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Steam coil not heating

So I have a old factory with a bunch of ceiling mounted vertical discharge fan coil units. I'm having a issue with 1 not heating. These units are from the 70's and had been shut down for a few years. After making repairs to piping and coils one unit has me scratching my head. Probably overthinking it. I have a steam supply 1-1/2 that supply's steam to 4 units. The last unit on my run is the problem. I have steam to the coil. After the coil it goes into a f+t trap, then a check valve where it is gravity drained into the condensate return line. I crack the union on the inlet side of the trap and get good steam flow. My issue is after 15-20 minutes of run time the condensate line on the outlet side of the coil starts cooling off causing my aqua stat that is on the condensate line to open due to not being hot enough. I thought the check valve was getting stuck causing the condensate to back up into the trap. I found there was some steam leaking back through the check from the main condensate line. There is a f@t trap located at the end of the supply after all the fan coils that gets piped back to the common return. Any thoughts are appreciated.


    EBEBRATT-Ed Member Posts: 15,317
    Sounds like undersized piping on the supply or not enough steam pressure. Depends how big the unit heaters are. It's the last unit on the line. Check boiler pressure, put a pressure gage on the steam inlet to the coil if you can. If not put one at the trap inlet. Also loosen a union in the condensate return with everything running and see if your getting a lot of back pressure. Check your condensate pump and tank and see if you have steam coming out of the vent pipe (you shouldn't) That's enough to get you started. Report back on what you find.

    1 1/2" won't feed a lot of unit heaters.
  • fitzyfitta
    fitzyfitta Member Posts: 5
    The 1-1/2 comes off the 4' main. There are 3 other 1-1/2 supply's off this main. My other 3 supply's are having no issues. They are all the same length and have 4 coils heating properly. I did break the union after the trap on the problem unit to see if the check valve was seating properly. The check is swinging and seating but leaking ( wheaping steam) from the trap at the end of the supply towards my trap which seems normal. No? I'll check the pressure off the top of my trap.
    EBEBRATT-Ed Member Posts: 15,317
    Ok. Your first post said "I have a steam supply 1-1/2 that supply's steam to 4 units".

    I would see what the pressure gage shows. I guess I would check the steam at the trap inlets and see if your getting steam there. Compare it to one of the working units. Could have a bad trap which you can rebuild, Plugged return?
  • Steamhead
    Steamhead Member Posts: 16,723
    Here's your problem:

    I found there was some steam leaking back through the check from the main condensate line.

    There should not be any steam in the return line. One or more of the other traps feeding into that return line is leaking steam. The steam is pressurizing the outlet side of the check and holding it closed. It would probably be a good idea to rebuild all the traps since they're all the same age.

    Also, check the air vent serving that return line- the steam may be closing it, or it may be stuck closed.

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    Towson, MD, USA
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