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Low limit on Weil-McLain Ultra-Oil?

Jud_Aley Member Posts: 15
I just had a used Weil-McLain Ultra-Oil UO-3 installed. It has a AuquaSmart 7600.
In its prior location it was used for CI radiators and an indirect Hot Water Tank.
In its new location it’s used only for CI radiators, no indirect, least not at this time, maybe in the future.
The high limit is set at 180 and the low limit is set at 150. The AuaSmart manual says that the low limit should be at 150 to prevent condensing in the flue. The flue is a clay tile lined brick chimney.
Configured this way the boiler seems to run often and in very short cycles when there is a call for heat or to maintain the LL.
My question is can I safely lower the LL to something like 100 or 120 to save on fuel and get longer run times when there is a call for heat? Or am I asking for trouble and condensation?
Thanks - Jud


  • Jamie Hall
    Jamie Hall Member Posts: 17,832
    No. Neither the boiler nor your flue is designed for condensing flue gas -- the condensate is insanely corrosive, particularly from oil.

    How big is it in relation to your load?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Big Ed_4
    Big Ed_4 Member Posts: 1,829
    Hmmm , Cast iron radiators you don't want to heat more then 160* , ... You may need to add an boiler bypass to run that boiler on those higher temperatures ...
    I have enough experience to know , that I dont know it all
  • hot_rod
    hot_rod Member Posts: 15,428
    Here is an example of what a boiler protection device and piping may look like. This is a high flow valve, non electric solution.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jud_Aley
    Jud_Aley Member Posts: 15
    Jamie--The boiler is appropriately sized for the load, its not perfect, but its close. Remember this was a 2nd hand boiler that replaced a 65 year old unit so it’s a big improvement.

    Big Ed 4- - So you are suggesting the HL be set to 160 because of the CI radiators? If the HL is set to 160 then what would be a safe LL?

    Bob- It has boiler protection piping. When I first spoke with the installer about doing this job it was one of the first things he brought up, said it was essential. As best I understand some of the hot supply water leaving the boiler gets mixed in with the cool return water to prevent thermal shock. The installer is a 2nd generation oil man, I trust this guy, he knows what he is doing.

    Im not sure if the installer set up the AuquaSmart to its current settings or that they are from its other location where it had an indirect HW tank.

    I’ve seen discussions here about warm start and cold start boilers and that got me wondering about this one.
    At some point Ill ask the installer his thoughts on it, but thought I would ask here first.
  • gaabbee
    gaabbee Member Posts: 43
    Sit down and time how long it takes your aqua stat to raise from 150 to 180 degrees. If you are only getting 3-5 mins your system is most likely over sized. (I'm going to guess it is) If this is the case you could look into adding a buffer tank. From the sound of it maybe you have a coil in the boiler for DHW? If that's the case I would consider getting a reverse indirect tank. This will add a buffer and take care of your hot water needs. By adding the extra mass (water) your boiler will run longer (10-15 mins) and burn less fuel if sized correctly. It will also help with condensing issues because the buffer tank will be kept above 130 degrees.
  • Big Ed_4
    Big Ed_4 Member Posts: 1,829
    Would not be my choice boiler for your system . You say they require min 150* low , You need*minimum 20* diff between the Low and the Hi limit which brings the supply to 170* plus any over ride . I would recommend temper the supply to the radiators . Taco makes a mixing valve with an outdoor sensor .. It will tame the beast for comfort ..
    I have enough experience to know , that I dont know it all