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Help me understand boiler curves and ODR

28W28W Member Posts: 141
I've done a careful heat loss analysis on my house, and have calculated the output of all the emitters at different water temperatures (using data from the manufacturers' websites). On the basis of this information, I selected an output curve on my boiler (Vitodens 100 with ODR).

Should the curve be selected so that the boiler sends just enough btus to the radiators to equal the heat loss at any given temperature (i.e., constant circulation)? It seems to me that in such a configuration, the boiler would never be able to "catch up" from a temperature setback (i.e., turning the heat down a few degrees at night), because to do so would require the boiler to produce btus in excess of heat loss. So, should the selected curve cause the boiler to send slightly more btus to the radiators than is required to replace the heat loss?


  • CMadatMeCMadatMe Member Posts: 3,066
    Set Back

    If you want to set back the Vitodens 100 then add the COMO-OT via the OT terminals and you can do it.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • CMadatMeCMadatMe Member Posts: 3,066
    Set Back

    If you want to set back the Vitodens 100 then add the COMO-OT via the OT terminals and you can do it.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • 28W28W Member Posts: 141

    Is my understanding of boiler curves accurate?
  • Paul48Paul48 Member Posts: 4,492
    Boiler Curves

    Your understanding is correct. The problem with night setbacks is, you cool the whole structure.
  • Empire_2Empire_2 Member Posts: 2,343
    Boilers and the pump required...

    Having done your homework on the total convectors in the structure, you now have a figure as to the heat output required in relationship to the new boiler.  Your boiler has to be able to cover the total loss of the structure at any time. 

    With a basic pump curve you are looking to cover the longest possible convector in your system and be able to have the volume to achieve designed output to that convector. 

    The boiler as a heat source must be able to cover the entire heat loss in your figures, and have the proper pump to deliver the heat, or BTU's as you are referring to.  Don't forget the boiler is your source and the pump will deliver the required heat to what ever area that needs heat. 

    Pump:  It must be able to handle the total heat loss (net BTU) and have the ability to move that heat to any one or all convectors one at a time or the entire structure.  Your "delta T" should be at 20*F, but not a rule.  Might I add HEATINGHELP.COM has a multitude of reading books to help decide what you seek.  ODR is a device that uses the outdoor temp/in relationship to total output to satisfy the load.  In essence it throttles down the boiler output to save fuel, and in turn saves money.

    The last part of your question...?  Since your boiler is sized for the total BTU output your structure, it easily satisfies any or all zones calling at once.  If only 1 zone is calling, boiler will quickly cycle on/off until that zone or zone's are satisfied.

    Hope this helps

    Mike T.
  • 28W28W Member Posts: 141
    So perhaps a TRV would be best

    My wife and I like our bedroom cool at night. We have a convector unit in there, so I'm guessing a TRV with a remote sensor would be the way to go.
  • CMadatMeCMadatMe Member Posts: 3,066

    The curve is the problem. The boiler has no idea you set back so it doesn't know you need to boost that curve coming out of set back. You need to add the Como-OT if you want to eliminate the problem.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • 28W28W Member Posts: 141
    Thanks for the explanation

    My house has a single heating zone, so that makes things even simpler. The total heat loss at a design temperature difference of 63 degrees (indoor = 70F, outdoor = 7F) is just under 30,000 btuh. I set my boiler to deliver 140F water to the radiators when the outdoor temp is 7F, which should deliver about 34,000 btuh, so there is a small buffer built in, and the boiler should be condensing in all but the very coldest weather. We will see this winter if the house stays warm enough.
  • CMadatMeCMadatMe Member Posts: 3,066
    Btu/hr and Water Temperature

    Are two different animals. Your emitters need an X water temp to emit the btu's created by the source. If that heat source isn't aware of a set back situation it is not capable of delivering the required water temp to get you out of set back in a reasonable time. In other words, mornings will be uncomfortable. On the Vitodens 100 simply remove your existing thermostat and replace it with the Como-OT which is wired to the OT terminals on the same wiring block the outdoor sensor is wired to.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • 28W28W Member Posts: 141
    edited September 2012
    I understand

    This is what I was trying (clearly unsuccessfully) to describe. If I set the curve high enough so that the boiler can overcome a setback, I lose efficiency because I'm heating the radiators more than needed.

    I looked up the wiring diagrams in my Vitodens manual. This looks like something a homeowner can do himself . . . no?
  • icesailoricesailor Member Posts: 7,265
    True but missing a piece:

    All of what you said is true but you are missing a piece.

    Once you have all the info you have posted and asked about, all will work.

    The question is about ODR and set back thermostats.and wanting the system to "catch up".

    Understand that setback thermostat manipulate the outside temperature. If the indoor set-point is 70 degrees inside with zero degrees outside. and you target boiler system is 180 degrees, all will be well if these settings are used, But, if you turn the inside temperature down by 10 degrees (to 60 degrees on the thermostat), you have theoretically raised the outside temperature by ten degrees. When the thermostat goes back to the set-up temperature, 70 degrees, because of the setback, the system needs to see a rise in system temperature that the OAT/ODR control has no ability to know about. The 180 degrees will heat the structure at zero degrees temperature, but not at 10 degrees below. It would at ten degrees above and have extra, but not when trying to recover from a setback at minus ten degrees outside.

    The thermostat mentioned looks like you only use it for one zone. It won't cover multizones. If it does or would, you have to install it on the zone that is the most important to you. Like the bedroom.

    My own personal limited experience with the setbacks are that if you get too close to the curve, you may find it comfortable but women don't like it because it may be too cold and the recovery is too long. You have to move the set-point curve up so that you run warmer system water. It's not the drop, it's the recovery that is the problem.

    You may not mind wearing a coat in the house but your wife or female partner may object. When my wife comes in from outside, she may not notice that it is cold in the house. But after she sits down, she is cold. If she is cold, there is no force on earth that will make her happy except seeing the reading on the indoor/outdoor thermometer on the table at the temperature she finds comfortable. And she knows what that is..

    There needs to be a device that acts as a clock thermostat on the ODT/OAT line to the controller. When someone finally makes one, everyone will be using it.

    Another way would be to use the DHW priority to overcome the setback.

    It doesn't matter how fast or slow you pump the water in the system. If it is not hot enough to overcome the temperature differential inside, the heated space will not recover.

    In my experience.
  • CMadatMeCMadatMe Member Posts: 3,066

    The device you are talking about exisits. For the Vitodens 100 that would be the Como-OT for Vitodens 200 and some other mod cons out there, that logic is built into the boiler controls but seldom used by most installers.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • icesailoricesailor Member Posts: 7,265

    Be careful that you don't end up stepping over a $10.00 bill to pick up a dime ($0.10).

    If I only had a penny in my pocket, and I found another penny, I doubled my wealth. If I had a dollar bill in my wallet, and I found a dime, my value increased by 10%.

    The smartest ODR control can not figure out infiltration from high wind. I've seen many buildings that will heat to 70 degrees inside when it is 10 degrees outside with no wind blowing. But wouldn't go over 60 degrees inside when it is blowing 35 knots outside and it is 25 degrees outside.

    Progress not perfection.
  • 28W28W Member Posts: 141
    For sure . . .

    I think what you are saying is, don't get so fixated on running the boiler at the absolute lowest curve to squeeze out that last dime's worth of efficiency, that you make life miserable for yourself.  Run a slightly hotter curve if needed, to provide some extra btu capacity for those cold, windy days, etc.

    My system was installed in July, so this will be the first winter it heats my house.  I plan to experiment a bit to find the best boiler curve.
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