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Viessmann 4-Way Mixing Valve

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theowwm
theowwm Member Posts: 13
Hi Everyone, I'm getting ready to do a boiler replacement. Existing unit is an Atola AR 199k BTU (circa 1991). The heating load on the existing boiler is a mixture of baseboard and radiant under-floor loops. The new boiler is a Vitocrossal CU3A 199k BTU. This unit is essentially a drop-in replacement. There is an existing Viessmann 4-way mixing valve on the radiant load. The controller on this valve is not compatible with the CU3A, so I had to contact Viessmann to get the part number for the add-on module. During that conversation, the factory rep questioned the use of a 4-way valve. His concerns were vague, but he thought it should be a 3-way. I'm attaching a diagram of the existing system for reference. I'd appreciate any thoughts on this.

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  • GGross
    GGross Member Posts: 1,048
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    The original 4-way valve provided return temp protection for the old boiler and mixed down for the radiant citcuits. The new boiler does not need return protection, it is condensing and will run more efficiently with lower return water temperature. Due to the mass of the boiler, and the fact that you are making DHW with it (high temp application generally) you will want a 3-way valve to correctly control the outgoing temperature to the radiant floor.
    Mad Dog_2Paul Pollets
  • hot_rod
    hot_rod Member Posts: 22,144
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    do you even need a mixing valve, what temperature does the fin tube need to meet the load? Run the boiler on reset for the required radiant temperature, DHW on priority

    If you could run just one lower temperature, below 130 the boiler efficiency jumps. Beef up the fin tube to run at the radiant SWT

    I think you can plug a port on a 4 way, but it will require some repiping. Doesn’t that boiler have multiple temperature output in the control?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGrossMad Dog_2
  • theowwm
    theowwm Member Posts: 13
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    hot_rod said:

    do you even need a mixing valve, what temperature does the fin tube need to meet the load? Run the boiler on reset for the required radiant temperature, DHW on priority

    If you could run just one lower temperature, below 130 the boiler efficiency jumps. Beef up the fin tube to run at the radiant SWT

    I think you can plug a port on a 4 way, but it will require some repiping. Doesn’t that boiler have multiple temperature output in the control?

    The existing valve is connected with unions on every port. It wouldn't take too much to re-pipe it. Removing the valve altogether or using a different type of valve is pretty appealing. The Atola supported the 4-way with no add-on. The CU3A requires an add-on module that is hard to get (long lead time), and very expensive. The current water temp for the radiant floors is 135, and the baseboard is 120. This seems backwards to me, but the radiant load is 3 times that of the baseboard, so the temp at the manifold might be higher. I updated the diagram with the 3 way valve. I assume this is the way it would be configured. Thanks.

  • hot_rod
    hot_rod Member Posts: 22,144
    edited February 2023
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    Gosh it seems just running the baseboard at 133, same as the radiant simplifies all this?

    Why does the condensing Vitocrossal even need a mix valve? Seems the boiler is ideal for direct pipe, low temperature applications.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Alan (California Radiant) Forbes
  • theowwm
    theowwm Member Posts: 13
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    Viessmann pointed me back to the Application Guide as well. Specifically to Application 6, which includes mixing valves where radiant loads are combined with baseboard units. This diagram from the application guide is essentially what's in place, except that all of the radiant loads are serviced by a single pump and mixing valve.

  • hot_rod
    hot_rod Member Posts: 22,144
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    That is the correct drawing if you need higher temperature for the baseboard.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • theowwm
    theowwm Member Posts: 13
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    I wanted to post a follow-up on this topic as I have the new valve in and working as expected. First, I ended up replacing the valve and the actuator. I also installed the mixing valve extension module. The new valve and the extension module were required. The actuator probably was not required as the older one was the exact same voltage, current, mounting, and range of movement. Both it and the four way valve were less that a year old, but the new actuator came in the kit, so I just replaced it. The Extension module was simple to install and was auto recognized by the boiler. I also installed a check valve on the return input side of the mixing valve. I did this because I noticed that if I switched the boiler to DHW only, and the mixing valve is in the open position (does not return to closed automatically), hot water would flow back through the valve to the boiler return. The check valve stopped this. Everything is in and working great. Thanks to all who commented.