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Viessmann LLH w/ sensor - function.

TAG Member Posts: 751
edited February 2019 in Thermostats and Controls
First -- Does Viessmann supply the sensor with the header? Or is this an additional part. Also -- I get that it's measuring the temperature on top of the header. But why ? Is not the boiler measuring the return temp anyway ? Or am I incorrect and the boiler does not have a way of measuring the return water temp? If it does not .. I get why it would like the header temp.

Looks like the 200 uses a 7134 791 header (80/50)


  • SuperJ
    SuperJ Member Posts: 609
    edited February 2019
    I can't speak to Viessmann specifically, but the LLH allows mixing (okay since it's hydraulically decoupling the two loops) to occur, potentially on either the supply to the building (when building flow > boiler flow), or the boiler return (boiler flow > building flow).
    Normally you want the supply to the building held at setpoint, so the boiler needs a sensor on the (potentially) mixed supply so it can compensate for any temperature reduction from mixing building return into the supply.

    Example (with sensor, where building flow is higher than boiler flow):
    Setpoint 160
    Building return 140
    Boiler return 140
    Boiler supply 170
    Building supply 160 (hot boiler supply is slightly diluted and mixed down by excess building return flow).

    Without the sensor the numbers would be something like this:
    Setpoint 160
    Building return 140
    Boiler return 140
    Boiler supply 160 (boiler uses SWT in the absence of system sensor)
    Building supply 155 (5deg less than setpoint, do to mix down without compensation).

    If you don't have it setup this way the higher your building flow, the further below setpoint it will be. Because your boiler flow doesn't go up with the system flow, so the supply gets more and more diluted. You can compensate by jacking up your boiler's ODR, but cold temps and high flow don't always coincide (say a sunny afternoon on a cold day, flow could still be low even if it's really cold out). Or you can compensate by speeding up the boiler primary pump to avoid mixing the supply, but now you've killed your boiler's efficiency because it's mixing hot boiler supply into the return, reducing the boiler's ability to condense.

    This all applies to closely spaced tee's as well. (supply sensor goes after the tee's.

    IMHO, this sensor, and setting the primary boiler flow correctly are the most overlooked things in modcon install and setup (besides the ridiculous lack of outdoor sensor in some cases).

    Primary pump speed: min speed that lets you maintain setpoint to the building, and keep the delta from getting higher than the manufacturer recommends at high fire (or min boiler flowrate).
  • TAG
    TAG Member Posts: 751
    Get the idea of the LLH .... think I understand what you are explaining. I'm thinking the sensor was for the return to the boiler -- it's actually for the supply out to the building?

    Still can't see if the sensor is part (included) with the LLH -- or a separate part. My installer who mentioned the Viessmann typically does 100's w/o the sensor or LLH ?
  • Gordy
    Gordy Member Posts: 9,537
    The sensor is a separate purchase.
  • TAG
    TAG Member Posts: 751
    Thanks -- I'm leaning on installing the smallest Viessmann in my studio as a test case. It's overkill quality wise ..but I cannot afford to screw the main house up