Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

External Demand 143?

Viess Member Posts: 58
Does anybody know if the External Demand #143 is the same as thermostat demand? Viessmann doesn't use the word thermostat in any of their diagrams. External Demand post 143 is located in the Pump Control Module for my Vitodens 200 WB2B-35. I assume that's where you tie the thermostat wires into? Any help ASAP would be appreciated. Thanks. 


  • Viess
    Viess Member Posts: 58
    Here is the link to the Install Manual.

    Go to page 43. Thanks.

  • CMadatMe
    CMadatMe Member Posts: 3,086
    Viess May I Ask

    DId you purch this Vitodens from an internet company?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • External Demand and TT

    External demand allows the boiler to go to a fixed temperature.  This will by-pass out door reset.  This boiler does not require a thermostat.  External demand is typically used for snow melt, pool, spa..  etc,   Thermostats should be connected to zone valves.
  • Viess
    Viess Member Posts: 58
    Thanks Gilo.

    That might explain why when the contractor fired it up for the first time it would run for a bit then shut off. I believe he said that the main circulating  pump just keep running even after the boiler quite firing. It was a holiday and we couldn't get anyone on the phone to answer any questions. The state rep did eventually call us back late that evening but at that point the installer was just trying to program the boiler. So I'm pretty sure  he didn't ask about wiring the thermostat wires to the external demand 143 block. This baby was the first one of its kind to be installed in the entire state. Not many Viessmanns boilers up here period. Our Rep did give us an East coast RI number and Ontario number to call in the morning. I do appreciate the advise Gilo, thanks.
  • scott markle_2
    scott markle_2 Member Posts: 611

    Better than zone valves (in my opinion) would be flow modulation via TRV's.

    If your going to have zone valves you should probably be using a low loss header, unless you leave a large enough un-valved zone that will satisfy the the boilers minimum flow requirement.

    If it's a radiant system consider manifold actuators that cut some but not all of the loops to a given zone, This will maintain some flow at all times. Get your head around the idea of constant circulation, output (supply temp) is determined by outside temperature, indoor feedback in most euro systems is usually achieved by non-electric thermally actuated valves (TRV's).

    Programing a Vito take some practice and patience(finding the right curve etc.), make sure it's done by someone who is committed to truly understanding how this system is designed to work. Too many of these boiler are installed at a high outdoor reset curves with zone valves. Grafting one of these boilers to a conventional system is a bit of a wast.
  • Viess
    Viess Member Posts: 58
    edited November 2009
    Up and Running.

    Thanks again Gilo. 
This discussion has been closed.