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2-Pipe versus Primary/secondary

In my quest to become acquainted with hydronic heating I've come across 2-pipe systems and primary/secondary arrangements

Could someone help guide me as to when one would be preferred over the other? 

The primary/secondary seems "cleaner" for installation but each secondary loop requires its own circulator in addition to a more detailed calculation/understanding of the water temperature as it moves thru the primary loop.  While the 2-pipe system is a little more straight forward and requires zone valves and/or balancing valves.

Thanks in advance for the help.

DBA (aka "damp head" ... but trying to get more "wet")


  • bob_46
    bob_46 Member Posts: 813
    BIG Question

    Damp Head, If you want to become a "wet head" I suggest you immerse your head in

    a ........... couple of good books. The short answer to your question is " It Depends".

    There are so many variables in architecture, heat emitters, heat sources, pumps, etc. that

    there is no best. One fundamental design criteria that seems to be overlooked these days

    is trying to keep the heat load and pressure drop as close to the same in each loop.

    Good luck with your quest.
  • bob_46
    bob_46 Member Posts: 813
    edited September 2010
    BIG Question

    Double post, sorry
  • Piping

    I pipe primary/secondary (p/s) to keep the boiler happy.  Let's say there are multiple zones and only one small zones calls for heat, the flow through that zone will not pull all the heat out of the boiler's heat exchanger, even if the boiler is modulating on low fire - the boiler will start to grumble. P/S piping always allows good flow through the boiler.

    On the other hand, there are often older homes or apartments with one big zone where p/s piping would be a waste of time; pipe it 2-pipe since you get full flow on a call for heat.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Piping:

    For what it is worth, I pipe all new and replacement boilers with a primary loop for the boiler and make secondary loops from the primary. That way, I know I will always have good flow through the boiler no matter what secondary zone is runing and it seems to solve all problems.

    If it is a one zone replacement, I just use the closely spaced tee thing. If there is a indirect added, I use another set of closely spaced tees.

    Never had a problem. If there were origonal design problems in the old system, they will not effect the qualities of the new boiler and piping.

    Hydronics is very forgiving. You really need to work hard to screw it up. The fun is figuring out what is wrond and if it can be reasonably fixed.
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