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closed system layout: do I need a loopback/mix circuit?

installing a Munchkin 50K in a detached garage radiant insulated monoslab.  Surfing all over shows SEVERAL different strategies for piping and gadgets.  I am pretty handy, I used to sell pipe/valves/fittings in a previous life.  I just don't understand the concept of a '"mix'" leg on a closed system.  Is it necessary?  or just a good idea?  or??  please see attached .jpg out of the Munchkin manual.  My question is centered around the "'closely spaced tees'" portion of the diagram.  I am guessing in theory it is like a car that allows a little exhaust into the intake in order to warm the engine faster (and give the unburned fuel a chance to burn)  but...  looks to me like the "'path of least resistance'" would be for the heated fluid to run right back to the boiler (especially since the circ pump is pulling on that leg) ??  I am not one of those that has to know every little detail... I can respect that it just needs to happen.  But curiosity is killing me  :)  To add to my confusion, there are diagrams and pictures floating around the web of closed systems that do NOT include this branch of the circuit.  Here are the NOTES that accompany the diagram: 

1. This drawing is meant to demonstrate system piping concept only. Installer is responsible for all equipment and detailing required by local codes.

2. All closely spaced tees shall be within 4 pipe diameters center to center spacing.

3. A minimum of 6 pipe diameters of straight pipe shall be installed upstream and downstream of all closely spaced tees.

4. The minimum pipe size for connecting an indirect water heater is 1”.

5. The minimum pipe size for connecting the unit is 1.25”.

6. Circulators are shown with isolation flanges. The alternative is standard flanges with full port ball valves. Purge valves can be used with the circulator flanges as an alternative.

7. The anti-scald mixing valve is recommended if the DHW temperature is set above the factory setting of 119oF.

8. Install a minimum of 12 diameters of straight pipe upstream of all circulators.

9. Winterization: When winterizing the unit, put a drain valve on both the supply and return between the union and the shut-off connection.


  • Ironman
    Ironman Member Posts: 7,181
    edited December 2012
    Hydraulic Separation


    I like your handle.

    The purpose for using the closely spaced Tee's is hydraulic separation. Simply put, that means that each circuit can maintain its own flow rate while transfering heat from one to the other.

    The Munchkin has a high head heat exchanger, which means a high resistance to flow. It is critical that a proper flow rate be maintained when it fires or a premature failure will occur in the heat exchanger from metal fatigue. We recently replaced one that someone else installed that was two years old.

    The system piping is typically low head and thus requires a low head pump. This is why hydraulic separation is needed with this type of boiler: it's to protect it from over-heating the heat exchanger due to low flow.

    If you are well-versed in hydonic calculations and pump selection, you may be able to get by with one large circ. that takes into account the total system head and required GPM provided there is only one zone connected to the boiler. That usually would require a circ. that costs more than twice as much as two smaller ones and uses more than twice the energy.

    Other than that, the Munchkin MUST be piped primary/secondary as shown and two circs. used. Make sure the boiler circ. is what they spec'd in the manual for that size boiler.

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Xparatrooper
    Xparatrooper Member Posts: 2

    I tried to thank you a couple days ago...  but my login was on vacation :)    Great information!!  I surfed the link you provided and learned a bunch!
  • Ironman
    Ironman Member Posts: 7,181
    De Nada

    Glad to help.

    One other caution: the boiler must be setup with a digital combustion analyzer by someone who is trained in its use.

    Best Wishes
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
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