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Can you really size a mixing valve for the crossover bridge?

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Luketj
Luketj Member Posts: 2
edited February 2023 in THE MAIN WALL
I’m in the middle of Dan’s book, “Primary-Secondary Pumping Made Easy,” and in it he suggests putting the mixing valve on the crossover bridge as opposed to just before the pump on the secondary circuit. One of the selling points is that you can save money by getting a smaller one because there is often less flow on the crossover bridge. My question is can you really size it for the crossover bridge since you are still getting some secondary flow through it as well, not to mention if you are piping it to account for jet flow, you’d be right back to the same size mixing valve you’d have if you had left it in the secondary. Or does having a mixing valve on the crossover bridge eliminate the need to worry about jet flow?

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  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,839
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    Don't know what kind of job you're doing but crossover bridges are seldom used except on large commercial or industrial jobs. Their purpose is to assure the same water temperature in distribution pipes which are really long. What are you trying to do?

    But the answer is yes, the mixing valve sized for a crossover bridge is sized for the piping that leaves the crossover to feed the zone. But a lot depends on your water flow and water temp requirements. The bridge and the zone can be different temps, or the same temp and flow rates and pipe sizes and mixing valve sizes depend on knowing all the information.
    Mad Dog_2Luketj
  • Luketj
    Luketj Member Posts: 2
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    Thanks for the response, I’m not working on anything right now, just trying to get some of these concepts down.
  • hot_rod
    hot_rod Member Posts: 22,476
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    All valves and devices in the flow path should be sized by the flow rate that will pass through them.
    All valves should have a Cv number on them, on the box, or in the submittal sheets. You do not necessarily size a valve by the pipe or connection size.

    Ideally the Cv number would match the exact flow rate. That is not always possible so you use a spreadsheet or formula to calculate the pressure drop through the valve, at the designed flow rate.

    Here is an example of one. In the last box, put in the design flow rate, also the Cv of the valve you are considering, read the pressure drop.

    So now whatever is causing flow in that piping circuit, it has to overcome the pressure drop.

    In hydronics, the circ pump supplies that “head”

    In domestic water piping, the incoming pressure needs to be able to overcome the pressure drop.

    The sheet can also be used to determine the Cv of a valve if you know flow rate and measured pressure drop.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Luketj