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Radiator Traps - Low Temp on Low Pressure System

Upon checking some radiator traps recently in a school, I was finding that several of the thermostatic traps were much colder that excepted, between 160 and 190 degrees at the body. Downstream was a little cooler by 10 to 15 degrees, and upstream about the same. System pressure is between 7 and 10 psi with temps around 235. The valve from the steam main feeding the radiator was opened to 100% (done over WiFi).
When I listened to the traps by way of ultrasonic probe, many sounded like they were cycling, shutting on and off.

As I understand it, radiator traps that cold would be constantly open, allowing the excess amount of condensate to move through the trap until enough steam was present to heat the disc and closing the valve.

Any ideas as to what state the traps are in from the info given? Are they actually just failed open allowing for both steam and condensate to flow through? And why aren't the traps themselves heating up more 190 degrees when steam up 230 degrees is being run to the radiator?

Also, a few of the traps in the series tested were at temperatures above 215, some blowing and some not.

Any insight would be helpful!!


  • Jamie Hall
    Jamie Hall Member Posts: 20,488
    Running at 7 to 10 psi they're probably ruined. They aren't meant to run at those pressures. I would bet they are failed closed. I'd also bet that the ones which do heat are failed open, and are pressurizing the return line.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dopey27177
    dopey27177 Member Posts: 885
    Systems set at pressures above 2 PSI generally have zone valves installed to different parts of the system.

    The outlet pressures from the zone valves never exceed 2 PSI.

    Check out the system for zone valves. Are the heat exchangers in the room (radiators with fan units or are they plain radiators.

    Discharge temperatures from radiators will vary based on the size of the radiator and the differential in heating run time.

    In milder weather steam can enter the radiator at a high temperature and because the radiator does not heat all the way the condensate temperature will be colder. Conversely when it gets colder the system runs longer and the radiator hesats all the way the condensate will be hotter.

  • Joel_ISC_20
    Joel_ISC_20 Member Posts: 2
    I appreciate the insight. Thank you both!
    EBEBRATT-Ed Member Posts: 13,149

    Traps failing open will be the same as the steam temperature.

    Traps that are cold are probably failed if you have steam to the radiator, If you can here the trap cycling it should be good

    Traps are affected which may be good are affected by bad traps over pressurizing the return lines.

    If you can get the steam pressure lowered and still heat the building it would help the system calm down
  • retiredguy
    retiredguy Member Posts: 686
    edited November 2020
    From the information given, an exact answer to what the problems you are experiencing is not possible. A good explanation can only be obtained by actually being on the job and checking more parameters. I will try to give you some direction on things to check.

    You said that the zone valve was wide open as per the wifi. Did you check to see if the valve is actually wide open. You will also need to know the size of the zone valve, the mfg , the type of valve and the actuator model. There may also be a problem with the internals of that valve; broken stem, bad seat, wrong actuator, and etc.

    What type of radiation is this valve connected to; standing radiation, finned tube, fan coil, univent, etc. Is the inlet hot and if so how far along the radiation is it hot?. Do you know if the condensate returns have pressure in them. The trap itself may have fatigued or worn out, it could be plugged with debris as could the condensate line leaving the trap. If the traps are more than 10 years old all the traps in the system should be checked and most will probably need to be rebuilt or replaced.

    Could you list the size and the model of the thermostatic trap so we can get an idea of the type it is.

    The boiler pressure seems to be a little high unless the building is extremely large. I would guess that the boiler's limit should be set at 5 psig cut off and 2-3psig cut in unless the system has been designed for a higher operating pressure. How is the room or building temperature controlled? Do the boilers cycle on steam pressure or do they cycle on/off by a temperature control system? If this school has had numerous additions of upgrades there may be a myriad of operating problems. My last question is where are you located?