Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Water hammer after installing TRVs, Differential Valves not helping

Hi, a multifamily property I'm working with recently installed TRVs in their hydronic radiators, to correct a heating imbalance. The buildings have about 9 units each, mostly 2-story apartments.

Shortly after installing the TRVs, severe knocking / water hammer began, especially on mild days. We believed that the cause was excess pressure, due to some of the TRVs closing. I should also note that in most buildings, only about half of the zones have a TRV installed. Most apartments have 2 stories, and only 1 of the 2 stories has a zone valve installed.  

We installed a differential-pressure bypass valve in each boiler to correct the water hammer, however the water hammer (if that is the correct term), has not gone away. I'm thinking that the differential valve is either not set properly, or is not sized large enough.

Does anyone have experience with something similar to this? I would appreciate any thoughts.  Thanks!


  • Bill_17
    Bill_17 Member Posts: 68
    Water hammer

    Dan, water hammer in TRVs is only caused by incorrect flow direction.  Measure the supply and return temperatures of each valve as compared to the flow arrow on each valve body.  Since the noise transfers throughout the system, one single incorrectly mounted valve can be heard almost everywhere. 
  • Dan_NYC
    Dan_NYC Member Posts: 2
    edited February 2012
    temperature measurement

    Thanks, it sounds like some of the TRVs might be in backwards. One TRV is installed in each apartment, would the knocking be coming from mainly near that TRV, or would it sound more or less the same throughout the system?

    Also, the TRVs are each installed in a pretty short hydronic baseboard ratiator, about 4 feet long usually. Would I be able to measure any temperature difference from one end to the other, with the valve open? Otherwise I guess I could only measure it with the TRV closed, when the knocking has occurred.

    Also, I think in some cases the TRV may be installed correctly with regards to flow direction and positioning of the operator, but be at the wrong end of the radiator, not the supply side. I doesn't seem that this would make much of a difference, especially since each apartment has many radiators. Is this not something to worry about?
  • Bill_17
    Bill_17 Member Posts: 68
    Water hammer

    Sometimes it is hard to tell exactly where the noise is coming from, but it should be loudest near the offending valve(s).  If necessary, close the valve and let the pipes cool and see which side of the baseboard heats up first, that might be easier then trying to measure the small difference between supply and return temperatures.  You might find that an entire riser's flow is opposite what is expected, that happens often. 
This discussion has been closed.