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do these F&T traps fail open or closed?

Most of the steam traps on my radiators are of this type (they have thermostatic disc inside).

I read on this site that the only way to test them is to open the output side while in operation and if you have steam coming out, it's bad.
However, what if it fails closed? How would you know it's bad?



  • Grallert
    Grallert Member Posts: 644
    It's been my experience that these will more often fail open causing hammering and banging down the line. hey will fail open sometimes but not as often. The temperatures across the trap should tell the story. If that valve is open there will be steam leaving it and it will reflect that in its surface temps. If it is closed the outlet should be much cooler than the inlet.
    Miss Hall's School service mechanic, greenhouse manager,teacher and dog walker
  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    If it's failed closed the associated radiator or section of line won't heat, since it can't vent the air.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ScottSecor
    ScottSecor Member Posts: 855
    We've found they can fail in the open or closed position. Most often, the open position.

    Can be difficult to pinpoint which trap failed in a larger system (think public high school with 2000 students). Most often a small group of classrooms next to each other might be under-heating. Someone swaps the trap and/or or the zone valve (Danfoss, Honeywell, etc.) and problem continues. Only to fin out that the offending trap is actually closer to the end of the line and is wide open. In this case, the blown (open) trap is causing the return main to pressurize and not allow the adjacent traps to work properly. Some tell tale signs are the classroom with the bad trap typically overheats, you can often hear the steam blowing through the trap, and the return pipes in the crawl space below will be hotter than normal just after the failed trap ties into the return.

    We did a lot of this type of work in schools long before imaging cameras and even infrared heat temperature guns were available. I'm guessing with today's thermal imaging technology things would be a lot easier. I also suspect my knees would feel much better with today's technology. I don't miss those seemingly endless crawlspaces that I used to visit regularly doing trap maintenance.
  • Harry_6
    Harry_6 Member Posts: 141
    Aside from thermal imaging, if you have access to the pipe a couple feet downstream of the trap, feel to see if it is steam-hot. If so, the trap is probably leaking through (assuming the radiator is hot). If the radiator is cold, remove the trap cover and see if it starts to heat. Although thermostatic traps usually fail open, some, like the 1980's Hoffman 17-C, occasionally had the spot weld holding the thermostatic elements to the cap fail, dropping everything to the bottom and closing the port. But beware, if that happens you might have a radiator completely full of water dumping out of the trap at you.