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What outlet temperature is ideal in a residential boiler?


I am a fairly clueless homeowner with a Lochinvar Knight WHN155 boiler. Recently I had an issue where the boiler locked out because the outlet temperature increased to 160 F which was set as the automatic shutdown limit. The manual describes how the outlet temperature is used, but... what actually is the outlet temperature? Is there a problem if it is reaching 160 F? It is usually stable at 130 F. I have no idea if this is a "correct" or ideal temperature for the outlet temperature sensor to read and would appreciate any insight. If it is not a healthy temperature, where/how would I adjust it? Thanks for your time.


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    Unfortunately, there is no "ideal" outlet temperature for a hot water boiler. It depends -- completely -- on what kind of radiation is attached, and how much there is of it, and what the heating load of the structure is.

    Many modern systems are able to vary the outlet temperature of the boiler by modulating the firing rate so as to match the heat load of the structure to the ability of the radiation to satisfy the load. Most commonly this is done with "outdoor reset". Other systems -- where the boiler can't modulate -- instead modulate the water circulating in the system, but run the boiler from time to time to add heat to the circulating water. Older systems, and a few less well done modern systems, simply turn the boiler on and off, running up to a particular temperature to satisfy the heat load of the structure and then shutting off for a while.

    For some systems, 160 would actually be rather low. For others, perhaps such as yours, it is too high.

    Which really doesn't answer your question, does it? In your case, where normally the outlet temperature seems to be around 130 and, I presume, heats the structure the way you want it at that temperature, something happened to the circulation so that not enough water was circulating through the boiler, and the temperature rose. Unfortunately (again!) there are a number of possible causes for this. The boiler may simply need a thorough cleaning and possibly descaling. The circulating pump may have failed or at least not be running well. A valve which shouldn't be closed may have gotten at least partly closed. The thermostat (strictly, aquastat) which tells the boiler when to run and not run may have failed. And so on. In any event, if your system normally runs happily at 130, and the automatic shutdown was activated at 160, something else isn't right.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Alan (California Radiant) Forbes
    The outlet temperature is the water temperature after it leaves the boiler. From what you describe, the last person that programmed the boiler set the space heating temperature not to exceed 160F. If you consistently get the error message, have a professional find out what the problem is. It could be a pump not working or a bad sensor.
    Do you have radiant heating in the floor?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • TomS
    TomS Member Posts: 62
    Your boiler should be equipped with an outdoor reset. If so your outlet boiler temperature will vary with the outdoor temperature. The warmer it is outdoors the cooler your boiler temperature will be and conversely the colder it is outdoors the warmer your boiler temperature will be. There is a diagram in the instruction book indicating this.
    HVACNUT Member Posts: 5,887
    Agree with all of the above ☝️.
    I would also check the setting for the operating limit. The high limit should be set 20°- 30° higher than the operating limit. 
    Is there an indirect water heater? 

  • Zman
    Zman Member Posts: 7,605
    Some more detail would be helpful. If the boiler actually "locked out" at 160 meaning you had to manually reset it, that is a problem that should be taken seriously. If it simply turned off at 160 because the setpoint temp was satisfied, that is normal operation.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,468
    the ideal the ideal temperature is the one that exactly matches the ever changing load. With indoor and outdoor reset you can get close. Then a proper sized boiler to prevent excessive cycling
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream