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Spirovent/ boiler fill: Seems like various theories.

TAG Member Posts: 755
Did my fist CI Buderus boiler back in the early 90's: Followed the pumping away "module". First thing on the hot side of the boiler was the "T" with purge valve. Followed by the 1.5" isolation valve ... 1.5" Spirovent w/ the expansion tank and boiler fill connected to the bottom of the Spiro.

I'm seeing various setups when using the wall boilers and LLH .. Some connect the boiler feed to the bottom of the LLH and the -- other on the feed after the LLH. Not everybody uses the Spirovent ?

With the low mass heat exchangers wanting the pump -- pumping into the boiler. Is that changing things? --- Why not the Spirovent before the pump on the main boiler circuit and just attach the water feed and expansion tank to the bottom as before? What am I missing.

Spent some time looking over the Viessmann 200 manuals ... not really spelled out!


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,124
    edited January 2020
    As long as you understand the Physics of water regarding Boyle's Law and Henry's Law, just use them to your advantage. Looking at all the Viessman piping diagrams, the Pump is pumping away from the expansion tank. If the pump is pumping into the return of the boiler, it is because it is just after a bypass of hot water from the supply in a "Primary /Secondary" or Low Loss arrangement of some sort. The key is getting the lowest pressure in the system to be near the air vent. and to get the hottest water in the same location is a big plus. Now the hottest water is obviously in the boiler and in many cases, the boiler has an air vent internally in the cabinet near the top of the water circuit.

    When you design the near boiler piping you need to account for the minimum water flow required by the boiler to maintain the proper temperature rise through the heat exchanger. To get that minimum flow for proper ΔT you must sometimes have a boiler circulator that is separate from the system circulator.

    That said, your near boiler piping to the system circulator should have the Micro Bubble air separator in the hottest water and the lowest pressure pumping away from the location of the expansion tank (Point of no pressure change) Your near boiler piping design is for you to determine the best location for all the components in the proper order.
    This is how I design those systems. The hottest water in the "System" loop is just after the supply from the boiler enters the "System" The lowest pressure in the "System" is on the inlet of the "System" Pump. By placing them in the same general location you get Boyle's Law and Henry's Law to work to your advantage. Same thing as on a conventional system. Just look at each Loop separately to analyze and create the best design. SYSTEM LOOP, DHW LOOP, BOILER LOOP. Each is a pumping loop in itself. after you put them together on paper, just check to see that they are not fighting each other or forcing more water through a common pipe than it can handle.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • hot_rod
    hot_rod Member Posts: 22,268
    A LLH, hydro sep, hydraulic separator are really all the same thing.

    The hydroSep is just a supersized set of closely spaced tees. Two sets of tees actually, a primary and secondary side.

    The advantage is that one device does your hydraulic separation, air purging, dirt separation, and magnetic separation, a true 4 in 1 device. For the cost and labor saving $$ it is a non brainer most installers have learned.

    Many of the mod con manufacturers show LLH as an option to closely spaced tees because it is hard, although not impossible to screw it up :) Some boiler manufacturers are now offering seps with boiler packages.

    Really the expansion tank could connect at any of the 4 ports, the entire device IS the PONPC.

    I like the tank, and fill at one of the bottom connections as it sees the coldest fluid, which may extend the life.

    But not directly below as dirt will settle on the diaphragm.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream