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  1. #1
    Join Date
    Jul 2012
    Location
    Ottawa, Canada
    Posts
    7

    Confused Hydronic system: 3-way valve piping

    Hi.
    I was wondering if anyone knew why I have a cross connection from supply to return at every 3 way valve in my HW & CW systems.
    They are constant volume systems with 3-way valves at each branch.
    Thanks for any help in advance.

  2. #2
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,725
    Is it setup for diverting or mixing? It's most likely the bypass of the coil I assume.
    UA 100

    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

  3. #3
    Join Date
    Mar 2009
    Location
    Several Miles from Sane
    Posts
    1,471
    Since it is a "Constant Volume" pumping system(s) the bypass created by the 3-way valve lets that "Constant Volume" delivered by the pump(s) to continue to flow through the bypass port of the 3-way valve. Normally that is done so that the Pump(s) don't "Dead Head" if more than a certain percentage of the valves are closed, there by keeping the "Volume" (flow) constant through the piping system.

    On a "constant Volume" system when 2-way valves are used if more than about 35% of the valves close the pump discharge pressures increase and cause unwanted issues on the other parts of the system.
    If sense were so common everyone would have it !

    All opinions expressed are my own. Any advice provided is based on personal experience, generally accepted fact or publicly available information. As such, it is worth exactly what you paid for it, not a penny more not a penny less !!

  4. #4
    Join Date
    Jul 2012
    Location
    Ottawa, Canada
    Posts
    7
    Used for mixing.

  5. #5
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    I second Cagey57, your constant speed pump is going to pump the same amount of water no matter if you are cooling or heating a single fan coil or all of them... so your water can bypass all the fan coils that are shut off and run back to the return. Variable speed pumps won't need the 3 way valves... 2 way will do.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  6. #6
    Join Date
    Feb 2012
    Location
    New York City
    Posts
    106
    If there are balance valves on the bypass legs (there should be to mimic the pressure drop of the coil) then you can add VFDs to your pumps and go to variable flow pumping and save a boatload of money on pump energy. Typical paybacks 1.5 to 4 years. A little longer if you have to isolate the piping and insert a shut off valve in or cut and cap the bypass leg. You can replace the three way valves with two way as they fail at end of life or with equipment replacement.
    Last edited by 19characterusername; 07-22-2012 at 08:51 AM. Reason: iPad
    I hope I helped!

    Feel free to contact me and other users here for more detailed assistance within the context of a contract. Advice and opinions are provided here "as-is" without any warranties, guarantees or acceptance of liability for use, misuse, or accuracy of advice given. (Gotta keep my legal team off my back at work.)

  7. #7
    Join Date
    Jul 2012
    Location
    Ottawa, Canada
    Posts
    7
    Variable speed would be nice. A few years back a consultant was hired to give us system options, I'm sure the price came in at close to 500k.

    Constant flow seems to work fine with 1 chiller running (which is about 85% of the year). Chilled water temperatures great in the distribution system until system variables come into play. Like when OAT shuts off pumps for fresh air units, or summer loads bring on multiple chillers.

  8. #8
    Join Date
    Oct 2001
    Posts
    6,966
    chillers and boilers need to see constant flows no matter what position the 3-way is moving to as the AHU reaches setpoint the valves moves to shut coil entering and by pass that water into the returns thru the by pass.seeing 2-wy valves on chilled water sytems is a flag where engineers go with the idea water will pass thru the coils to make the loop into that return..the chiller picks up low flow returns and reacts unloading or tripping flow switch or going off on freeze protection
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

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