Don’t Get Caught in the “MasterTrap”!

Recently a contractor needed help with a job involving an old two-pipe steam system. It was a real adventure. The mistake he made was classic and one worth sharing with you. He described the problems he was having with the system: “The building was heating unevenly and banging. I visited the building and discovered that steam was pouring out of the vent pipe on the condensate pump.”

He figured the banging was caused by all the excess steam coming from the receiver vent, and the uneven heating was caused by the steam heading toward the vent line on the receiver instead of traveling towards the radiators. He installed a float and thermostatic Bear Trap right in front of the receiver’s inlet which stopped the steam from blowing out the vent line, but the heating system continued to bang and heat unevenly. In fact, if anything, after installing the trap the uneven heating became worse and the banging seemed louder! He was under the gun because he assured the customer that he knew what the problem was, and that he also knew the solution. At that point, the contractor called his Hoffman representative for help.

We hear and see the contractor’s problem all the time. Other than preventing the steam from blowing out the receiver’s vent pipe, his approach does nothing to fix the real problem and it will likely make the problem worse! To understand why his solution will not work, you must first understand how a two-pipe trapped system is designed to work.

Steam is produced in the boiler and then heads out towards the radiation. In front of the steam is air, which is always present and needs to be vented before the steam will enter the mains and radiators. In a one-pipe system, the air is vented through the main vent and the individual radiator vents. However, with a two-pipe system there are radiator traps installed on the outlet side of each radiator. These traps perform several functions.

  • They are normally open to vent air from the radiator and pass it into the return system.
  • They snap shut in the presence of steam, preventing it from getting into the return piping.
  • They open to drain the condensate that forms when the steam in the radiator condenses.

The air that passes into the return piping is eventually vented out of the system through the condensate pump’s vent pipe. The system is designed to operate with steam in the supply pipes and radiators, and only with air and condensate in the returns. As the condensate forms in the mains and radiators, the traps open and allow it to gravity drain back to the vented receiver. Once there, the float in the receiver rises and turns on the pump. The condensate is forced back into the boiler to start the cycle all over.

The big difference between what we just described, and what our contractor friend saw, is the steam blowing out the receiver’s vent pipe. The first question the contractor should ask is: What would allow steam to show up at the condensate receiver’s vent pipe? Remember that one of the functions of a steam trap is to trap steam! When it doesn’t, there is nothing to prevent the steam from crossing over into the return side of the system and once there, the pressure in the return piping will start to approach the same pressure as in the supply mains. This will stop the steam in its tracks because it needs this pressure differential to move. It’s like turning the circulator off in a forced hot-water system and expecting the system to still operate. Try to think of a two-pipe system as a ladder. One side of the ladder is the supply and each rung is a radiator with a steam trap. The other side of the ladder sees only the air and condensate from each rung.

When our contractor friend installed the “master trap” in front of the condensate receiver’s inlet, he only made a bad situation worse. The float and thermostatic (F&T) trap made sure that the returns would be pressurized. There was no pressure differential across the system’s original F&T traps, and this caused the condensate to back up in the distribution mains, causing more problems. The solution to this system’s problem-and many others that plague two-pipe steam systems-is to replace the radiator traps that have failed in the open position.

If you are experiencing problems with a steam system, or have any steam questions, give your HoffmanMcDonnell & MillerSteam Team representative a call.