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thermostatic mixing valve for fan coil?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by mmudd, Oct 17, 2012.

  1. mmudd

    mmudd Member

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    Central MO
    I'm ready to plumb my water to air hx in forced air plenum. I have manual adj mix valve that has large temp range of 110 to 176 degrees. I was thinking of using the mix valve in the hx zone to limit temp of water to make storage last longer depending on weather conditions outside . Instructions with valve say supply temp should be 20 degrees higher than desired output. I assume that means if I am at 180 temp with boiler, valve will supply 160 degree water to fan coil. That would make storage go farther, and in moderate weather be ok, but thinking that would not supply enough heat when temps are cold. Has anybody else done this? Any thoughts on if this is not such a bright idea? I've seen some caleffi drawings where motorized valves /outdoor reset was used. Not sure if those valves have different mix perameters. I had it plumbed up previously without mixing valve and it worked great, just trying to get more time out of pressurized storage which I am adding now and reason for re plumb.

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  2. RobC

    RobC Minister of Fire

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  3. BoilerMan

    BoilerMan Minister of Fire

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    The 20 degree differential is so the valve can accuratly modulate the h/c ports to get the desired mixture temp within a certain degree of accuracy ie. 3-4* this is not important in your case, you will act as the outdoor reset, when it gets colder, you turn the knob for hotter water. To extend your storage the most, consider two WAHXs then you get more (not twice though) the temp drop through them, needing cooler water to still give the desired temp rise to the air in the plenum. It's like oversized radiators, you can get the same amount of heat out, but with cooler water in. The more surface area exposed to the air in the plenum the bigger the deltaT will be.

    TS
  4. Bob Rohr

    Bob Rohr Minister of Fire

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    Yes a 3 way thermostatic is a very common way to mix down temperatures. It is very commonly used in radiant systems.

    As Taylor mentioned, you will be the "operator". As colder temperatures outdoors require warmer supply tremperature, then you will need to adjust the valve.

    Install the valve near the boiler not the fan coil so you limit the loss thru the piping. Be sure the valve has a Cv rating to pass enough flow for the load of the coil without adding too much pressure drop.t

    A motorized type mixing valve allows you to add an actuator to do the adjusting for you. It does add additional cost however.

    Here is a link to som e good info on mixing devices

    http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_7_us.pdf

    Attached Files:

  5. mmudd

    mmudd Member

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    Jun 7, 2012
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    Loc:
    Central MO
    Thanks for your help guys.

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