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Three port thermostatic mixiing valves

SantilalSantilal Posts: 30Member

When you look up the Cv of a three port TMV, is the pressure drop and flow based on flow from one inlet to the outlet or both inlets to the outlet, or does it not really matter. I am thinking about the circuit where a TMV is used for boiler protection - the biggest pressure drop is likely to be across the TMV, but I can't get my head around if it makes a difference if one or both ports are open?

Also, we use metrics in NZ but most Cv's I have seen are based on imperial measures - are there conversion factors to convert Cv's or do you just have to develop your own?



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  • hot rodhot rod Posts: 3,606Member ✭✭✭
    The metric flow factor unit is Kv

    Kv would be cubic meters per hour (m3/hr.) of a 16• c water with a 1 bar pressure drop.

    Cv is GPM of 60•F water with a 1 psi pressure drop

    Kv = 0.862 x Cv

    The thermostatic valves built for boiler protection have larger flow rates and are usually a fixed temperature cartridge.

    The pressure drop either Cv or Kv is the flow thru the valve to the outlet.

    Keep in mind a valve with a Cv of 3 can flow more then 3 gpm, but you will have additional pressure drop.

    Here is a little spreadsheet we use to calculate flow, pressure drop, and gpm. I'll see if we have one in metric units if you would like.
    Screen shot 2013-06-30 at 9.55.46 PM.png
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  • SantilalSantilal Posts: 30Member


    Thanks for that. From your answer it appears that the Cv is not dependent on either or both hot and cold ports being open or closed.


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  • hot rodhot rod Posts: 3,606Member ✭✭✭

    what comes out of the mixed port is the blend of the H &C port. If it needs more hot, that port opens as the cold closes down. The wax cartridge inside move the spool up and down to vary then opening into the valve from the H & C
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