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Viessmann return water temperature

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DC123
DC123 Member Posts: 69
Is there any way to figure out the return water temperature on a Viessmann boiler (CU3A)? It only gives me the system temp and the flue gas temp - is this actually the more important number in terms of determining where you are on the efficiency curve? The manual says flue gas temp can vary from 9-27F over the return water temp, but I'd like to figure out what my delta T is a little more effectively.

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  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    Are you looking for something strictly for setting up pump speed and calculations, or a perminant temp readout. I'm not familiar with the CU3a control. But it is a sweet boiler!
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,607
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    If you wan't to know your delta T, measure it with a thermometer
  • DC123
    DC123 Member Posts: 69
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    Yes, re pump speed. May end up getting a couple temp probes, but figured the boiler should have it already measured. But if flue temp is as good or better to work with, I can just use that instead. Seems odd that viessmann wouldn't report such a basic parameter.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    edited December 2017
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    I really like these

    https://www.supplyhouse.com/Azel-Technologies-DS-60P-Dual-Zone-Digital-Temperature-Gauge-with-Two-Universal-Sensor-Probes

    Used many of them, have internal battery which lasts about 1.5 years and/or you can feed 24 volts a.c. to them from boiler controls.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited December 2017
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    ^ I use the same Azel units on my boiler....


    Solid_Fuel_Man
  • DC123
    DC123 Member Posts: 69
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    Thanks, that looks like a reasonable one. Any sense of whether flue gas temp is a better parameter than return water temp to evaluate?
  • NY_Rob
    NY_Rob Member Posts: 1,370
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    The mod-con efficiency charts (that I've seen) are all based on return water temp, so to me that's what counts.
  • Alan (California Radiant) Forbes
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    Solid Fuel Man:
    Excellent call on the dual sensors! I gave you an "Awesome" vote.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    Solid_Fuel_Man
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    edited December 2017
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    This is my own control panel as in my avatar. I have radiant supply and return, as well as indirect tank temp and boiler return temp. I just wish there were not 50 bucks a piece! But very good quality none the less.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    Canucker
  • Canucker
    Canucker Member Posts: 722
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    Very clean set up @Solid_Fuel_Man I like it
    You can have it good, fast or cheap. Pick two
    Solid_Fuel_Man