Oscillating hydronic system

Discussion in 'HVAC Heating & Cooling' started by alternety, Jul 20, 2014.

  1. alternety

    alternety Like an engineer

    Apr 2, 2006
    I have a hydronic system. Five manifolds. A large system with half the circuits not being used. Upstairs - yes. Basement and garage - no. The system has been operating for more than 5 years.

    I have always had some noise in the system. A banging from the manifolds and detectable in rooms. It has been getting worse. It seems to correlate with the turning on and off of circuits based on demand. If you look at the little enclosed display rods that are supposed to show flow rate on on each circuit, they just jump up and down wildly.

    The system has ball valves on the supply to each manifold, and a rotary valve on the return. I am not sure what type of valve it is on return. But it seems to be a variable adjustment. Originally running an antifreeze that ate my boiler core, I am concerned that more components may be damaged. I flushed the system with fresh water, but the piping did really not let me get the entire system clean. I am going to fix that this summer.

    I could use a suggestion for a flush and antifreeze for when I get to that point.

    I am looking for input on most any of these issues. I have to get things resolved before freezing temperatures. I have several outside loops.
  2. alternety

    alternety Like an engineer

    Apr 2, 2006
    No one seemed particularly interested in this problem but I thought I would explain what happened.

    In normal operation, there can be a minimum of one small loop creating demand. The way I use it, less than 1/3 of the entire heated areas are ever turned on. The pump was sized for the whole system; and maybe oversized. And that was for antifreeze, which is no longer used. Net result; it was capable of creating a flow way in excess of what is used. The original design was to use a variable speed pump. The Grundfoss pump available at the time had a trivial TRIAC speed control on a regular pump and no actual operational controls (I was going to add a PLC or something for loop control) . The noise could be heard through most of the house. Hence, a fixed pump.

    There is a flow bypass device between source and drain portions of the manifold. That, by itself, is not sufficient to limit the flow capacity in the system. I could hear flow noise from the piping and small distribution manifolds. That was not heard when the system was first installed.

    The issue was those valves on the return paths from the distribution manifolds. When the flushing of the diseased antifreeze was done, these valves were fully opened. But never put back to where they were at installation. Neither I nor the hydronic person I had come in to fix it, realized just what the situation was with those valves.

    When a change in the call for heat occurred, the the pressure across those thermal valves caused them to "fluttered" during the transition each way. The control system did not have sufficient time delay range to allow the valves time to fully open or close. So the pump started before they were very far open, and continued when the were closing. That was what caused the noise. The exact mechanism in the distribution manifolds; I am not sure. It may have been the velocity or it may have resulted from a differential pressure being generated between valves in the same grouping (on and off together).

    With a smarter system control system I may have been able to resolve this with some parameter changes. Solenoid operated valves might have been useful together with decreasing flow at the pump or return valves. Without coordination with the controller, fast acting valves would probably just generated some serious water hammer in the system.

    I am not certain this is a really good configuration. There are some serious flows probably happening across those return valves. They are almost closed and I am concerned about erosion. Particularly as I have not actually seen the internal damage that may have been caused by the diseased antifreeze. Same with the thermal valves. Maybe next summer when I feel one of the closets does not have enough water on the floor I will take a thermal valve out and see what condition it is in. They are an old version, but I can replace them in the manifold with newer designs. Unfortunately that will also require reversing the input and output lines to the manifolds. A smaller circulation pump could reduce some the the potential damage there.

    That's it. Just sharing.
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