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Nesbit steam coil

I’m wondering if anyone has had problems with steam not traveling through coils on Nesbit unit ventilators.  80% cold, 20% coil hot with a 200deg condensate line. The only way I can imagine steam getting around 80% of the coil would be internal failure of the copper header which has a divider between steam and condensate. That would provide a short path and make the condensate line hot.  I have had a few good steam guys look at this and they can’t seem to figure it out.  I cut open a spare coil that had a leak and the internal tubing is in good shape,clean… one surprise, a brass tube with distribution holes within the steam tubes for even heating or maybe low noise? Tube in tube.  The teacher has had it with me, I have to replace this 50 year old coil without any visible problem


  • Steamhead
    Steamhead Member Posts: 16,729
    I bet there is a bad trap in an adjacent unit that is pressurizing the return line. That would explain why the return is that hot but the coil is only heated to 20%. Where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    EBEBRATT-Ed Member Posts: 15,335
    edited December 2023
    It depends how the coil is constructed.

    I have run into this several time and it fools everyone who tries to figure it out.

    I am talking about "non freeze steam coils"

    There is some good information attached on steam coils

    Non freeze steam coils are a pipe within a pipe. (no such thing as a coil that cant freeze) They have no return bends. 1 tube carries steam and 2 tube carries condensate. When they freeze internally the coil "short circuits" The steam header will be red hot and the condensate outlet and trap will be hot. But when you put your hand on the coil surface while heating you will get hot and cold spots depending on where the damage is. You can't fix it without replacing the coil.

    If the coil has return bends that is a different issue. Don't know what type you have. Non freeze coils with 1" distributing tubes are better than 1/2"
  • Thompa222
    Thompa222 Member Posts: 3
    Thank you Ed! Very helpful.
  • clammy
    clammy Member Posts: 3,094
    Ed s dead on w non freeze coils personally here the only real coil type to use on air duct installed steam coils ,I de be checking traps and any control valves on the steam supply . There should be a vacuume breaker some where either on coil piping or on the inlet piping . In the past I ve had to replace a few but in there defense they where close to 50 years old and had there share to repairs and had about zero maintance w bad control valves and basically just ran all,out . I know from working on Nesbit stuff that it always seemed that ever unit was a bit different like there is and was some parts sourcing issues ,I say this from working many years ona job w 5 20 ton roof tops all the same heating and cooling outputs but each one had different controllers or gas valve quite a mess and there factory tech service let a lot to be desired . But it always seems they had a nitch in there market being engineers could spec all sorts of non sense and Nesbit would comply to their request not matter how silly and nonsense . There hot cold deck multi zones where just so much fun to work on not .
    Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
    EBEBRATT-Ed Member Posts: 15,335

    I was thinking of the rooftop "multizone" systems similar to hot deck cold deck, I guess. Luckily, I never ran into too many of those, but they were always a problem. Carrier used to make them and there were others.

    With roof top AHUs with steam coils, we would keep the control valves below the roof if possible and the traps as well and just have the two pipes on the roof. We would put an additional trap on the downstream side of the control valve as well to help prevent freeze ups.

    It worked well that way, but the valves usually ended up over a machine or something making them tough to work on.