Startup/troubleshooting HW fin tube system

Mick76

We are having issues with excessive knocking sounds coming from the perimeter fin tube radiation.



The fin tube is supplied/returning to the lower levels of a 30-story hot water heating system: supply and return loops at subcellar level, feeding CS risers which extend up four floors to feed banks of copper fin tube runs (aluminum fin).



The knocking sounds seem most prevalent in areas with an extended straight run (roughly 30-40 feet or more).



Any tips on ways to go about diagnosing and correcting the issue are appreciated.



Some additional info:



- The HW system operates on an OA reset (110-160 HW temp, based upon 60-0 OA temp).



- Individual fin tube runs/banks are controlled via standalone Danfoss valves (sensing line strung below nearest element).



- Pressure at bottom of system (HW pump discharge) is roughly 145#.

Brad White

Expansion Noises

The fact that you are using ODR control minimizes this, but my hunch is that the forces of expansion are still messing with you.



A couple of mental diagrams for you:



1) The risers are at least 325 feet high, given your 145 psig bottom pressure and allowing 5 psig at the top. The risers, left to themselves, would expand 3.75 inches when heated from 60F to 160F. That expansion has to be taken up somewhere, preferably at mid or third points with an anchor on the bottom.



(A project of this size must have had a Professional Engineer on the job, so go by what they say. This is general information.)



The binding could be occurring at any level.  A concern I have is that if you are using ODR control now, you have not yet seen your highest temperatures, this early in the season. Maybe you have as a test run, but if not, the problem will amplify if this is expansion at work, when it gets colder out.



2) Lateral forces- The 40 foot runs might well expand nearly a half-inch across that length at the same temperature differences. Some loops, such as MetraFlex or Flexonics are a nice way to break up a riser or run into manageable segments which are allowed to "float".



As a general principle for your understanding, expansion occurs away from the heat source proportionately and the length is taken up at the far end.

For example, a supply riser anchored at the bottom, will grow by 3.75 inches as said above.  At the mid-point, the expansion would be half that, at the 1/4 point, 25% of that and at the top, the total amount.  This assumes that the heated water starts at the bottom and goes upward.



A second principle is support and anchoring.



Typically, supply pipe risers would be supported and anchored at their bases and guided as they go upward. (If expansion loops are used, anchoring differs with the type but lets keep this simple for discussion.)  But as the pipe rises and expands, you do not want to clamp/anchor the pipe until the next true anchor point. You want the pipe to float and in so doing, not impose stresses on itself or the structure beyond its own weight, ideally.



Conversely, a return pipe wants to "hang" from the top and be allowed to float and be guided as the pipe expands downward. 



Remember that expansion cannot be resisted, but it must be allowed for. And go by what the project PE says, this is just general information.