Harford Loop
Photo credit: Dan Holohan

The Hartford Loop isn’t just for steam jobs in Connecticut. It has a grand history and it has saved a lot of lives. It’s one of those things in the world of heating that is beautiful for its simplicity. Here’s what you should know:       

1. 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 water line 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 2 in. below the boiler’s lowest operating waterline.

2. 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 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. 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. Whatever pressure appears inside the boiler will appear inside the equalizer pipe — if you size the pipe properly. The two forces balance each other and the water stays in the boiler.

3. If a wet-return line broke, the water would flow from the boiler and the boiler would either crack from dry-firing or explode if someone added water while it was dry-firing. Keep in mind there were no low-water cutoffs during the days of coal-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.

4. Around 1919, the Hartford Steam Boiler Insurance and Inspection Co. got tired of paying the claims on all those broken boilers, so it came up with the idea of this special piping configuration and mandated it for anyone who wanted insurance on a 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 it had a very positive impact on our industry.


How it works

5. With the Hartford Loop, 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 the boiler. It wasn’t perfect but it was much better than what they had before.

6. The Hartford Loop connects to the equalizer with a close nipple because 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. The returning 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 act like gun barrels in this case and you should always avoid them. Stick with a close nipple or use a “Y” fitting.

7. Always follow the boiler manufacturer’s installation instructions when setting the centerline of that close nipple. They know the safest point. This is not a place to get creative.

8. If the system has a dry return (as most one-pipe steam system do), you still need a Hartford Loop, even though no return piping is below the boiler’s waterline. This is because 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 into the 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 create a loop seal. Now steam can’t enter the return line and cause water hammer.

9. Base the size of the equalizer on the size of the boiler. Boilers up to 900 sq. ft. Equivalent Direct Radiation (DOE heating capacity) should have an equalizer no smaller than 1 1/2 in. If the boiler is rated between 900 and 6,400 sq. ft. EDR, use a 2 1/2 in. equalizer.

Boilers sized over 6,400 EDR should have an equalizer that’s 4 in. If your equalizer is too small, it won’t balance the pressure on the return and your boiler waterline will be very unsteady.

10. If you’re using a condensate return pump, you 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 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 backward 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 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. 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 turn your steam into water and increase your customer’s fuel bills. It’s best if you pipe the return pump’s discharge into the bottom of the header drip, well below the boiler’s waterline.

See what I mean about it being beautifully simple?