Do’s and don’ts for one-pipe steam systems

Volume 2/ Issue 4/ July 2015

Over the years we’ve heard numerous questions on steam systems, from what causes water hammer to how to size traps to selecting boiler feed tanks and condensate pumps. We’ve compiled our answers in the following do’s and don’ts for a one-pipe steam system.

1. DON’T size a replacement boiler by using the heat loss calculation method, the label method, the “looks a lot like” method or any other rule-of-thumb method.

DO size a replacement boiler by counting all the radiation in the house. The total becomes the net EDR (equivalent direct radiation) rating of your replacement boiler. Steam doesn’t care about heat loss or anything else–only the amount of the length of piping. If the boiler is too small, part of the building will never heat; if it’s too big, the new boiler will short cycle, consume more energy and require more service calls.

2. DON’T assume that you can pipe the new boiler exactly the same way as the old boiler.

DO pipe the boiler according to the manufacturer’s instructions. New boilers are very different from older boilers. The sections are narrower, the exit holes are smaller and less numerous, and the steam chest is almost non-existent. Pipe according to the directions, and save yourself a lot of headaches.

3. DON’T just throw in a couple of bottles of cleaning chemicals when you are finished piping a new boiler.

DO skim the boiler according to the manufacturer’s instructions. Oil from the foundry, as well as the cutting oils used in the field, creates surface tension on the top of the water in the boiler. This sets up a foaming/priming condition that creates wet steam, leading to water hammer, lack of heat, and unhappy customers.

4. DON’T use small radiator vents as end of main vents! Worse, don’t use a pipe plug.

DO use the largest main vents you can get. Venting the air from the main separately, and quickly, significantly improves the system’s balance: with a large vent, the steam heads toward the end of the main before it starts filling the risers. Hoffman Specialty Vents Number 4A, 75 or 76 are the right choice.

5. DON’T size the horizontal run-out to a one-pipe riser based on the inventory method.

DO use a steam pipe-sizing chart that tells you the exact size needed to support the radiation connected to that riser. The pipe that connects the steam main to an up feed riser i s called a horizontal run-out. Its job is to simultaneously supply steam to the riser and let condensate returning from the radiators drain back into the steam main. If the steam is moving too fast, the condensate can’t drain back into the main; instead, the steam will drive it toward the radiator vents.

6. DON’T use adjustable vents based on the radiator’s location.

DO use adjustable vents based on the radiator’s size. When coal-fired systems were converted to oil or gas, the system operated differently. Adjusting to this new “on-and-off ‘ system required balancing the air venting of the larger radiators against that of the smaller radiators. It had nothing to do with radiator location. With at least one large-capacity main vent installed at the end of each main, the adjustable vents simply control the venting rate of the larger radiators compared to the smaller radiators.

Hoffman Specialty® Adjustable Radiator Vent Series A1

Hoffman Specialty® Adjustable Radiator Vent Series A1

7. DON’T raise the setting of the pressuretrol, thinking it will solve a heating problem!

DO keep the Pressuretrol® or Vaporstat® set as low as possible, because:

  • Low-pressure steam moves faster than high-pressure steam. If you want steam to reach the end of the main quickly, lower the pressure and make sure you install a large-capacity main air vent.
  • All radiator vents have a “drop-away” pressure rating. This is the maximum pressure at which the float inside the vent can fall down without stopping the vent from re-opening. lf the pressuretrol setting is too high, the vent closes and stays closed, and any air remaining in the radiator can’t vent.
  • One-pipe steam radiators were sized to heat the house on the coldest day of the year with less than 1 psi of steam. For every square foot of EDR, the radiator will emit 240 BTU/H when the room temperature is 70°F and the steam temperature is 215°F.

Remember: Your Hoffman Specialty representative is always willing to help you solve your steam heating problems. Call them next time you need help.

Pressuretrol and VaporStat are registered trademarks of Honeywell International Inc.