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aquastat confusion

dbessey Member Posts: 12

Ok..I have two aquastats on the pipes leaving the hot water boiler. One has the probe in the pipe, and is set to 190 F. It seems to me to be the one that shuts off the boiler burner when the temp of the water in the boiler reaches 190 f, which it does fairly often upon heat up

The other has a surface probe, is just a few inches away and is set to 110 F. I think it turns the circulator on. Unfortunately, the sequence of events tends to go like this on heat up

1) burner comes on
2 temperature in boiler hits 190 F and the burner turns off
3) Ten seconds later, the circulator comes on and the water flows...until the temp in the boiler gets cool enough that it shuts off
4) burner comes back on and the process repeats until the water in the loops is warm enough to keep the circulator going

Should the temp on the circulator aquastat be lower?
should the probe for the circulator aquastat be in the pipe rather than on the surface?




  • dbessey
    dbessey Member Posts: 12
    I should clarify
    This occurs when the system is attempting to bring thehouse back up to 68F from the overnight setting of 64F

    It seems when the system is cool and trying to get up to temperature, it has these episodes. Then , once the water is up to temp and the boiler is coming off and on to maintain the temp, it goes
    1) water at 90F to 105F , pump off
    2) house calls for heat
    3) burner comes on, water temp rises to 110 or slightly higher
    Pump comes on
    Water temp continues to rise to about 120F to 130F and the pump remains on
    House heats up

    Burner turns off
    Pump continues to pump until temp of water hits 110F
    Pump turns off

    Everything as it should be..so it seems to me

    Is there a way to set up the boiler so that it behaves better when it is trying to bring the house ( and water) up to temp after a cooler night setting?

  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    What type of radiation do you have? Cast iron? Large diameter pipes from an old gravity system perhaps?

    Some pictures would tell us.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • dbessey
    dbessey Member Posts: 12
    Hi All

    Ok, so i spent a bit more time sitting with the boiler and I am fairly certain tha the problem is the aquastat that turns the circulator on and off (honeywell L6006C) It’s the surface mounted style (not immersion)
    When the burner comes on, the pipe that the aquastat is mounted to, can reach 200F before the aquastat “clicks” and turns on the circulator. By this time, the burner has turned off on high temp.

    Is there a trick to mounting these to make the contact with the pipe better?? Should i just replace? Does honeywell have a model that i can use that has a capillary tube so that i can make sure the bulb is tight to the pipe and just mount th. Controls nearby??


    HVACNUT Member Posts: 5,887
    Can you post a pic of the boiler?
    The L6006 can likely be removed and install a triple or at least a dual aquastat in the existing well.
    Maybe an aquastat with LWCO and condensate protection.
    Post a few from 6 ft away so we can see your set up.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    You have a typical old, non-integrated system. A high limit and a low limit. Proper temperature reading is inaccurate.
    If it were me, after verifying the high limit works properly, I would leave it in, set at 200 degrees, leave it as a high limit safety, and wire a modern aquastat in series-with all the bells and whistles, set its high limit to 180, low limit off (unless you need it for domestic hot water, and enable circulator hold off and thermal purge.
    Your obvious problem is the low limit would be better off in the boiler (as would the high).

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