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 water line 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.