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odr with gravity - no pump!

donaldmcdonaldmc Posts: 31Member
Old 1913 crane boiler - open gravity hot water system. Can a outdoor reset work on this? The thermostat goes right to the burner control and a honeywell temp limiter is in the boiler, set to 175. Any ODR out there that will work WITHOUT a circulator or a secondary controller?


  • SWEISWEI Posts: 7,356Member
    Boiler ODR control

    Will work, but you'll bottom out around 130ºF return temp.  Look at the Taco SR501, Tekmar 256, or Heat-Timer HWE-SS.
  • icesailoricesailor Posts: 7,265Member
    Gravity and ODR:

    In a sense, you already have ODR with the system you describe. The system only gets hot enough to equal the heat loss to the outside. The system temperature should never get any hotter than the requirement to heat the building. That's what ODR is really based on.

    If you had a circulator, it becomes something else but if there is no limit or operating controller and only a high limit control, the room thermostat is actually sensing the outside temperature. Gravity flow is the original constant circulation. The flow through the big pipes is equal to a circulator forcing the water through small pipes.
  • Mike Kusiak_2Mike Kusiak_2 Posts: 604Member
    edited November 2013
    Natural ODR

    Icesailor brings up a very good point. A gravity system inherently functions as an ODR system without an external sensor or controller.

    Think of it this way. The heating system constantly supplies heat to the living area because of the large thermal mass of the radiators and the gravity circulation of the water contained in them. On the other hand, there is a constant heat loss to the outside dependent on outdoor temperature. So there is a sort of balance between the heat supplied and the heat loss.

    Now say that the outdoor temperature drops. The heat supplied remains the same, but the heat loss now increases so the thermostat senses a drop in indoor temp and kicks on the burner. The burner runs long enough to bring the system water temp up to the point where the heat supplied equals the new higher heat loss and the system is in balance again. The system water temperature will rise no higher than is necessary to meet the heat loss at a given outdoor temperature, which is exactly how an ODR system works.

    Honestly, I don't believe you will gain anything by adding an ODR controller to the existing gravity system
  • icesailoricesailor Posts: 7,265Member
    edited November 2013
    Heat Anticipators:

    And the heat anticipator in the thermostat is the adjustment to make the burner run longer or shorter times. The anticipator in a sense, is the adjustment of the ODR cycle adjustment.

    How far we've come to get back to where we started.

    If you have a "virgin", untouched working gravity system with all the great big pipes, the flow will be equal and the same 1GPM goes through the radiators with little resistance in the system. Add a loop and upset the balance, and have an area that doesn't heat as fast as another, add the circulator. It overcomes the resistance of the bad part. The difference in concept is no different in an old gravity system than in the new wall hung SS boilers that will be lining the sidewalks in 5 to 10 years, waiting to be recycled.

    How far have we come?
  • SWEISWEI Posts: 7,356Member
    It would be interesting to log data

    to find out whether a boiler ODR control produces less under/overshoot on a gravity system.
  • Mike Kusiak_2Mike Kusiak_2 Posts: 604Member
    edited November 2013
    Anticipator setting

    A lot depends on the anticipator setting. If the cycles are too long then the burner will run too long and the water temp will overshoot what is necessary to meet heat loss. Short cycles will produce a more constant temp but of course more wear and tear on the boiler.

    By the way, this natural ODR doesn't only apply to gravity systems. Any hot water system with high water content and big cast iron radiators  can work similarly. Maybe not quite as well as the gravity situaton because there may not be constant circulation, but water temp will still tend to track outdoor temp, provided the boiler is not set up with an aquastat to maintain a constant temp.
  • SWEISWEI Posts: 7,356Member
    High mass systems of yore

    respond so well to mod/cons and ODR.
  • icesailoricesailor Posts: 7,265Member

    With everyone using multi-speed circulators. start at #2 and go up or down.
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