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outdoor reset and thermostat setback

sunlight33sunlight33 Member Posts: 120
I have a Viessmann 200 that uses outdoor reset to modulate the boiler, usually there is nobody at home 13-14 hours a day for six days a week. Thermostats are programmed to 68 when no one is at home, then 71 just before I come home at night. I understand that a bigger setback often goes against the outdoor reset, but for 13-14 hours away time would a bigger setback be justified in saving fuel or should I just leave it the way it is now?

My second question is regarding the heating curve of this boiler. My bedroom is a separate zone and I set the night time temp to 68 and 71 in the morning, in reality the room temperature struggles to reach 70 (sometimes only reaches 69) after 90 minutes of preheating. I could solve it by either increasing the "slope" or "shift" on the heating curve, but I want to know which one should be adjusted first.


  • TimcoTimco Member Posts: 2,826
    Delta T

    What is the Delta T of the zone that never gets hot enough? If too great you may just need more pump on that zone. Adjusting the curve affects the whole boat.
    Working on steam and hot-water systems isn't rocket's actually much harder.
  • sunlight33sunlight33 Member Posts: 120
    what is delta T?

    How do I find out the delta T?
  • TimcoTimco Member Posts: 2,826

    How hot is water leaving heading to that zone, and how hot is it where it comes back into the room? Difference is Delta T. If you have hot water going out and cool water coming back you're only utilizing a portion of the total tube in your floor. You need hot water across the loop.
    Working on steam and hot-water systems isn't rocket's actually much harder.
  • sunlight33sunlight33 Member Posts: 120
    I see

    I am not sure if this info can be found in the boiler display option, how is it found normally?
  • TimcoTimco Member Posts: 2,826

    Digital tester and pipe clamp. Or thermal imager. Or feel them for basic results.
    Working on steam and hot-water systems isn't rocket's actually much harder.
  • gennadygennady Member Posts: 680
    Night setback

    You have to have remote viessman control unit instead of thermostat set to reduce mode with schedule programmed in. Regarding second zone, you need another control unit with mixing valve installed and programmed. Just shutting boiler or zone valve on outdoor reset system will lead to very long time to get to desired zone temperature after turning zone on.
    Gennady Tsakh

    Absolute Mechanical Co. Inc.
  • ChrisChris Member Posts: 3,056

    Are you setting back the thermostat or the heating circuit via the boiler. What is the nighttime setback in the boiler control and for what time period. The control can be programmed for a heating schedule just like a thermostat. Sounds like your setting back a room t-stat and not the boiler. If you are how does the boiler know you set back? It doesn't so it doesn't know to shift the curve itself.

    Is this a Vitodens 200? Vitola 200?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • sunlight33sunlight33 Member Posts: 120
    sorry i wasn't clear

    Both the boiler (shift of the heating curve) and thermostat are set back to the same temp.
  • sunlight33sunlight33 Member Posts: 120
    Regarding my first question

    I spent last Sunday doing air seals on windows, floors and electrical boxes, and the room felt a lot better now.

    Any thoughts on my first question?
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