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Need replacement aquastat for L4080B 1105

Need a replacement for old aquastat L4080B 1105, on a late 1960s Hydrotherm HC100 propane Boiler. No domestic hot water made by this boiler, just hydronic baseboard heat. Circulator control is R8845U 1003.

I suspect a faulty aquastat causing extreme short cycling. I'd like an aquastat with a larger adjustable differential.

As an aside, what does the little lever do as indicated by the red arrow in the picture?

Thank you


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,725
    The lever is just a stop to prevent you from setting it too high.
    In theory your high limit is a safety device. They pretty much all are 10° diff.
    You shouldn't be constantly bouncing off the high limit.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 303
    > @STEVEusaPA said:
    > .
    > You shouldn't be constantly bouncing off the high limit.

    Not sure that I understand that. If the temperature goes up to the high Limit the flame stops. If the temperature drops to the high limit minus the differential, doesn't the flame start again? Even if no call for heat?

    Would this aquastat serve my purpose - a high limit and an adjustible differential? For example set the high to 175° with a diff of 25, shouldn't short cycle.

  • Jamie Hall
    Jamie Hall Member Posts: 19,150
    What @STEVEusaPA is getting at with his comment on bouncing off the high limit is that if the system does this relatively quickly -- short cycles -- it may not be the aquastat. It may also indicate a variety of problems with circulation in the system...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 303
    I might be chasing "Ghost Flow" too. Could be the relay board is goosing the circulator every so often or not shutting off when no thermostat call. I'm 90% sure it is not.... But not ruled out.

    I've definitely heard short cycling.

    Checking all possibilities.
    HVACNUT Member Posts: 4,624
    Is that the only zone?
    What's the outdoor temperature? It will short cycle more often in mild weather.

    The limit shows it's set at about 185. On a heat demand, both the burner and circulator should start.
    The burner(s) will run and cycle on and off though the limit, less the differential, until until the demand is satisfied.
    That burner circuit should be wired to dry contacts on the RA845.

    The circulator will run continuously on a heat demand, unless theres a B aquastat somewhere, until the heat demand is met.

    On a cold start, once the boiler starts making some temperature you'll typically want a 20° difference between the supply and return temps.

    Its likely the boiler is oversized and will probably short cycle even at design day temperature.

    There are aquastats (like a Hydrostat) available that can bring all the control wiring into it. Thermostat, burner circuit, circulator line voltage, eliminating the RA845.

    It can also use thermal targeting to adjust the supply temperature according to the rate of boiler temperature rise and drop on a heat demand, and economy setting that will further decrease short cycling.
    Change the well to a Hydrostat and now it has a Low Water Cut Off.

    Outdoor reset is another option.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 303
    It short cycled no matter the season. Even in standby with no call for heat (unless the thermostat is faulty).

    This system always maintains ~175°F, even when no call for heat.
    HVACNUT Member Posts: 4,624
    edited October 2018
    Can you post some pics from farther back showing the burner compartment with controls and wiring, the boiler wiring schematic, as well as the wiring in the RA845?

    Does the circulator run during the short cycles?
    If so it could be the thermostat, a short, or bad contacts in the RA845.

    If the circ does not run during the short cycles.
    The L4080 is set to 185°, and with a differential of 10° the circuit will close again to fire the burners at 175°, so it seems as if someone along the road wired the boiler to maintain a constant temperature, or theres a "B" (close on temperature drop) aquastat somewhere.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 303
    The aquastat mounted now is "L4080B 1105". Is it the "B" type you refer to?

    In the attached pic, unless I misunderstand is it set to 175°? Where the pointer is aiming (red arrow)? On the wheel it looks like 200° is further down clockwise. The lever (blue arrow) is at a lower temp?

    I do not see any other controls, besides the "R8845 U 1003". It's NOT "RA845", if that makes any major difference.

    I'll have to sketch out how it's wired; likely I will be there this weekend. I want to replace the thermostat (CT87K) , it "could" be the issue although the problem existed with an old round mercury STAT prior to it.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 303
    This is in my late mother's house..... I'll have to babysit the system to see if the circ runs during short cycles. Not cold enough yet for heat to run.
    HVACNUT Member Posts: 4,624
    Now I'm really interested in that switching relay.
    When you can, pics of all will definitely help.
    And sorry about your mom.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 303
    Thank you.

    Here's a link

    I have pic of relay. Will draw out wiring when I next go on site.

    I appreciate your and everyone else's help.