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Outdoor reset and Old Boilers

I have an old boiler in a 19th century house. When heat is required the burner fires, the circulating pump starts, and the temp of the circulating water depends on the amount of time the burner fires and the house needs heat. Isn't this what outdoor reset tries to accomplish?
I'm trying to understand how modern boilers work. It seems that the water is preheated to a certain temp, let's say 180 degree before it is allowed to circulate. From a comfort and energy saving perspective why would they not allow the circulating water to vary in temperature like it does in my ancient boiler?


  • nicholas bonham-carter
    In the modern application of ODR, the water temperature is regulated by the need for the appropriate water temperature needed to heat the radiators enough to satisfy the heating needs of the interior of the building. This water temperature setting is determined by an outdoor temperature sensor, so that in mild shoulder seasons, the radiators are less hot than in the colder temperatures of the depth of winter, but probably never as high as 180 degrees, if properly designed, and installed-more likely 140 degrees.
    I think that this adjustment of water temperature is mainly only available with the sophisticated ModCon boilers now available, and they save fuel, in many installations with older radiators.
    The water temperature of your system will vary, with the needs of the thermostat to be satisfied, rather than the needs of the structure to be heated enough for the outside temperature.
    The aim, in many cases is to have a smaller boiler, which fires almost all the time, burning more efficiently, satisfying the heat needs of the structure.
    Sadly, many of these systems have been badly designed, and the promised efficiency goals are not met.—NBC
  • Steamhead
    Steamhead Member Posts: 17,000
    ODR works by measuring the outdoor temperature and tailoring the boiler's water temperature to whatever is needed at that outdoor temperature, by varying the time the burner fires during a heating cycle. So the boiler is not preheated to full temperature before the circulator comes on.

    Some more-modern controls will hold the circulator off until the boiler is warm enough so the flue gases won't condense.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • hot_rod
    hot_rod Member Posts: 22,468
    Here is some reading that helps clear up supply water temperature SWT requirements throughout a heating season.

    You can pull data for your area to determine what % of the year you are at design condition. In many, most, areas a small % of the years requires design temperature from the boiler. Assuming it is designed correct and keeps up with the load on the coldest (design) days.

    An outdoor reset control would lower SWT on those lower load days.

    When systems are upgraded to condensing boilers they can be modulated over a wide temperature range range. Your boiler would have a minimum operating temperature to keep in mind, usually around as 130° return is the minimum °

    it may be possible to run your boiler down to a 150 SWT on low load days. That would increase efficiency and may lessen short cycling.

    Some number crunching is needed to see the homes actual load, the heat emitters output, and the boiler sizing.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream