
The Dead Men didn’t always use this piping arrangement. They used to bring the return directly back into the bottom of the boiler without 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. They solved this problem by using a check valve in the wet return (that’s the pipe below the boiler water line).
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. Whatever pressure appears inside the boiler will also appeared inside the equalizer pipe if you size that pipe properly. The two forces balance each other, and the water stays in the boiler.
Now, you may wonder what happens if a return line breaks. Well, water will flow from the boiler, and the boiler will either crack (dry-fire) or explode (if someone adds water while it’s dry-firing). That’s a sobering thought, isn’t it? Keep in mind there were no low-water cutoffs during the days of coal- and wood-fired boilers. E. N. McDonnell of McDonnell & Miller fame invented the first low-water cutoff in 1923. He had to wait for the invention of the oil burner before his product made any sense.
In 1919, the Hartford Steam Boiler Insurance and Inspection Company got tired of paying the claims on all those broken boilers, so they came up with the idea of this special piping configuration and mandated it for anyone who wanted insurance on their steam boiler. Before long, everyone was calling it the Hartford (or Underwriters) Loop. If you look at the records of boiler failures before and after 1919, you can see that the Loop had a very positive impact on our industry.
Here’s how the Hartford Loop works. If a return line breaks, water can only back out of the boiler to the point where the wet return line connects into the equalizer. The Loop works like a siphon that runs out of water. The point where the Loop connects to the equalizer is higher than the boiler’s crown sheet, and that’s what provides the safety. Since the water couldn’t instantly vanish from the boiler, it bought the Dead Men some time to notice the problem and save that wood- or coal-fired boiler. The piping arrangement wasn’t fail-safe, but it was a vast improvement over the old way of returning condensate directly into the bottom of the boiler.
Should you use a Hartford Loop nowadays? I sure think so! Your low-water cutoff should protect the boiler against a sudden loss of water, but if you have a gravity-return system, a Hartford Loop is the cheapest insurance you can buy to back up that low-water cutoff should a return rupture and water suddenly leave the boiler. Low-water cutoffs are great, but believe it or not, there are a few out there that don’t get blown-down once a week. Really!
The Loop connects into the equalizer through a close nipple and here’s why. Steam rises up through the equalizer, just as it does through the boiler sections. When the relatively cool return water meets that steam at the close nipple, the rising steam bubbles quickly condense. Naturally, the return water rushes in to fill the void left by the collapsing bubbles, and this creates a slight water hammer inside the tee connecting the return to the equalizer. If you use a long nipple instead of a close nipple, the returning water will have more room to move, so it creates more water hammer through its inertia. Long nipples acts like gun barrels in this case, and you should always avoid them. Stick with a close nipple, or use a "Y" fitting.
Generally, you should connect the close nipple about two inches below the bottom of the gauge glass, but check the boiler manufacturer’s specs on this. The centerline of the close nipple has to be below the waterline, but above the boiler’s crown sheet.
And don’t install the close nipple two inches below the center of the gauge glass because that’s probably too high. You see, when the water leaves the boiler as steam, the level in both the boiler and the equalizer will drop. If it drops below the close nipple, steam will push down through the equalizer, enter the wet return and create water hammer. This usually happens near the end of the firing cycle, and it causes a very noticeable racket.
If the system has a dry return (as most one-pipe steam system do) you still need a Hartford Loop. Consider a one-pipe steam system for a moment. The main works its way from the boiler header, around the basement, and then drops for the first time when it returns to the boiler. There are no return pipes below the boiler water line, so there’s no danger of losing the boiler water should an above-the-water-line return pipe break.
But keep in mind that the near-boiler piping has an equalizer that keeps the water from backing out of the boiler when steam pressure builds. If you drop directly from the end of your steam main (the return) into the middle of your equalizer, you’ll set up a condition where steam might have access to the return line through the equalizer as the waterline in the boiler steams down. By bringing your return line down to the floor, and then rising up into your equalizer (in other words, by building a Hartford Loop), you ensure that steam in the equalizer can never enter the return line and cause water hammer.
You base the size of the equalizer on the size of the boiler. Boilers up to 900 square feet EDR (D.O.E. Heating Capacity) should have an equalizer no smaller than 1-1/2". If the boiler is rated between 900 and 6,400 square feet EDR, use a 2-1/2" equalizer. Boilers sized over 6,400 EDR should have an equalizer that’s four-inch in size. If your equalizer is too small it won’t balance the pressure on the return, and your boiler water line will be very unsteady.
If you’re using a condensate return pump, you really don’t need the Hartford Loop. As soon as you add that condensate- or boiler-feed pump to a steam system, you open the returns to atmosphere. At that point, the equalizer stops being an equalizer, and becomes just a drip line for the header. All the return water must flow into the pump’s receiver, and from there, into the boiler.
The pump has a check valve at its discharge to keep the boiler water in the boiler. Should the check valve fail, the water from the boiler will simply flow into the pump’s receiver, and start the pump. The pump will move the water back in the boiler and then shut off. Then it will do it again – over and over.
Should the receiver spring a leak and the check valve fail at the same time, it’s possible for the boiler to lose its water, so you can use a Hartford Loop if the spirit moves you, but there is one large drawback. Every time the pump starts, it will shoot water under pressure through the close nipple and into that bull-headed tee in the header drip. Some of that water will probably fly up into the boiler header where it will create some bodacious water hammer.
That’s why I think you’re better off piping your pump discharge into the bottom of the header drip, well below the boiler’s waterline.
(Henry Gifford, who is well-known in energy circles, developed what he calls the Gifford Loop. It is a variation on the Hartford Loop and you can read more about that here: HPACGiffordLoop.pdf .)
Want to learn more? Check out The Lost Art of Steam Heating Revisited.