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TRV's in 2-pipe direct return systems?

Standard TRVs can be used with hot water or two-pipe steam--in both cases they replace the supply valve. <I>Expect limits of around 15 mbh for a 3/4" body; 18 mbh for 1"+; 10 mbh for 1/2".</I>

If the balance is currently even decent, I would not move the balancing valves as unlike a hot water system, steam does not always have heat available and balance valve settings can have benefit.


  • Abro
    Abro Member Posts: 37
    TRV's in 2-pipe direct return systems?

    Can TRV's be installed in 2-pipe direct return systems? If so , once they are installed, should we fully open all of the balancing valves? Thanks.
  • Abro
    Abro Member Posts: 37

  • Abro
    Abro Member Posts: 37
    hot water

    This is a hot water system, not steam. My concern is just that, since the system is designed to be balanced with the balancing valves, will the addition of some 300 TRV's screw the system up in way that I can't forsee. I don't know why there would be a problem, but someone once told me that TRV's shouldn't be used in a 2 pipe direct return system that uses balancing valves. Thanks.
  • ALH_4
    ALH_4 Member Posts: 1,790

    The beauty of TRV's is that they are continuously self-balancing. You will want a differential pressure bypass (VFD?) on the circulator(s) because the TRV's could potentially be mostly closed.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Two-pipe, direct return is the preferred piping strategy for TRVs. Reverse return piping is not necessary and [usually] only adds to cost.

    TRVs are self-balancing but of course there are limits. In a system of this size, I can only assume that the balance valves serve general areas of the building (each with a number of TRVs).

    If the original engineer did a reasonable job of sizing the piping and there have been no gross changes, then there should be no problems.

    If there is already good general balance between the areas served by the balancing valves, then leave them be and install the TRVs and differential pressure bypass.

    If all of your balance valves are the same size with very similar piping in each branch, then you can just open them fully and install the TRVs and differential pressure bypass.

    If there is not good general balance between the areas served by each balance valve then it takes some more work. (Balance problems within areas served by a single balance valve are immaterial as long as the radiation or piping is not utterly inadequate.)

    Think of such a system with hundreds of TRVs served by a number of large brances like a district system of buildings.

    You have a district source serving buildings (balance valves). Under ideal circumstance, the district source can provide the exact amount of energy required by the buildings, but for this to happen the district authority must know the general share of the load posed by each building.

    Determine the amount of radiation connected to each balance valve and its share of the total load.

    The district collectes info from all of the buildings, sizes its mains and circulators and gives each building a tap that assuming 20F delta-t will provide the required flow at a specific pressure drop. Every building is expected to have similar pressure drop in similar conditions.

    Now you must calculate the head loss in each building at the provided flow rate.

    Since you have balance problems, you'll find high variability and standouts among the buildings with regard to pressure drop at the provided flow.

    If some building can't receive its share at the given flow then the best solution is to correct the piping in that building--someone probably "did something" in there...

    If the problem is that some buildings have very little head loss while others are near the specification then you need to balance.

    Buildings with very low relative head loss can draw more than their share of flow from the system, starving those with relatively high head loss.

    The best way to solve the problem is to replace the balance valves with differential pressure bypass valves. If every building is not given a differential pressure bypass then the district must still install a system differential pressure bypass.

    With bypass for each building, adjust each valve for the head loss at the required building flow and everyone will be comfortable and happy.

    If you do not install differential pressure bypass at each building, you can attempt to adjust the existing balance valves. The more precise the balance valves and the more precise your head loss calculations the better your chance of rapid success. Of course you must still install a system differential pressure bypass.
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